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The Geology and Mineral Potential of PAPUA NEW GUINEA
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Edited by Anthony Williamson and Graeme Hancock
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Edited by Anthony Williamson and Graeme Hancock
The Papua New Guinea Department of Mining
wishes to thank the World Bank Technical
Assistance Project in the Mining Sector for its
support in making this publication possible.
Compiled by
Dr Greg Corbett
Edited by
Anthony Williamson and Dr Graeme Hancock
Publisher
Papua New Guinea Department of Mining
Supported by
World Bank Technical Assistance Project
in the Mining Sector
Graphic Design
Lian Rigano
Production
Alan Caudell and Associates
Copyright
© Papua New Guinea Department of Mining 2005
ISBN
9980-81-490-X
Photographs
The publishers wish to thank the following companies and individuals.
Highlands Pacific Group
United Pacific Drilling
Ok Tedi Mining Limited
Lihir Gold Limited
Greg Corbett
Trevor Neale
Rocky Roe Photographics
INDEX
Acknowledgements
4
Foreword
5
1.
Papua New Guinea – An Overview
6
2.
Mineral Discovery and Mining in PNG
9
3.
The Mining Sector’s Contribution to the PNG Economy
14
4.
Geological Framework of PNG
22
5.
Geological Terranes and Mineralisation
30
6.
Tectonics and Mineralisation
42
7.
Mineralisation Styles
50
8.
Mineral Projects and Mines
60
9.
Environment
138
10. Other Relevant Agencies of Government
140
11. References
142
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Acknowledgements
Many geologists contributed to this publication, some as independent consultants,
employees or contractors. It is only fitting that due acknowledgement be given for
their individual efforts as far as is possible.
The main contributor was Dr Greg Corbett, who was primarily responsible for the
first draft of the book whilst consulting to URS Australia Pty Limited, the principal
contractor for the work. Dr Corbett has extensive practical knowledge of Papua New
Guinea mineralisation and geology. He has worked extensively throughout the Pacific
Rim as a geological consultant.
More specifically, Dr Corbett’s ideas are reflected in the sections entitled Geological
Framework of PNG, Geological Terranes and Mineralisation; and Mineralisation Styles.
The latter section contains a wealth of data that provides technical background on
epithermal and porphyry related mineralisation using examples from Papua New
Guinea.
The section entitled Tectonics and Mineralisation is a combination of interpretations
from Dr Corbett, Professor Hugh Davies, Dr Robert Findlay and Dr Richard
Rogerson. The editors have taken liberty in amalgamating the contributions and
this proved to be the most difficult section to present to the reader.
Professor Davies gained his PhD many years ago based on new tectonic theories in
Papua New Guinea, and is currently employed as head of the School of Earth Sciences
at the University of PNG. Dr Findlay, until recently, was employed in the regional
geology section of the Geological Survey Division of the PNG Department of
Mining. Dr Rogerson has extensive regional geology experience in PNG, having
worked in the country for many years. He is currently employed by the West
Australian Department of Minerals and Energy.
Dr Corbett’s experience is again relied heavily upon through his compilation of the
section entitled Mineral Projects and Mines. The editors and Dr Findlay had further
input into this section.
The Compendium of Prospects, presented on the companion CD, was compiled
by experienced Papua New Guinean field geologists Kassy Akiro, Jerry Garry, and
Khon Digan, with input from Peter Macnab.
The remaining sections were compiled by, or had input from, the editors,
Anthony Williamson and Dr Graeme Hancock . Dr Chris McKee and Lawrence
Anton made an important contribution via the seismic interpretation and earthquake
data maps and profiles.
4
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Foreword
It is widely recognised that Papua New Guinea (PNG) is well endowed with natural
resources, in particular, economic minerals that are sources of gold, silver, copper,
nickel and chromite.
PNG has been on the world mineral map since the early 1970s after its first porphyry
copper mine came into production. Subsequent to that, the geological understanding
of the nature of gold mineralisation in epithermal and porphyry settings has
deepened, with the result being the development of a second large copper-gold mine
and several major gold mines in PNG. There are currently three world-class mines
that collectively produce in excess of 65 tonnes of gold and 200,000 tonnes of copper
each year, so the opportunity for a return on investment in the PNG mineral sector
is proven.
In addition, one of the objectives of this publication is to highlight to potential
investors in the PNG minerals sector that there remain numerous untested, or
incompletely tested exploration targets, not to mention the as yet undiscovered
resources.
The Minerals Tenement administration system used by the Department of Mining is
based on the Mining Act (1992) and provides the investor a high degree of security
over their investment. The country has a competitive fiscal regime that operates
under terms and conditions that most accountants and financial advisers would be
familiar with.
The regional geology sections of this publication may appear repetitive to the
informed, but our intent is to provide sufficient information within each section in
an effort to stimulate the readers’ imagination.
One of the most obvious comments I would like to make based on my experience
with the minerals sector in Papua New Guinea, is that management persistence,
support and patience is an essential element for success in any PNG operation.
This publication details numerous prospects that are yet to reach economic thresholds
due to the fact that they have not yet been fully explored, mainly as a result of
management decisions to curtail exploration due to disillusionment of a project’s
potential. Some may see this as a lost opportunity for one but a gain for another. The
point is that the rugged, deeply weathered tropical terrain of Papua New Guinea does
not easily yield up its riches.
KUMA AUA, OBE
Secretary
Papua New Guinea Department of Mining
The Geology and Mineral Potential of
PAPUA NEW GUINEA
5
1. Papua New Guinea - An Overview
BRIEF FACTS
Full name:
The Independent State of Papua
New Guinea
Land area:
474,000 square kilometres
Population:
5.3 million, 85% of whom are
rural based
People:
95% Melanesian, 5% Polynesian,
Micronesian and Chinese
Language:
805 indigenous languages plus
Pidgin, Motu and English
Legal system:
Based on English common law
Government:
Constitutional Monarchy with a
Parliamentary Democracy
GDP:
K11.63 billion (approx US$3.6
billion)
GDP per head:
US$675
Inflation:
4%
Major industries: Gold, oil, copper, coffee, silver,
copra, palm oil processing and
logging
Trading partners: Australia, Japan, USA and China
GEOGRAPHY
Papua New Guinea lies in the southwest Pacific, just
below the equator, between Asia and Australia. It
comprises more than 600 islands and covers
474,000km2. The country has a wide variety of
landscapes. Rugged highlands over 1000m high form
the core of the mainland and are flanked by rainforestclad foothills and savannah. Extensive swamps with
navigable rivers dominate the western mainland to the
north (Sepik River) and south (Fly River) of the
highlands. The major islands of New Ireland,
Bougainville and New Britain are surrounded by
striking coral formations. Numerous active volcanoes
and geothermal areas stud the islands and are often
scenes of unpredictable natural violence — in 1994,
the once-beautiful New Britain town of Rabaul was
destroyed by the Tuvurvur eruption.
CLIMATE
The country extends from the equator to latitude
10° south and the climate is typically monsoonal —
often being hot, humid and wet all year-round. There
are defined wet (December to March) and dry (May
6
to October) seasons, but both are subject to regional
variation (especially in the islands). Rainfall, for
example, varies remarkably: Port Moresby, on the
southeast coast, usually experiences an annual rainfall
of 1000mm (39 inches) while Lae, on the north coast,
has over 4500mm (176 inches). In extreme rainfall
areas, such as West New Britain, the annual rainfall
can exceed 6m (20 feet) per year. Temperatures on the
coast are reasonably stable all year (between 25° and
30°C; 77 to 86°F) but the humidity and winds are
changeable. Temperatures are noticeably lower at
higher altitudes, and it can be very cold in the
highlands.
Cyclones occasionally affect the eastern islands. Papua
New Guinea is subject to El Nino influenced climatic
variations. Most recently, an extensive drought during
1997 caused the Fly River to become un-navigable,
interrupting the operations of the Ok Tedi Mine.
POPULATION
Papua New Guinea is essentially a southwest Pacific
Melanesian culture with Polynesian and Micronesian
influences, sandwiched between Asia to the west and
north and the western cultures of Australia–New
Zealand to the south. Archaeological evidence
indicates that people arrived about 50,000 years ago
and began farming 30,000 years ago, making them
probably the world’s first farmers, domesticating crops
such as sugar cane.
The population of 5.3 million people can be loosely
divided into four regional groupings:
• Highlanders — living in the mountainous part of
Papua New Guinea, and make up approximately
30% of the population of Melanesian origins
• Papuans — from the south coast, where the
Polynesian influence is most apparent
• New Guineans — from the north mainland coast
• Islanders — from the offshore islands of mostly
Micronesian and Polynesian influences.
Internal migration, which began with plantation
labourers in the 19th century and continued with the
modern mobile work force, has blurred these original
distinctions. The number of foreign nationals in
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Papua New Guinea - An Overview (cont.)
Papua New Guinea has declined from a preindependence peak of 50,000 to the current 24,000.
The principal expatriate groups comprise refugees
from West Papua living in camps near the border,
expatriate professional workers, plantation owners and
missionaries.
GOVERNMENT
Papua New Guinea became an independent
parliamentary democracy on 16th September 1975.
There are three levels of government — national,
provincial and local.
The government is formed in the National
Parliament, the Head of State is the Prime Minister
and the ceremonial Head of Government is the
Governor General. The National Parliament,
sometimes referred to as the House of Assembly, has
109 seats; 89 members are elected from open
electorates and 20 from provincial electorates. In the
past, the members have been elected by popular ‘first
past the post’ style vote to serve five-year terms. The
constitution has recently been amended to introduce
limited preferential voting. Elections were last held
during 15–29 June 2002 and April and May 2003,
being completed in May 2003. The next elections will
be held no later than June 2007. There is a multiparty system and most governments are formed from
a coalition of parties and independent members.
EDUCATION
Papua New Guinea recognises the importance of basic
education, with about 70% of children attending
primary (community) school and about 30% passing
onto secondary school. Secondary education is
delivered mainly in the government-operated
National High School System, supplemented by many
mission schools and several international schools.
Tertiary institutions that train professional geologists,
engineers and metallurgists include the University of
Papua New Guinea at Port Moresby and the
University of Technology at Lae.
LANGUAGE
Each of the provinces has a provincial assembly,
governor and a bureaucracy to handle provincial
matters. A further 150 local councils function at
village level where there is an overlap with the
traditional clan system in which elders (bigmen)
provide leadership.
In a country comprised of isolated mountain or island
communities, over 800 discrete languages (tok ples)
have arisen. ‘Police Motu’ was adopted as the ‘lingua
franca’ of Papua by the early Papuan administration
from the simplified Motuan tongue historically used
as a trading language on the south coast of Papua. In
New Guinea, Melanesian Pidgin evolved in the postGerman administration times by integrating words of
many languages, including German, Malay and
Kuanua (Tolai), with basic English (in which many
words were given altered meanings). Pidgin has
continued to evolve as a dynamic language, such that
with the integration of the Papua and New Guinea
administrations at independence, and the greatly
increased mobility of the population in recent
decades, the use of Motu has largely died out except
around Port Moresby, where Hiri Motu is the
traditional language. Pidgin is almost universally
understood and used between members of different
language groups. English has been adopted as the
country’s official language and it is widely spoken
amongst educated people. There are people who
understand English in most areas of PNG.
JUDICIARY
RELIGION
The highest judicial body is the Supreme Court. The
Chief Justice is appointed by the Governor General on
the proposal of the National Executive Council after
consultation with the minister responsible for justice.
Other judges are appointed by the Judicial and Legal
Services Commission.
Christianity is the dominant religion (96%) with the
main denominations being Catholic (27%), Lutheran
(19%), United Church (11%), Seventh Day Adventist
(10%), and other protestant (19%). Other significant
minorities include the Baha’i Faith (1%) and
indigenous beliefs (1%).
The Geology and Mineral Potential of
PAPUA NEW GUINEA
7
Papua New Guinea - An Overview (cont.)
CURRENCY
The national currency is the Kina (K), which is made
up of 100 toea, with each name being based upon
traditional shell monies. In September 2004, the Kina
was valued at US$0.30 and AUS$0.43. There is an
adequate network of locally owned and foreign
(mainly Australian) banks operating in the major
centres.
TRAVEL
The isolated mountain and island communities of
Papua New Guinea and the mining industry have a
long and enthusiastic relationship with air travel (see
section entitled ‘Mineral Discovery and Mining in
PNG’). The national carrier Air Niugini provides
national and international services, many of the latter
in code share arrangements with other airlines. Key
services and connections include:
• Australia - to and from Cairns (1.3 hours flying
time), Brisbane (3 hours flying time), Sydney (4
hours flying time, ex-Brisbane);
• Europe - via Air Niugini services to Narita, Manila
and Singapore;
• North America - via Australian or through Asian
connections.
Air Niugini also connects to Honiara in the Solomon
Islands.
Domestic Air Niugini services link most of the major
regional centres. Several helicopter companies service
the mineral and petroleum exploration and mining
industries using aircraft ranging from the smaller
Hughes 500 and Longranger, to Mil 8 helicopters.
INTERNATIONAL TREATIES
Papua New Guinea is a party to the following treaties:
Antarctic Treaty, Biodiversity, Climate Change,
Desertification, Endangered Species, Environmental
Modification, Hazardous Wastes, Law of the Sea,
Marine Dumping, Nuclear Test Ban, Ozone Layer
Protection, Ship Pollution, Tropical Timber 83,
Tropical Timber 94, and Wetlands.
Channel chip sampling on a remote prospect site.
8
The Geology and Mineral Potential of
PAPUA NEW GUINEA
2. Mineral Discovery and Mining in PNG
INTRODUCTION
For thousands of years the indigenous people of Papua
New Guinea have mined and traded stone
implements and ochre, and used clay to make pottery.
Gold was first discovered in Papua New Guinea in
1852 as accidental traces in pottery from Redscar Bay
on the Papuan Peninsula (Fig. 2.1). This summary of
prospecting in Papua New Guinea is taken from Nye
& Fisher (1954), Cotton (1975), Nelson (1976),
Lowenstein (1982), Williamson (1982), Loudon
(1984) and Davies (1992).
EARLY EXPLORATION —
PRE-WORLD WAR I
By the 1870s, gold prospectors, who had migrated
northwards along the east coast of Australia,
progressed to the islands of Papua New Guinea
(Fig. 2.2). In February 1873, Captain Moresby of
HMS Basilisk reported traces of gold from the vicinity
of what is now Port Moresby, and this was exaggerated
in a speech to the Colonial Institute in London the
following year. At that time, Papua New Guinea was
unclaimed by any European powers. Aviation pioneer
Lawrence Hargrave found a speck of gold and a
specimen of copper at the furthest point of D’Albertis’
exploration voyage in the Ok Tedi River in 1876. In
1877, gold was again reported from the vicinity of
Port Moresby and a small rush prospected the Laloki
River without success — many of the miners died
from malaria. In 1884, Britain established a
protectorate over Papua, and Germany colonised
northern New Guinea.
Papua New Guineans, returning from labouring on
Queensland plantations, may have identified the first
meaningful quantities of gold in PNG at Sudest
Island. David Whyte produced 142oz of gold from
there in 1888 and sparked a rush of miners from
Australia, resulting in a further 15,000oz produced to
1898. The rush of 400 miners soon exhausted the
shallow alluvial and eluvial gold on the island, so the
British Administrator, William MacGregor (later Sir
William), used HMS Swinger to take miners
prospecting on nearby islands, eventually finding gold
on Misima in October 1888. By 1895, gold had also
been identified on Woodlark Island. As the miners
began to prospect the Papuan Peninsula, gold was
identified at Mambare River in 1896, Gira in 1897,
and Yodda in 1899 (where platinum and osmiridium
were also discovered). The prospectors moved
The Geology and Mineral Potential of
PAPUA NEW GUINEA
progressively to Milne Bay, Cloudy Bay and
eventually, by 1909, the rich Lakekamu River alluvials
on mainland PNG were identified. Lode mining for
gold began on Sudest Island as early as 1890; miners
began to work the lodes at Woodlark in 1900 and on
Misima in 1904. These operations remained active
until World War II. Further details are included in the
discussions of those projects later in the text.
At the Astrolabe mineral field near Port Moresby,
massive copper ore was mined from 1907 to 1926 at
the Laloki and Dubuna Mines and transported by
light rail and aerial ropeway to a smelter near the
Tahira Inlet wharf. Two other mines provided ore for
another smelter from 1938 to 1942 (Davies, 1992;
Williamson, 1982).
In German New Guinea, Ernst Tappenbeck
discovered gold in the lower Ramu River in 1898 and
a German syndicate worked gold in the Waria River
from 1901 to 1904. Some prospectors entered the
Waria Valley from Papua with the blessing of the
German administration, and an area was reserved for
the Rudolf Wahlen syndicate. In 1910, a CanadianAustralian, Arthur Darling, identified gold in what
became the Morobe Goldfield. However, the focus of
the German administration in New Guinea was more
on scientific endeavours than prospecting, as distinct
from the Australian administration in Papua, which
saw gold mining as a valuable source of revenue. Table
2.1 summarises gold production in Papua New
Guinea to 1926.
Goldfield
Production (oz)
Date field
proclaimed
Sudest
Misima Island
Woodlark Island
Gira River
Milne Bay
Yodda River
Keveri River
Lakekamu River
Astrolabe (Port Moresby)
Morobe
10 035
138 049
200 348
67 242
14 230
76 822
4 770
37 170
3 300
23 005
Total (rounded)
575 000
1888
1889
1895
1898
1899
1900
1904
1909
1906
1923
Table 2.1 Gold production from Papua New Guinea to
30 June 1926 (after Nelson, 1976 and Lowenstein, 1982).
9
Mineral Discovery and Mining in PNG
(cont.)
Fig. 2.1 Prospects, mine sites and localities cited in this book.
Fig. 2.2 Pre-World War I gold field proclamations and discoveries.
10
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Discovery and Mining in PNG
THE WAR AND INTER-WAR
YEARS (1914–45)
At the start of World War I, Australia took possession
of the German colony of New Guinea, which was
administered from Rabaul at that time. In 1922, New
Guinea was made a Mandated Territory of Australia
by the League of Nations. In the same year, a mining
ordinance was put in place to legalise prospecting.
After Arthur Darling’s death in 1921, William ‘Shark
Eye’ Park rediscovered Darling’s gold find at Morobe
in 1922, and together with Jack Nettleton began to
work gold in secret at Koranga Creek, in what is now
the Morobe Goldfield. By 1923, miners began to
flock to the field, which was proclaimed that year.
The number of expatriate miners grew rapidly to 219
in 1926. Also in 1926, William Royal and Dick
Glasson climbed past substantial waterfalls to discover
the incredibly rich alluvial gold deposits in Upper
Edie Creek, winning up to 240oz/day from a single
sluice box (Lowenstein, 1982). At that time, it took
eight days for labourers to carry supplies from the
coastal port of Salamaua to Wau, consuming part of
the cargo along the way.
(cont.)
A milestone in prospecting and gold mining came in
1927 with the first Lae to Wau aeroplane flight,
leading to the next stage in development of the
Morobe Goldfield. From 1932, Bulolo Gold
Dredging, floated by a precursor of the international
Canadian gold mining company Placer Dome,
constructed eight dredges at Bulolo and Wau from
dismantled parts flown in using three Junkers aircraft.
This resulted in a total airlift of 39,417 tons of freight,
for production of 1.3 million ounces of gold, until the
planes were destroyed by Japanese fighters in 1942.
The Morobe Goldfield reached its peak production in
1938 when 700 expatriate and 6,218 national miners
produced 404,000oz gold. Dredging resumed after
the war and continued until the last dredge closed
down in the mid-1960s. By the mid-1980s, the field
had produced 3.5 million ounces of alluvial gold and
0.5 million ounces of hard-rock gold (Nelson, 1976;
Loudon, 1984).
Prospecting by Ned Rowlands in the Eastern
Highlands led to the discovery in 1928 of gold near
Kainantu, while in 1930 the Upper Ramu River was
declared a provisional goldfield, and from 1934 the
Sepik and Torricelli regions were explored.
Table 2.2 PNG gold production 1928-1951.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
11
Mineral Discovery and Mining in PNG (cont.)
Government patrol officers who entered new
territories panned to test drainage systems for gold,
and between 1933 and 1939 Jim Taylor and John
Black identified gold downstream from Porgera. The
government sanctioned epic prospecting expeditions
such as the 1933 patrol of the Leahy brothers (the film
of which still remains).
Between the two World Wars, gold mining
represented a significant source of income to the
Papua New Guinea administration. (Table 2.2)
POST-WORLD WAR II
After World War II, prospectors moved to follow up
on the Porgera discovery in 1948, but only Joe
Searson remained to work the alluvial gold. A road
completed by army engineers linking Wau to Lae
greatly aided mining in the Morobe Goldfield.
Jack Thompson, the Chief Government Geologist,
promoted mineral exploration in PNG and in the late
1950s initiated geological surveys. International
mining companies extended these surveys to evaluate
the Papuan Ultramafic Belt on the Papuan Peninsula
for lateritic Ni–Co deposits.
At Porgera, during the 1960s, Searson focused on the
hard-rock potential, forming a syndicate to finance
initial adit development. With the help of the
Administration, he attracted other explorers such as
Bulolo Gold Dredging and later Mount Isa Mines
(MIM), which began to drill test the Waruwari hardrock resources at Porgera. Eventually, in 1983,
continued geological studies by a consortium of Placer
(now Placer Dome), Renison Goldfields Consolidated
(RGC) and MIM identified the Zone VII high-grade
mineralisation, dramatically improving the economics
of the project.
THE 1960s PORPHYRY
COPPER BOOM
At the time Searson was promoting Porgera to major
mining companies, the science of porphyry Cu–Au
mineralisation was beginning to emerge. In 1962,
Ken Phillips of Conzinc RioTinto of Australia (CRA)
applied a geological model, based on Philippine
porphyry deposits, to Papua New Guinea mineral
exploration. In 1964, following the advice of Jack
Thompson and a 1930s report of alluvial gold and
lode copper on Bougainville, Phillips identified the
Panguna porphyry Cu–Au deposit. Panguna went
12
into production in 1972, and had produced 30Mt of
copper and 9.6 million ounces of gold by its closure at
the end of 1988.
The Australian Bureau of Mineral Resources (BMR)
provided geological services to PNG from 1948 to
1972. The BMR contribution to the geological
understanding of PNG was significant. In the late
1950s, BMR geologists discovered the Yandera copper
mineralisation, were responsible for the preparation of
most of the 1:250,000 scale geological maps of PNG,
and in 1962 discovered the Ramu lateritic Ni-Co
deposit. The Ramu project has been periodically
evaluated ever since. Again in 1966, the BMR
geologists recognised mineralised float in streams
which subsequently led to the discovery of the Frieda
porphyry Cu–Au system. MIM acted on reports of
the BMR discovery to take up ground covering the
Frieda prospect. Further exploration of the Frieda
area led to discovery of the Nena high sulphidation
Cu–Au mineralisation in 1979.
Regional exploration continued elsewhere in the
rugged jungles of Papua New Guinea. In 1968,
Kennecott Copper Corporation geologists followed a
cupriferous float train from the junction of the Ok
Menga and Ok Tedi drainages to identify the Ok Tedi
porphyry Cu–Au intrusion at Mt Fubilan. After its
success with the discovery of Panguna, CRA outfitted
a ship (the CRAEStar ) with its own laboratory and
helicopter, which was used to prospect the western
Pacific rim for porphyry Cu–Au deposits. Many
anomalies identified during this time are still being
explored (e.g. Wafi), and surprisingly, some still
remain to be followed up.
From the mid 1960s to the early 70s, vast areas of
Papua New Guinea were subjected to first-pass
prospecting for porphyry copper style mineralisation.
This work was carried out at a time of relatively low
gold prices. Thus, the exploration programs gave little
or no consideration to gold as a possible exploration
target.
THE 1980s GOLD BOOM
When Papua New Guinea gained independence in
1975, Panguna was the only major mine operating.
As the price of gold rose in 1974 and again more
spectacularly in 1979, there was increasing
international recognition of the gold potential of
Papua New Guinea. This led to a significant increase
in applications for Exploration Licences.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Discovery and Mining in PNG
A moratorium was placed on granting new
applications in 1980 to enable the Department time
to assess and process the Applications. The
moratorium was lifted in November 1982, resulting in
a flood of new applications from international
companies and consortia (e.g. Niugini Mining –
Kennecott Joint Venture, CRA, RGC), and many
other junior exploration companies. The 1980s saw
the start of a new gold rush in the country (Fig. 2.3).
As mentioned above, much of the earlier porphyry
copper exploration did not include assaying for gold
(e.g. CRAEStar) as it was not considered economically
significant at that time. The new gold discoveries in
areas such as Kerimenge, Hamata and Hidden Valley
(all in the Morobe Goldfield), as well as Mt Kare and
Tolukuma, were the result of a new generation of
helicopter-supported reconnaissance which explored
much of Papua New Guinea for gold mineralisation.
The exploration efforts were given a conceptual basis
by the Geological Survey of Papua New Guinea which
promoted the application of new tectonic theories
(Rogerson et al., 1988). Geochemical studies by the
BMR (Wallace et al., 1983) played a distinct role in
discovery of the giant Ladolam
gold deposit on Lihir Island,
while the application of new
conceptual geological models
led to discoveries such as
Rafferty’s at Wafi in 1989
(Corbett and Leach, 1998).
(cont.)
Guinea for gold mineralisation first took hold in
1982, and was subsequently validated with several
new discoveries being made over the ensuing few
years. In addition to the examples cited above, the
gold rush at Mt Kare (1987–91), the bonanza gold
grades produced from early mining at Porgera Zone
VII (1991–02), and good results from drilling the
Minifie Zone on Lihir Island could be added to
substantiate the high prospectivity for gold
mineralisation of PNG. All of this has encouraged
further prospecting and exploration.
The 1987 stock market crash brought the 1980s gold
exploration boom to an abrupt end in Papua New
Guinea and indeed throughout the world. Mergers,
acquisitions, a declining commitment to explore, and
lacklustre investor sentiment in the mining sector,
reigned throughout the nineties. Only recently has
there been an increase in exploration activity in PNG.
Detailed accounts of the discovery and development of
mines (e.g. Lihir, Tolukuma) and many exploration
projects (e.g. Wafi, Hidden Valley) are presented later
in this document.
Reappraisal of the oxidised lowgrade gold mineralisation
surrounding the previously
mined (underground 1911-41)
high-grade mineralisation at
Umuna, on Misima Island in
1976-7 by Peter Macnab, led to
its redevelopment as an opencut mine in the late 1980s.
Tolukuma, discovered during
helicopter-supported regional
reconnaissance geochemistry in
1985, saw mine construction
begin in May 1995. The mine
now operates without a road
link, depending entirely on
helicopter support.
The
perceived
high
prospectivity of Papua New
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Fig. 2.3 Exploration expenditure and licence applications per year.
13
3. The Mining Sector’s Contribution to the PNG Economy
INTRODUCTION
The economy of PNG is
dominated by a subsistence and
cash agriculture sector, yet since the
early 1980s mining and petroleum
continued to be the driving force in
the
economy,
contributing
significantly to total exports,
government revenue and GDP.
Mineral product exports alone
accounted for 55% of total
merchandise exports in 2003,
equivalent to some PNGK 4.2
billion (approx US$1.1billion)
(Fig. 3.1).
Employment directly attributable
to mining is estimated to account
for 5% of the total available
workforce; furthermore this figure
represents about 20% of the total
formal rural workforce (Table
3.1). The indirect employment
figure derived from support
services, contractors etc not
classified as ‘mining’ but engaged
on mining related projects,
obviously makes the overall
employment figure due to mining
significantly higher.
SECTOR
Fig. 3.1 Export value in US$.
Fig. 3.2 Real GDP Growth 1995-2005.
Source: Bart Philemon, Economic and Development Policies; Department of Treasury
2003, Mike Manning (INA) 2004.
EXPORTS
%
Nominal
GDP %
2003
2003
EMPLOYMENT %
ESTIMATED 2000
Agric./Forestry/Fisheries
Mining
Petroleum
Manufacturing
Construction
Wholesale/Retail Trade
Transport/storage/comms
Electricity/gas/water
Business Services
Community/Social/Others
Other
Total
24
55
21
100.0
26.6%
17.3%
7.8%
9.3%
4.8%
9.7%
4.8%
1.3%
3.4%
12.7%
1.8%
23
5
15
7
17
33
100.0
Table 3.1 Sectoral Contributions to the PNG Economy.
Source: Dept of Treasury; Lavantis, 2000.
14
Since the early 1990s, minerals and petroleum sector
products have consistently comprised around 70% of
total merchandise exports; comprised over 20% of total
government revenues and; was between 16% (Nat Stats
Office) and 25% (Dept of Treasury) of GDP, on
average (Fig. 3.2).
As mentioned previously, systematic mineral
exploration of Papua New Guinea commenced in the
1960s with attention largely directed towards finding
porphyry copper deposits. By the mid-1970s, three
world-class deposits and several smaller systems had
been discovered. In the 1980s, attention shifted to
gold exploration in previously known as well as virgin
areas, and resulted in recognition of two world-class
deposits, each containing >200t of gold, and
numerous smaller deposits of economic interest.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Mining Sector’s Contribution to the PNG Economy (cont.)
Fig. 3.3 B=Bougainville; O1=Ok Tedi Gold; O2=Ok Tedi Copper; M=Misima; P=Porgera; T=Tolukuma; L=Lihir,
Black line = Bougainville closure in 1989.
Fig. 3.4 Year 2003 ranking of top ten gold producers in PNG and Australia.
Papua New Guinea has been ranked as the 11th
largest gold producer in the world over the past few
years and is a significant copper producer, with a very
real potential to exceed the present production level
for both commodities. Silver is a commercial byproduct from most of the mines. Accordingly, most
exploration and mining carried out within Papua New
Guinea is, and has been, for gold and copper.
Mineable reserves of nickel, cobalt and chromite have
been identified, but remain to be exploited. Sizeable
mineral sand prospects are known to occur, but have
not received much exploration attention over the last
The Geology and Mineral Potential of
PAPUA NEW GUINEA
20 years. Manganese has also been mined in a small
way in the past.
It is fortunate that there has been a positive turnaround
in interest in the mining sector since 2003, which will
afford PNG a renewed opportunity to grow and
diversify its economy. This is timely, as the average
lead-time for the 6 major mines which either still are, or
were in production stands at 14.7 years. The average
time between mine commissioning is 5.9 years
(including Kainantu which is scheduled to commence
production in 2005). The five remaining advanced
projects (Simberi, Hidden Valley, Wafi, Ramu and
15
The Mining Sector’s Contribution to the PNG Economy (cont.)
Fig. 3.5 Years 2003 and 2004 gold production.
Frieda) have been subject to an average of 26 years of
systematic exploration and are yet to reach
development. In other words, there has been a long
lead time between discovery and development in PNG.
The mines of PNG are some of the largest in the
region and have collectively produced over 60 tonnes
of gold each year for the last 13 years. The interplay
between geological (high prospectivity/large
occurrences/ease of discovery) and commercial (high
capital/operating costs) are the main reasons why the
mines are large. Smaller, higher grade mines will be a
part of the sectors’ future in PNG.
Projected gold and copper production to 2013 should
exceed 70 tonnes and 195,000 tonnes per year
respectively, appreciating significantly after 2007,
because it is anticipated that there will be three new
mines commissioned over the coming three years.
FISCAL PROVISIONS
APPLICABLE TO THE MINING
SECTOR
A summary of the mining fiscal terms for new mining
projects are presented in the Table below. These
16
provisions came into effect in January 2003.
Income tax rate
Dividend withholding tax rate
Accelerated depreciation
allowance
Royalty rate
Deductions of exploration
expenditures
Additional profits tax
Ring fencing
Mining levy
Capital gains tax
State equity
Fiscal stability
30%
10%
25% DB Pool
2%
200%
Abolished
Relaxed
None
None
Under review
Optional at 2% premium
Table 3.2 Mining Fiscal provisions.
Royalty
The holder of a special mining lease or a mining lease
must pay a royalty to the state equivalent to 2% of
the net proceeds of sale of minerals (calculated as
net smelter return or fob export value, whichever
is appropriate).
Royalty distribution
At least 20% of the royalties from a project are
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Mining Sector’s Contribution to the PNG Economy (cont.)
Fig. 3.6 K=Kainantu; S=Simberi; HV=Hidden Valley; W=Wafi; F=Frieda.
distributed to the landowners of the project area; the
remainder is spent within the mine impact area and
the province in which the project is located. These
royalties are not wholly distributed as cash. They are
to be primarily spent on an approved Community
Sustainable Development Plan.
General taxation provisions
Persons taxable under these provisions are taxed the
same way as other businesses in Papua New Guinea.
Such entities are liable for corporate income tax.
Import duties
Project companies (or their subcontractors) may
import specialised goods for exclusive use during
mining operations at a reduced tariff for large-scale
projects.
2002 tax review
The Papua New Guinea fiscal regime was reviewed in
2002 with the view to proposing a more attractive
regime for the sector. The result of the review elevated
the internal rate of return (IRR) for both ‘model’
copper and gold mines within Papua New Guinea, as
shown in Table 3.3. The following changes have been
implemented as a result of the 2002 tax review.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Double deduction of
pre-production exploration
costs
Most mining tax systems allow pre-production
exploration costs to be expensed or deducted in a
relatively short time period. Papua New Guinea’s ring
fencing provision has in the past limited the write
down of exploration expenses. In order for the
country to better promote itself as a nation that
welcomes and supports mineral exploration, a double
deduction for the incurred expenditure has been
offered. The current system has been modified to
allow a 200% deduction allowance for exploration
expenditures incurred after 1 January 2003.
The first 100% is allowed as a deduction against
assessable income in accordance with the current
deduction rules. The second deduction would only
arise after a mine commences commercial operation.
Depreciation
In order to improve investor returns, Papua New
Guinea has moved to using a 25% declining balance
depreciation pool arrangement for all assets for any
new mineral development.
17
The Mining Sector’s Contribution to the PNG Economy (cont.)
Country
Foreign investor’s IRR (%)
Total effective tax rate (%)
15.7
15.0
13.9
13.8
13.5
13.5
28.6
36.6
40.0
42.7
39.8
45.3
13.5
13.0
12.9
12.7
12.7
12.6
45.0
50.2
46.1
36.4
41.7
49.9
12.5
12.4
11.9
11.7
11.4
11.3
46.1
47.8
54.4
46.5
43.1
49.9
11.2
11.0
10.8
10.1
9.3
8.9
3.3
52.2
49.6
57.8
63.8
62.9
62.4
83.9
Lowest taxing quartile
Sweden
Chile
Argentina
Papua New Guinea 2003
Zimbabwe
Philippines
2nd lowest taxing quartile
South Africa
Greenland
Kazakstan
Western Australia
China
USA (Arizona)
2nd highest taxing quartile
Indonesia (7th, COW)
Tanzania
Ghana
Peru
Bolivia
Mexico
Highest taxing quartile
Indonesia (non-COW 2002)
Poland
Papua New Guinea 1999
Ontario, Canada
Uzbekistan
Ivory Coast
Burkina Faso
Table 3.3 Comparative fiscal regimes for a model copper mine in selected jurisdictions
(indexed on Foreign Investor IRR, Otto, 2002).
The proposed regime has the additional benefit of
removing the requirement for mining companies and
the Internal Revenue Commission (IRC) to retain
complex asset registers in order to determine the
amount eligible for deduction in any one year.
Loss carry forward time limit
The loss carry forward time limit was increased to 20
years in 2000. Investors viewed this period as attractive,
but it presented mining companies and the IRC with
the administrative requirement to maintain records of
losses for the full 20-year period. As a result of this
issue, many nations have now moved to eliminate any
maximum time period for the carry forward of losses.
Similarly, Papua New Guinea has now removed any
time limit on the carry forward of losses.
18
Premium for entities wishing to
make use of the Fiscal
Stabilisation Act
Papua New Guinea currently offers fiscal stability under
the Fiscal Stabilisation Act requiring payment of a
small premium for this benefit. Papua New Guinea has
a 2% company tax premium for the offer of fiscal
stability for the duration of the financing period.
THE MINING ACT, MINERAL
PERMITS AND IMPLEMENTATION
The principal legislation in Papua New Guinea that
regulate mining activities are the Mining Act 1992
and the Mining Safety Act (Ch. 195A), administered
by the Department of Mining.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Mining Sector’s Contribution to the PNG Economy (cont.)
Under the Mining Act, the State owns "all minerals
existing on, in, or below the surface of any land in
Papua New Guinea, including minerals contained in
any water lying on any land in Papua New Guinea."
A person must not carry on exploration or mining on
any land unless duly authorised under the Act.
Consequently, the Act sets out the procedure whereby
the State’s Minister for Mining can issue various types
of leases or licenses (mining tenements) to interested
companies on application, to enable them to engage
in various exploration and/or mining activities in
Papua New Guinea.
Papua New Guinea citizens are allowed to carry out
non-mechanised mining of alluvial minerals on land
owned by them (using handtools and equipment but
not pumps or machinery driven by electric, diesel,
petrol or gas-powered motors), provided that the
mining is carried out safely and in accordance with the
Mining Safety Act, and that the land is not the subject
of another tenement (other than an Exploration
Licence).
Licence types
The various types of mining tenements (licences
and/or leases) issued under the Mining Act on
recommendation from the Mining Advisory Board
include:
Exploration Licence (EL)
Mining Lease (ML)
Special Mining Lease (SML)
Alluvial Mining Lease
granted for a term not
exceeding 2 years and
may be extended for
periods up to 2 years;
granted for a term not
exceeding 20 years,
which may be extended
for such period not
exceeding 10 years;
may be granted for a
term not exceeding 40
years and may be
extended for such
period not exceeding
20 years;
generally used by Papua
New Guinea citizens
for small-scale mining
activities.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Exploration Licence (EL)
The area of land in respect of which an EL may be
granted must not be more than 750 sub-blocks (one
sub-block = about 3.41km2). When applying for an
extension of the term of the EL, not less than half of
the area held at commencement of that term must be
relinquished. Where the area of an EL has been
reduced to not more than 30 sub-blocks, the EL
holder will not be required to make any further
relinquishment on renewal. Where, as a result of
relinquishments, the area has been reduced to not
more than 75 sub-blocks, the EL holder may apply to
the Director to waive or vary the requirement to
relinquish, but the total area permitted to be held after
such waiver shall not exceed 75 sub-blocks.
An EL authorises the holder to:
• enter and occupy the land that comprises the EL
for purposes of carrying out exploration for
minerals on that land;
• to extract, remove and dispose of such quantity of
rock, earth, soil or minerals as may be permitted by
the approved work program;
• take and divert water situated on or flowing
through such land and use it for any purpose
necessary for exploration activities subject to, and
in compliance with, the Environment Act 2000
which is administered by the Department of
Environment and Conservation;
• do all other things necessary or expedient for the
undertaking of exploration on that land.
Applications for the grant or extension of an EL must
comply with two main requirements, namely the
technical and financial capacity to undertake an
approved work program.
Minimum annual expenditure in connection with an
approved program is prescribed in the Act. An
approved program may be varied at any time on
written application to the Director based on one or
more of the grounds specified in the Act.
An EL holder is also required to lodge the following
reports in duplicate with the Director:
• a Bi-annual Exploration Report and a Bi-annual
Expenditure Statement calculated from the date of
grant, on expiry, on cancellation and also on
19
The Mining Sector’s Contribution to the PNG Economy (cont.)
making an application to surrender the EL;
• an Annual Report calculated from the date of grant
of the EL;
• a Relinquishment Report in respect of the period
up to the date of relinquishment or surrender of
the whole or any portion of an EL, or on expiry or
cancellation of the EL.
Mining Lease (ML)
An ML shall not be more than 60km2 in area. The main
difference between an ML and an SML is the scale of the
operation. In any event, the EL holder has the exclusive
right to apply for a ML (or SML) over ground covered
by the EL.
Special Mining Lease (SML)
An SML is generally issued to the EL holder for largescale mining operations. The EL holder must also be
a party to a Mining Development Contract with the
State. Before the grant of an SML, the Minister is
required to convene a Mine Development Forum to
consider the views of those persons and authorities
that the Minister believes will be affected by the grant
of the SML. Those represented at such a forum
include the applicant for the SML, the landholders
affected by the application, appropriate National
Government departments, and the Provincial
Government in whose province the SML application
is situated.
Mining Development Contract (MDC)
Under the Act, the State has the discretion to enter
into an agreement consistent with the Act, relating to
a mining development or the financing of a mining
development held under a mining tenement. Some of
the factors that the Minister may consider in
determining whether the mining of a mineral deposit
should take place under an MDC between the State
and a tenement holder include: the size or distribution
of a mineral deposit, the method of mining or treating
it, the infrastructure required for it and its financial or
economic attributes. An SML applicant is obligated to
enter into an MDC, but an ML applicant may elect to
enter into a MDC.
20
Avoiding uncertainty concerning
the administration,
interpretation and enforcement
of existing regulations in the
Mining Act
As mentioned above, the Department of Mining is
charged with the responsibility of both regulating and
promoting the mining industry. The primary
legislative authority providing the mandate for this
task is enshrined in the Mining Act 1992 and the
Mining Safety Act Chapter 195A. The Mining Act
sets out the general law relating to minerals and
mining activities whilst the Mining Safety Act
provides for the regulation, and inspection of mines
and actual works undertaken.
The administrative structure as set out in the Mining
Act to implement the Act involves the following:
(a) The Minister for Mining;
(b) A Director, who is the Head of the Department
of Mining;
(c) A Mining Advisory Board (MAB) which consists
of:
(i) the Director, who is the Chairman;
(ii) three (3) officers of the Department
appointed by the Director;
(iii) three (3) persons appointed by the Minister;
(iv) one (1) person appointed by the Minister on
the recommendation of the Premier’s
Council;
(d) A Registrar of Tenements who is also an officer of
the Department and serves as the Executive
Officer to the Board; and
(e) A Chief Mining Warden and other Wardens who
are also officers of the Department.
The officers or persons appointed to be on the Mining
Advisory Board must have qualifications and
experience in mining, geology, finance, law or related
fields.
The Mining Advisory Board’s functions are to advise
the Minister on such matters as the Minister may refer
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Mining Sector’s Contribution to the PNG Economy (cont.)
to the Board, and such other matters as specified in
the Act (eg. make recommendations to Minister on
various applications for grants / extensions of mining
tenements).
Grant and Extension of the Term
of a Tenement
(b) recommend the refusal of the application; or
After considering the recommendation of the Mining
Advisory Board, the Head of State (acting on advice of
the National Executive Council) has the authority to
grant a Special Mining Lease (SML). The Minister for
Mining is reponsible for granting an Exploration
Licence (EL), Mining Lease (ML), Alluvial Mining
Lease (AML), Lease for Mining Purpose (LMP) and
Mining Easements (ME) based on a recommendation
from the MAB. On the grant or extension of the term
of a tenement, the Registrar advises the applicant of
the Minister’s decision and requires the applicant to
submit the prescribed annual rent within 30 days, and
in the case of the grant of a tenement, requires the
applicant to lodge within 30 days the prescribed
security deposit as required by the Act. Where the
applicant complies with these requirements then the
Registrar will issue to the applicant the title document
to the tenement. If the applicant fails to comply with
these requirements then the Minister may cancel the
grant or extension of the tenement.
(c) defer further consideration of the application and
request the applicant to amend the application or
provide further information or revised programs
or proposals within a reasonable time specified by
the Board.
Payments of various fees for rents, royalties and
security deposits are required under the Act. These
fees are prescribed in Schedule 2 of the Mining
Regulations.
Mining Advisory Board’s
Recommendation
The Board considers each application for the grant or
extension of the term of a tenement together with the
program or proposal submitted by the applicant and
reviews the Registrar’s report, the Mining Warden’s
report, the report of officers of the Department who
are responsible for technical assessment of
applications, and any other reports submitted by a
Provincial Government affected by the application.
The Board also considers any objections that may
have been lodged against the applications. After
consideration of all these matters then the Board can
do the following:
(a) recommend the grant or extension of the
application; or
Port Moresby.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
21
4. Geological Framework of PNG
INTRODUCTION
Papua New Guinea’s unique geology and substantial
mineral resources result from its position on the
Pacific ‘Rim of Fire’, the interactive tectonic boundary
between the cratonic Indo-Australian Plate to the
south and the oceanic Pacific Plate to the north (Fig. 4.1).
This tectonic boundary occurs as a complex
arrangement of active subduction zones and associated
island arcs extending as a crustal-scale suture, east and
south through the Solomon Islands, Vanuatu and Fiji
to New Zealand, and west into Indonesia and on to
the Philippines and Japan. The boundary and arcs
have not always had the current configuration, and
changes in the tectonic setting through time have
provided complex overprinting relationships that are
reflected in the geology and have influenced the
mineralisation of Papua New Guinea. This can be seen
on the accompanying Gravity map (Fig. 4.3).
Over many millions of years, Papua New Guinea has
undergone uplift and deformation as a result of
collision of up to 100mm/y (Tregoning et al., 2000;
Hill and Hall, 2002) between the northward-moving
Indo-Australian Plate and westward-moving Pacific
Plate. This is one of the fastest plate movements on
Earth and projection of this motion back through
geological time provides an indication of the degree of
shortening inherent in the description of Papua New
Guinea as a classic terrane of craton – island arc
collision (up to 100km per million years).
The geological framework of Papua New Guinea
comprises a series of geological terranes (discrete
geological regions) that are commonly separated by
geological elements (structures, etc.) (Fig. 4.4). The
geological elements are discussed in this section with
more detailed descriptions of terrane geology
presented in the following section. The terranes are
presented as individual metallogenic units in the
discussion in the compendium of
prospects. The terminology used
to describe the individual
structures and terranes reflects
the increased understanding of
plate tectonic theory, as applied
to Papua New Guinea over the
past 30 years or so.
The components that define the
tectonic setting of Papua New
Guinea include:
• The Australian Craton, which
underlies the Fly Platform
and much of Papua New
Guinea as a rigid continental
block extending to the south.
Fig. 4.1 Papua New Guinea in relation to major geological elements of South East
Asia and Australia.
22
• The New Guinea Orogen,
represented
by
the
mountainous spine of Papua
New Guinea, formed as a
collision zone and can be
divided into the Western
(Highlands and Ramu–Sepik
regions) and Eastern (Papuan
Peninsula
and
Islands)
Orogens. It is a composite
terrane of metamorphosed
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Framework of PNG (cont.)
Fig. 4.2 Geological framework of Papua New Guinea and conceptual cross-section across the western New Guinea Orogen (after
Crowhurst et al., 1996).
sediments that have undergone fold thrust belt
deformation, island arc magmatic extrusive and
intrusive rocks, and obducted oceanic crust.
Basin (Dow, 1977). The platform is essentially
unaffected by the Cainozoic deformation that is
apparent in terranes to the north.
• The Melanesian Arc comprises a series of nowdismembered island arcs which lie to the north of
the New Guinea Orogen, within the segmented
oceanic Pacific Plate margin.
The Papuan Thrust occurs as a partly mapped
(Cecelia and Hegigio Thrusts), partly inferred colinear series of shallow north-dipping thrust planes
along the edge of the foothills of the Southern
Highlands. It is thought to represent the basal thrust
separating overlying deformed Papuan Fold Belt
sediments from the underlying, minimally deformed
sediments of the Fly Platform (Rogerson et al.,
1987a). The thrust is inferred to continue south of
mainland Papua New Guinea, offshore and
underlying of the Eastern Fold Belt, in eastern Papua
New Guinea.
• The Pacific and Caroline Plates, which have been
subducted into the Manus and Kilinailau Trenches
respectively, are locally obducted onto the Orogen.
GEOLOGICAL ELEMENTS
The following text discusses the geological elements
from south to north in the western part of the
country, then central, followed by the eastern part,
and finally the islands to the north-east of the
mainland.
The Fly Platform (Rogerson et al., 1987a) comprises
Proterozoic–Permian Australian Cratonic basement
overlain by Triassic–Neogene sediments of the Papuan
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Papuan Fold Belt lies immediately north of the
Papuan Thrust in the Southern Highlands Province.
In this region, the essentially undeformed sequence of
Miocene limestone and younger sandstone and shale
to the south becomes deformed by NE-facing thrusts
and associated folds to form a foreland fold–thrust
23
Geological Framework of PNG (cont.)
Fig. 4.3 Gravity map of PNG.
24
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Framework of PNG (cont.)
Fig. 4.4 Geological framework of Papua New Guinea.
terrane. The sequence is locally overlain by
Quaternary shoshonitic stratovolcanoes (Mt Bosavi,
Mt Murray) and contains large oil and gas fields.
Deeper erosion at the northern extremity of the fold
belt exposes underlying Mesozoic sandstone, siltstone
and shale as well as local intrusions, below the
Miocene limestone. The thickness of the entire
sequence within the fold belt is estimated to be 2km
of Cretaceous sandstone, overlain by 1km of Miocene
to Quaternary limestone, sandstone and shale (Davies,
1992).
The New Guinea Thrust is a corridor of arc-parallel
structures that form a terrane boundary separating the
Papuan Fold Belt from the region of intense
deformation to the north, termed the New Guinea
Thrust Belt. The Lagaip Fault is the most prominent
structure in the western portion of the thrust and
includes the Trangiso, Stolle and Figi Faults (Davies
1982). To the east, the Thrust probably encompasses
the Ambum and Kubor Faults (Davies, 1983), but is
not easily traced east of Quaternary basalt cover in the
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mt Hagen area (Dow, 1977; Rogerson et al., 1987b;
Smith, 1990). It continues west into West Papua as
the Tahin Fault, in each case separating uncleaved
from cleaved rocks (Davies, 1991).
The New Guinea Thrust Belt is a major foreland
thrust belt (Rogerson et al., 1987b) bounded by the
New Guinea Thrust to the south, the BewaniTorricelli and Finisterre Terranes to the north, and the
Aure Deformation Zone to the east (Rogerson et al.,
1987a,b). It roughly corresponds to the Western
Mobile Belt (Dow, 1977) and the Sepik Obduction
Complex (Smith, 1990). Many of the major
structures (Lagaip, Fiak–Leonard Schultz and Bundi
Faults) represent regional-scale thrusts and hosthanging wall obducted ultramafic oceanic crust
fragments.
The influence of the Australian Craton extends north
into the New Guinea Orogen within the New Guinea
Thrust Belt, in areas such as the Triassic Kubor
Intrusion Complex. Igneous activity within the belt
25
Geological Framework of PNG (cont.)
comprises marine and subaerial volcanism, and
associated intrusions, that vary from batholiths to
stocks and dykes. The Geological Survey of Papua
New Guinea (Findlay et al., 1997a) used earlier
classifications (Davies et al., 1996; Dow, 1977) to
separate the existing Maramuni Igneous Association
(Rogerson et al., 1987b) into a 30–22Ma Sepik Event,
and 17–10Ma Maramuni Event (Dow, 1977; Findlay
2002). They (op.cit) subsequently extended the latter
event to include Pliocene–Quaternary (6–0Ma)
magmatism. The variable ages and extent of erosion
account for the dramatic differences in the degree of
exposure of porphyry Cu–Au (Ok Tedi, Frieda River)
and epithermal gold (Porgera, Mt Kare, Nena)
mineralisation.
The Ramu–Markham Fault Zone has traditionally
been regarded as a terrane boundary between the New
Guinea Orogen and the inferred accreted Finisterre
Terrane, and so formerly defined part of the northern
limit of the New Guinea Thrust Belt. It occupies the
major topographic feature defined by the Ramu and
Markham Valleys, but is not easily traced westward
into the Sepik lowlands. Recent fieldwork by the
Geological Survey of Papua New Guinea (Findlay et
al., 1997a,b; Findlay, 2003) runs counter to previous
interpretations and suggests that no substantial
accretion has occurred, and that the Ramu–Markham
Fault Zone is not a terrane boundary, although some
major structures are present. This study, however, does
recognise the Ramu–Markham Fault Zone as a
significant boundary between the New Guinea
Orogen and the Finisterre Terrane.
The Bewani–Torricelli Fault System (Dow, 1977)
locally separates part of the Torricelli Terrane from the
New Guinea Thrust Belt in the vicinity of the Sepik
Basin.
The Finisterre Terrane lies at the north-eastern limit
of mainland Papua New Guinea. Although it has
traditionally been cited as a classic example of an
accreted terrane, recent fieldwork by the Geological
Survey of Papua New Guinea in the Finisterre
Mountains (Findlay et al., 1997a,b; Findlay, 2003)
believe that this area is not allochthonous accreted
from offshore to the north, but is relatively in situ and
correlates with rock units south of the
Ramu–Markham Fault Zone. The allochthonous
26
character of the Torricelli Mountains further to the
west on the north coast also requires re-evaluation.
The Aure Deformation Zone is a now-inverted trough
(previously also referred to as the Aure Trough)
characterised by marine sediments folded on N–Strending horizontal fold axes, which can be traced for
lengths of up to 100km. The intensity of folding
decreases moving upward from the early through
Middle Miocene and into overlying Quaternary
sediments (Dow, 1977). The Aure and Sunshine
Faults may represent deep basement structures that
locally define the western and eastern zone margins.
The latter continues into the adjacent Owen Stanley
Metamorphic Complex, delineating the northern
buried margin of the Wau Basin.
The eastern part of the New Guinea Orogen,
extending eastward along the Papuan Peninsula from
the Aure Deformation Zone, is divided into the
Eastern Fold Belt (south) and Owen Stanley Thrust
Belt (north). The latter comprises the Owen Stanley
Metamorphic Complex (south) and Papuan
Ultramafic Belt (north), separated by the Owen
Stanley Fault System.
The north-dipping Papuan Thrust is interpreted to
continue offshore from the western part of the New
Guinea Orogen where it is well documented, to the
eastern part of the Orogen, to separate the underlying
Papuan Plateau Palaeozoic crystalline basement from
the overlying Eastern Fold Belt (Rogerson et al.,
1987a).
The Eastern Fold Belt occurs southeast of the Aure
Deformation Zone as a narrow belt along the south
coast of the Papuan Peninsula, contiguous with the
Papuan Fold Belt. Although similar in structure, the
Papuan and Eastern Fold Belts may have developed at
different times. The Eastern Fold Belt comprises Late
Cretaceous to Middle Miocene folded and thrustdeformed marine sediments and ophiolites. It also
contains the Laloki massive sulphide and Rigo
manganese occurrences.
The Bogoro Thrust separates the Eastern Fold Belt
from the overlying Owen Stanley Thrust Belt to the
north. Near Port Moresby, the Bogoro Thrust places
gabbro of the Sadowa Complex, over Eastern Fold
Belt rocks. Further west, these rocks are thrust over
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Framework of PNG (cont.)
Pliocene sediments at the margin of the Aure
Deformation Zone (Rogerson et al., 1987b).
The Owen Stanley Thrust Belt is comprised of the
Owen Stanley Metamorphic Complex, which is
separated by the Owen Stanley Fault System from
overlying obducted oceanic crust of the Papuan
Ultramafic Belt that is located on the north coast of
the Papuan Peninsula. The mountainous spine of the
peninsula, rising to 4000m asl, is dominated by the
Cretaceous Owen Stanley Metamorphic Complex,
comprising a several hundred kilometre long belt of
medium to high-pressure, regionally metamorphosed
slate, schist and phyllite (Pieters, 1978). MidMiocene Morobe Granodiorite is exposed in the
deeply eroded western portion, while volcanics
dominate further east. In the Morobe Goldfield, gold
mineralisation is associated with Pliocene magmatism
of the Wau Basin, which transgresses the older
Miocene and Cretaceous rocks.
The Wau Basin hosts the Bulolo Graben, which
transects Miocene granodiorite and Cretaceous slate
of the Owen Stanley Thrust Belt. The area may be
interpreted as a setting of intra-arc extension on
structures such as the Upper Watut and Escarpment
Faults, in which gold mineralisation of the Morobe
Goldfield is associated with Pliocene felsic subvolcanic
units overlain by younger Pliocene sediments.
Extension on NW–NNW-orientated Upper Watut
and Wandumi bounding faults, constrained between
the Lakekamu Fault to the south and Sunshine Fault
to the north, facilitated graben formation.
The Owen Stanley Fault System crops out on the
northeastern side of the main range separating the
Owen Stanley Metamorphic Complex and Kutu
Volcanics to the south, from the obducted Papuan
Ultramafic Belt ophiolite further northeast. This
complex fault system comprises cuspate thrust
segments with southwest senses of displacement, and
linear interpreted, left-lateral strike-slip components
such as the Gira Fault. The linear faults may represent
extensions of the Woodlark Rift (see below).
Importantly, the fault system represents the
accretionary structure that sutured ophiolite over the
Owen Stanley Metamorphic Complex as long ago as
the Paleocene (Davies et al., 1997) or Late Oligocene
(Rogerson et al., 1987a).
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Papuan Ultramafic Belt (PUB) continues for
about 400km from the Finisterre Terrane to the
eastern Papua New Guinea mainland and comprises
the hanging wall of the Owen Stanley Fault System.
The PUB is a Cretaceous oceanic floor sequence
varying from a basal ultramafic zone, through a
gabbroic zone to basalt in the uppermost portions.
The pre-Eocene age is evident from the intrusion of
an Eocene tonalite into the PUB and from
unconformably overlying Middle-Eocene volcanics
(Pigram and Davies, 1987).
The geology of the Papuan Islands is a continuation
of the Eastern Fold Belt geology offshore, east of the
Papuan Peninsula to the D’Entrecasteaux Islands
(Goodenough, Fergusson and Normanby Islands) and
Louisiade Archipelago (Misima and Sudest Islands) as
well as Woodlark Island. The Papuan Islands lie on
two oceanic highs, the Woodlark Rise (north) and the
Pocklington High (south), separated by the E–Wtrending Woodlark Basin, which contains an active
spreading centre (the Woodlark Rift) that is
segmented by N–S transform faults. This spreading
centre continues westward to the D’Entrecasteaux
Islands group where metamorphic core complexes,
with associated peralkaline rhyolites, have developed
as a result of the westward rift propagation. The
extensional deformation may continue onto the
mainland as linear WNW structures within the Owen
Stanley Fault System in the Milne Bay area.
The Kilinailau Trench developed as a Paleogene,
south-dipping intra-plate subduction zone limiting
the oceanic Pacific Plate to the northeast from its
dismembered margin in which the Melanesian Arc
(New Britain, Manus, New Ireland, Bougainville and
Solomon Islands) formed by island arc magmatism.
The trench was the locus of Pacific Plate subduction
below the Australian Craton from the Paleocene, but
became jammed by a thick segment of oceanic plate
termed the Ontong Java Plateau, and ceased to be
active by the Mid-Miocene.
The Melanesian Arc, originally formed as a linear
island arc archipelago in the hanging wall of the
south-dipping Kilinailau subduction zone, has been
substantially modified since the Pliocene by continued
oblique collision of the westward-moving Pacific Plate
as well as the opening of the Manus Basin, resulting in
27
Geological Framework of PNG (cont.)
Drilling with a man-portable rig, Ramu laterite nickel prospect.
28
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Framework of PNG (cont.)
a repositioning of many of the individual island
elements (Davies, 1991).
On the northern side of the Papuan Islands, the
Solomon Sea Sub-plate , a remnant sliver of
Cretaceous oceanic crust, is constrained between the
north-dipping Pliocene New Britain Trench
subduction zone on the north side, and the more
poorly defined south-dipping inactive Trobriand
Trough to the south. The Solomon Sea Sub-Plate is
being actively consumed along the New Britain
Trench and is regarded as an important influence on
Pliocene island arc magmatism within the islands.
The Bismarck Sea Sub-plate lies to the north of the
trench (Tregoning et al., 1999).
The Bismarck Sea Sub-plate contains the New
Britain Island Arc formed in the hanging wall to the
New Britain Trench, and also the Manus Basin back
arc style-spreading centres. These spreading centres
are separated by a series of NW-trending transform
faults, one of which continues onshore on the
northeastern tip of New Britain at the Gazelle
Peninsula.
The New Guinea Islands Terrane comprises the
islands (Melanesian Arc) northeast of the mainland
(Rogerson et al., 1987a), but not including the
Papuan Islands. The islands of the Melanesian Arc
were mainly built up by subduction-related island arc
magmatism beginning in the Eocene. At that time,
the islands formed an archipelago that stretched
southeast as the islands of New Britain, Manus, New
Hanover, New Ireland and Bougainville, and thence
on to the Solomon Islands, Vanuatu and Fiji. While
recent data places the Finisterre Terrane (formerly
regarded as allochthonous and accreted from the
north) as a landward portion of the Eocene
Melanesian Arc, the relationship of this arc to the
Adelbert region to the west remains uncertain.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
A cessation of subduction and associated magmatism
in the Miocene (see below) led to deposition of thick
limestone caps on many islands.
Renewed subduction-related magmatism in
the Pliocene (see below) resulted in the
development of overprinting island arcs. The
Manam–Karkar–Umboi–Talasea–East New Britain
island arc (Bismarck Volcanic Arc: Dow, 1977;
Johnson and Jaques, 1980) stretches from the north
coast of New Britain to the northern margin of the
New Guinea mainland and is noted for the recently
active volcanism on Manam and Karkar Islands.
The Tabar–Lihir–Tanga–Feni Island arc lies to the
northeast of New Ireland as a linear Pliocene to
Quaternary volcanic arc and is noted for the presence
of shoshonitic magmatism and associated gold
mineralisation.
Transfer structures occur as NNE-trending
lineaments, interpreted as deep crustal fractures
possibly formed in association with Mesozoic craton
margin rifting (Dekker et al., 1990). These transfer
structures display protracted histories of activity,
localising Pliocene intrusions, mineralisation at Ok
Tedi and Porgera (Corbett, 1994; Hill et al., 2002),
and volcanoes such as Bosavi (Davies, 1991). These
structures also segment the fold–thrust belt into
portions, often with varying thrust extents (Hill,
1991), and changes in orientation across the transfer
structures. Intrusion ages on the mainland young
from north to south, suggesting that Papua New
Guinea might be moving north over hot spots
(Davies, 1991). Further east, transfer structures are
no doubt involved in extension of the Bulolo Graben
(as the Lakekamu Fault and proto-Sunshine Fault,
now deformed), and may localise the Wafi intrusionrelated mineralisation.
29
5. Geological Terranes and Mineralisation
INTRODUCTION
Geological terranes, discussed later as metallogenic
units, represent contiguous geological units
commonly separated by structural geological
elements. The terranes of Papua New Guinea
are described below, in sequence from west to east
(Fig. 5.1).
TERRANES OF THE WESTERN
OROGEN
Fly Platform
The Fly Platform is a broad, low-lying, relatively level
region south of the central cordillera in southwestern
Papua New Guinea, bisected by the Fly and Strickland
Rivers. Its basement is crystalline rocks of the
Australian Craton (Indo-Australian Plate) that is
overlain by a thick succession of sub-horizontal,
largely undeformed Triassic to uppermost Tertiary
marine sedimentary rocks of near-shore and shelf
facies. These are covered by a thick veneer of
Quaternary sediments comprising coarse molasse
derived from the rising central cordillera in the north,
fining southwards. Slight uplift in the north with
consequent southwards tilting of the Fly Platform has
resulted in the shallow incision of present-day streams
north of the Fly River. Laterite development is
common in many areas.
The central northern section of the platform is
blanketed by Quaternary pyroclastic and lahar
volcanics and reworked outwash derived from the
extinct Mt Murray, Mt Sisa and Mt Bosavi stratovolcanoes and associated parasite cones.
Mineralisation
Although no systematic survey of gold occurrences for
most of this terrane has been carried out, alluvial gold
of probable local origin is worked in the Ningerum
area south of Ok Tedi, and is also known to occur
further east in streams draining southwards onto the
Fly Platform from the Bolivip and Idawe Stocks.
Fig. 5.1 Geological terranes of Papua New Guinea.
30
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Terranes and Mineralisation (cont.)
Alluvial gold has also been reported from the
southwestern slopes of Mt Bosavi. The volcanics in
this region may have potential for near-surface
epithermal gold mineralisation.
The lateritised soils of the Fly platform have been
tested for bauxite, but the profile was found to be too
immature for economic concentrations.
Beach sands have been tested along the coastline from
the Fly River delta and further east into the Papuan
Gulf. Results from exploration carried out in the
1970s indicated sub-economic titano-magnetite
concentrations, but further work may be warranted.
Papuan Fold Belt
The Papuan Fold Belt is an elongate SE-trending
geological terrane dominated by a thick succession of
folded and thrust-faulted Upper Triassic to uppermost
Tertiary marine sedimentary rocks, occupying the
southern fall of the central cordillera in the western
mainland and merging in the southeast into the Aure
Deformation Zone. Unroofed intrusions of Upper
Miocene to Pleistocene age (including the Ok Tedi
and Porgera Mines and Mt Kare Prospect) occur in
the western section of the belt. A major geographical
feature of the Papuan Fold Belt is the Darai Plateau,
which forms an extensive belt of inhospitable karst
limestone country developed on thrust blocks of Late
Eocene to Late Miocene Darai Limestone.
Quaternary strato-volcanoes surrounded by thick,
widespread lahar outwash deposits rise 1500–2000m
above the surrounding countryside. Volcanic activity
is believed to have ceased, although fumarole hot
springs occur and oral history across the region
suggests a major eruption occurred in the Doma Peaks
area several hundred years ago. Some craters are
deeply eroded, but many volcanic landforms are still
well preserved. Extending southeast from the Mt
Bosavi massif and bordering the Fly Platform,
Quaternary volcanic centres including Recent cones
can be identified on aerial photographs.
Mineralisation
The Papuan Fold Belt hosts significant mineral, oil
and gas resources. The Porgera gold mine and Ok
Tedi Cu–Au mine are large scale, open pit operations,
The Geology and Mineral Potential of
PAPUA NEW GUINEA
and the Kutubu Oilfield is in production.
Exploration is being undertaken on the Mt Kare
epithermal gold and the Bolivip porphyry Cu–Au
projects. Many other mineralised intrusive stocks
throughout the belt have been prospected and good
potential still remains for identifying precious metal
mineralisation in areas surrounding intrusives of the
Belt.
New Guinea Thrust Belt
The New Guinea Thrust Belt occupies the northern
section of the Western Orogen, and is separated from
the Papuan Fold Belt to the south by the New Guinea
Thrust, which includes the Lagaip Fault. The Belt is
characterised by Upper Miocene foreland thrust
deformation, represented by strongly cleaved, subhorizontal to shallow north-dipping stacked sheets
and slices of regionally metamorphosed Mesozoic to
Early Tertiary fine-grained sediments. The latter are
thought to be the deep-water equivalents of the
Papuan Fold Belt sedimentary succession to the south.
Ophiolite slivers occur in late-stage thrusts. The
deformed sequence is overlain by volcanics and clastic
sediments.
The New Guinea Thrust Belt is divisible into two
zones:
• A northern zone of medium-grade metamorphic
rocks of possible pre-Oligocene Tasman Orogen
‘basement’ origin, which crops out in low
mountain ranges across the Sepik Plain. Intrusives
of the Sepik Arc magmatic event (30–22Ma,
uppermost Oligocene to earliest Miocene) are
exposed in the ‘basement’ country rocks. Further
to the east, another example is the Yuat Batholith
(22.5–14.2Ma), bounded by the Yuat Gorge and
lower Lai River.
• A southern zone of predominately low-grade
regionally metamorphosed sedimentary rocks that
occupies the northern flank of the central
cordillera and hosts the Maramuni Arc magmatic
activity. Maramuni Arc magmatism was much
more extensive than that of the Sepik Arc and was
active for some time after the commencement of
thrusting. The Maramuni Arc was initially
31
Geological Terranes and Mineralisation (cont.)
interpreted to have been emplaced in the
16–10Ma period (extending from uppermost
Early Miocene into early Late Miocene, but mostly
Middle Miocene), but is now described as
extending into the Pliocene (Findlay, 2003).
Intrusions have a complex relationship with thrust
planes, cutting across some, while being truncated
by others.
The two zones are separated along the foot of the
main cordillera by an E–W system of anastomosing
low-angle faults — the Fiak–Leonard Schultze Thrust
system — which extends east to the Ramu–Markham
Fault Zone.
The northern boundary is locally obscured by thick
Pliocene sediments that extend eastwards into the
middle Ramu River area, covering the projected
northwestern extension of the Ramu–Markham Fault
Zone and any eastward extension of the higher grade
metamorphics beyond the fault zone
Late-stage thrusts emplaced shallow-dipping sheets
and slices of obducted upper mantle and sea-floor
volcanics along the northern forefront of the central
cordillera in the Late Miocene, forming three
extensive ophiolite complexes — the Landslip Range
Eocene sea-floor volcanics and intercalated argillite in
the west, the April Ultramafics in the centre, and the
Marum Basic Belt hosting the Ramu (Kurumbakari)
Ni–Co laterite deposit in the east. These ophiolites
were obducted at a different time to the Papuan
Ultramafic Belt ophiolites of the Eastern Orogen.
Mineralisation
Magma-derived gold and copper mineralisation
developed in two periods within the terrane, an older
Sepik event (30-22Ma), and a younger ‘Maramuni’
event (17–10Ma).
Gold and some copper mineralisation are associated
with intrusions of the Sepik event, including the
following locations:
• Right May River area near the West Papua border;
• Waskuk, Yerakai and Garamambu areas near
Ambunti;
32
• the Hunstein Ranges;
• the Salumei and Cone Mountain Prospects
between the lower Salumei and Korosomeri Rivers;
• lower Maramuni River to Yuat Gorge; and
• lower Lai River area at the eastern extremity of the
arc.
The Sepik event mineralisation is mostly small to
medium in size, although some high-grade gold
occurrences have been noted. There is a strong
structural control to the localisation of mineralisation.
The prospects remain under-explored with only
limited amounts of drilling having been undertaken.
Mineralisation of the Maramuni event is related to
intermediate intrusions and occurs along the whole
length of the Belt. Notable prospects include the
Frieda, Horse Ivaal, Trukai, Nth Debom (porphyry
Cu–Au) and Nena (high sulphidation epithermal
Cu–Au) prospects. Numerous other prospects have
been drilled for gold or Cu–Au over the past four
decades in the Sepik region in the May,
Walio, April, Korosameri, Karawari, Maramuni
(Tarua), Yuat and Lai River areas. Several of
these prospects are either being explored or
are
under
application
for
exploration.
The Malaumanda quartz–sulphide–Cu–Au prospect,
on the Korosameri River, was recently drill tested.
Further east, gold or Cu–Au prospects related to
Maramuni event intrusions have been drilled in the
Simbai and Jimi Valley regions.
East from Simbai, major uplift of the cordillera south
of the Ramu River (the summit of Mt Wilhelm at
4509m in the Bismarck Range is Papua New Guinea’s
highest mountain) exposes Maramuni event intrusions
of batholithic proportions in the Bismarck Intrusive
Complex, Akuna Intrusive Complex, and the western
margin of the Morobe Granodiorite. Much of the
Cu-Au mineralisation throughout these regions may be
associated with Pliocene intrusions that overprint the
earlier mineralised Miocene intrusions (Rogerson and
Williamson, 1986). Mineralisation varies from
porphyry Cu–Mo–Au at Yandera within the Bismarck
Intrusive Complex, to intrusion-related structurally
controlled mineralisation at Kathnell, Kainantu and
Bilimoia, as well as gold bearing skarn at Mt Victor.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Terranes and Mineralisation (cont.)
Obducted ophiolites along the forefront of the
Bismarck Range, south of the Ramu River, expose
large areas of upper mantle ultramafic rocks (the
Marum Basic Belt), in which deep tropical weathering
of dunite has produced the Ramu (Kurumbakari)
Ni–Co laterite deposit. This project lies within a
granted Special Mining Lease and may be developed
by the investors after the completion of financing
arrangements and technical studies.
The Prince Alexander Mountains, centred on a core of
Jurassic metamorphic rocks and intrusions are
tentatively included in this terrane. The mountains
form a 100km long range splaying eastwards from the
southern edge of the central Torricelli Mountains.
A wide range of radiometric ages has been recorded for
the intrusions, including Middle Jurassic, Early
Cretaceous and uppermost Oligocene to Early
Miocene.
Lateritic nickel exploration was also undertaken in the
late 1960s and 1970s in the South Sepik region,
principally in the Hunstein Range and April River
areas, mainly by the testing of soils developed on
obducted slabs of partly serpentinised dunite and
peridotite mantle. Results of those initial surveys did
not yield any economic occurrences.
Mineralisation
Alluvial gold has been worked throughout the New
Guinea Thrust Belt, most notably in the Jimi Valley
and Simbai areas, westwards along the foothills of the
central cordillera and across the South Sepik region.
Volcanogenic Massive Sulphide deposits located in the
Jimi Valley have been prospected but an economically
viable occurrence has yet to be located.
Bewani –Torricelli Terrane
The Bewani-Toricelli Terrane extends along the NW
coast of PNG from the western limit at the West
Papua border through the Bewani, then Torricelli
Mountain ranges. The Kairiru Island Group, located
just offshore mid-way through the length of the belt,
is included in this terrane. In the western sector, the
terrane’s southern limit is defined by sediments of the
Pliocence Aitape Trough. In the North Sepik region,
widespread outcrop of Mesozoic metamorphics and
intrusions, interpreted from gravity data, suggests that
a continuous crystalline basement extends at depth
across the Sepik Basin from the present coastline,
south to the central cordillera.
The Bewani–Torricelli Mountains are dominated by
Eocene sea-floor volcanics and Late Oligocene island
arc volcanics, with widespread largely co-magmatic
intrusions that return radiometric ages in the broad
range of 73.2–17.3Ma (Late Cretaceous to Early
Miocene).
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Alluvial gold is being reworked into present-day
streams from widespread uplifted auriferous
palaeogravels (including Early to Late Miocene
conglomerates). This includes the flanks of the
Jurassic metamorphic and intrusive core of the Prince
Alexander Mountains, the intrusive core of the
Torricelli Mountains and, to a lesser extent, the
Bewani Mountains. Alluvial gold is accompanied by
traces of platinum in the Kilifas area of the Bewani
Mountains. Minor primary gold and base metal
mineralisation are associated with hydrothermally
altered intrusions, particularly in the western Bewani
Mountains. In the Maprik area of the Prince
Alexander Mountains, the mining of high fineness
alluvial gold provides a good cash flow for the local
people.
Finisterre Terrane
The Finisterre Terrane comprises the mountain belt
that extends for at least 550km along the north coast
of the New Guinea mainland from around the mouth
of the Sepik River and eastwards through the
Adelbert, Finisterre and Sarawaget Mountains.
The latter comprise the Huon Peninsula where the
belt reaches its widest dimension of 100km.
The Ramu–Markham Fault Zone has traditionally
been accepted as separating it from the New Guinea
Thrust Belt to the south, but recent work (Findlay,
2003) suggests the Finisterre Terrane is now part of
the New Guinea Thrust Belt.
This terrane has traditionally been regarded as a classic
example of an allochthonous terrane, formed at a great
distance and accreted onto the north coast of Papua New
Guinea by subduction of the intervening lithosphere or
strike-slip movement before the Early Miocene.
33
Geological Terranes and Mineralisation (cont.)
The allochthonous status of the terrane is now in
doubt. Recent mapping by the Geological Survey of
Papua New Guinea in the Finisterre Ranges has
identified intercalation of volcanics at the southern
margin of the terrane, with coarse-grained sediments
derived from metamorphic and igneous sources with a
‘continental’ provenance (Findlay et al., 1997a,b;
Findlay, 2003). This suggests that rocks of the
Finisterre Ranges were formed in a proximal position
to an emergent landmass similar to the nearby,
present-day central cordillera south of the
Ramu–Markham Fault Zone.
It is therefore
concluded by those authors that the Finisterre terrane
is autochthonous, probably comprising a chain of
offshore volcanic islands in the Oligocene.
In the Adelbert–Finisterre–Sarawaget Mountains,
Eocene and
Oligocene cherty argillite and
volcanogenic sediments grade up into massive
Oligocene back-arc or intra-arc rift related volcanics,
overlain in turn by thick Early Pliocene bioclastic
limestone. The volcanic rocks comprise submarine to
sub-aerial lava and agglomerate of the Huon
Supergroup (Findlay, 2003) including the Finisterre
Volcanics, which are intercalated with a continental
sedimentary wedge from the southwest and deep
oceanic sediments to the northeast, and interpreted
co-magmatic mafic and ultramafic intrusions.
Interpretations of gravity and seismic data suggest that
strike-slip movement on the Ramu–Markham Fault
Zone is of Recent origin, and the Finisterre–Sarawaget
Mountains (with their spectacular 4,000m high
summits and Pleistocene reef terraces up to 700m asl
at the northeastern corner of the Huon Peninsula) are
an upthrust nappe on a shallow north-dipping thrust
plane, which might represent a westward extension of
the north-dipping New Britain Trench along the
Ramu–Markham valley (Milsom et al., 2001).
Mineralisation
Gold mineralisation is not recorded in either the
Adelbert or Finisterre Mountains where there is
limited development of intrusions. Disseminated
chalcopyrite is associated with andesitic volcanic
horizons in the Finisterre Volcanics.
34
Aure Deformation Zone
The southeastern margin of the Papuan Fold Belt
merges with the Aure Deformation Zone that extends
from near Kerema on the gulf coast, north to the
contact with the Finisterre Terrane (Ramu–Markham
Fault Zone) between the Kainantu and Wau–Bulolo
regions. This terrane comprises tight, commonly
northerly trending sub-horizontal folds and parallel
faults developed by E–W compression.
The
Deformaton Zone is interpreted to have developed as
a northern branch of the Papuan Basin in the middle
Oligocene to Middle Miocene and was subsequently
inverted. Thus, the intensity of folding decreases
moving upward from the Early Miocene through the
Middle Miocene and into overlying Quaternary
sediments (Dow, 1977). This may reflect initial
strong compression in response to westward foreland
thrusting during formation of the Owen Stanley
Thrust Belt. The latter trends north under the
Finisterre nappe at its western extremity, reducing in
intensity over time as the regional tectonics changed.
The Aure and Sunshine Faults occur towards the
western and eastern margins of the Aure Deformation
Zone, and the latter continues into the adjacent Owen
Stanley Metamorphic Complex, delineating the
northern buried margin of the Wau Basin (including
the Bulolo Graben). These faults are assumed to
reflect deep basement structures.
TERRANES OF THE EASTERN
OROGEN (PAPUAN
PENINSULA)
Eastern Fold Belt
The Eastern Fold Belt is a narrow belt of folded
sedimentary rocks along the south coast of the Papuan
Peninsula. Cretaceous marine sediments, including
neritic arenaceous limestone with deeper water
carbonate, jasperoid and terrestrial turbidite,
deposited in an offshore continental rift environment,
have been well dated from abundant planktonic
foraminifera. Late Oligocene to Early Miocene tuffs
may represent the onset of island arc volcanism
elsewhere in the region, while younger bathyal
turbidites and carbonates are also recognised (Pigram
and Davies, 1987). These rocks were intensely folded
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Terranes and Mineralisation (cont.)
and metamorphosed during the Oligocene to Mid to
Late-Miocene.
Stanley Metamorphic Complex and the overlying
Papuan Ultramafic Belt.
Gabbro, of the Early Eocene to Mid-Oligocene
Sadowa Ultramafic Complex, occurs as thrust slices
within the Eastern Fold Belt rocks in the Port
Moresby area (Rogerson et al., 1987a). East of Port
Moresby, on the Sogeri Plateau, these rocks are
overlain by basalt and andesitic agglomerate of the
Pliocene Astrolabe Agglomerates.
The Owen Stanley Metamorphic Complex originally
developed as a thick pile of Cretaceous continentderived fine-grained marine sediments deposited in
the rifted margin of northern Australia, which was
subsequently tectonised to form a 375 x 80km thrustup belt occupied by the Owen Stanley Ranges (rising
to 4000m asl). Rock types include slate and phyllite
of pelitic, psammitic and lesser volcanic origin, as well
as marble, conglomerate and spilite; metamorphic
sweat-out quartz veins are common (Pieters, 1978;
Pigram and Davies, 1987).
Mineralisation
The Laloki massive sulphide deposits occur as
conformable lenses in association with a laminar grey
lutite marker unit within the upper portion of Late
Paleocene sequence of siliceous to calcareous and
carbonaceous mudstone and minor chert. These
rocks are overlain by Eocene biomicrite and chert
(Davies, 1961; Williamson, 1983; Rogerson et al.,
1981; Banda, 2001).
The Eocene hemipelagic Port Moresby Beds are host
to manganese of both stratiform deposits and
concretions. This origin is supported by evidence of
substantial planktonic test dissolution, organic
combustion and metal movement during diagenesis
(Finlayson & Cussen, 1984).
Owen Stanley Thrust Belt
The Bogoro Thrust marks the contact with the
underlying Eastern Fold Belt and the overlying Owen
Stanley Thrust Belt to the northeast. The Owen
Stanley Thrust Belt developed as part of the
accretionary wedge resulting from collision between
the continental and oceanic plates possibly initiated as
early as the Late Oligocene and continuing to the
Pliocene. Consequently, at the western margin, low
grade fossiliferous Cretaceous metamorphic rocks are
in fault contact with Middle Miocene Yaveufa Fm
west of Bulolo. The Owen Stanley Thrust Belt is older
than the New Guinea Thrust Belt, and so previously
metamorphosed and deformed rocks (Kaindi Schist)
are intruded by Maramuni Arc volcanic rocks such as
the 14.5–12Ma Morobe Granodiorite.
The Owen Stanley Thrust Belt comprises two main
NW–SE-trending linear rock units — the Owen
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Medium to high-pressure regional metamorphism,
associated with continent – ocean plate collision and
subduction, transformed some of the Cretaceous
sedimentary rocks and accounts for much of the
variation in rock types. Blueschist and granulite facies
rocks occur at the northern margin close to the Owen
Stanley Fault System contact with the overthrust
Papuan Ultramafic Belt. Yet, fossiliferous Cretaceous
rocks east of Wau-Bulolo are mid greenschist facies,
possibly indicative of separate collision events (Davies
et al., 1987). Elsewhere, zonation in the multiphase
regional metamorphism, contemporaneous with
deformation that formed penetrative cleavages, is
evident as belt-parallel lawsonite (north) and chlorite
(south) metamorphic isograds, with the latter also
containing albite, biotite and garnet (Pieters, 1978;
Rogerson and Francis, 1983).
Mineralisation
The Morobe Goldfield occurs within the Wau Basin,
which encompasses the Bulolo Graben, a 40 x 20km
Pliocene volcano-tectonic feature (Corbett, 1994;
Neale and Corbett, 1997; Corbett and Leach, 1998).
It is underlain in the Wau area by Cretaceous Kaindi
Schist, of the Owen Stanley Metamorphic Complex,
which is intruded by the mid-Miocene Morobe
Granodiorite batholith. The Bulolo Graben is
bounded by the NNW-trending Wandumi and Upper
Watut Faults and is constrained between the
Lakekamu and Snake River lineaments. Both the
Lakekamu and Snake River lineaments may represent
significant faults. Active hot springs on the Wandumi
35
Geological Terranes and Mineralisation (cont.)
Fault have deposited silica sinter and travertine. An
extensional regime is apparent at Hidden Valley where
mineralisation occurs within fractures formed in the
hanging wall to listric faults, while at Hamata the
same kinematics have promoted development of lodes
between reverse faults (see Morobe Goldfield). Gold
mineralisation is also localised by the:
• Escarpment Fault which crops out as a spectacular
10km long normal fault;
• parallel NW-trending Edie Structural Corridor;
•
N–S faults (Kerimenge, Slate Creek).
The Kaindi Schist comprises dark grey, strongly
foliated graphitic slate and phyllite, which varies in
composition to chloritic/chloritoid-bearing phyllite
(originally tuffaceous bands), quartzite and marble. It
is andalusite-bearing adjacent to contacts with the
Morobe Granodiorite. Some schistose portions
display a crenulate fabric, and metamorphic quartz is
common throughout all the metamorphic lithologies.
At Edie Creek, the relatively brittle chloritoid schist
provided a suitable host for mineralised veins, whereas
mineralised structures are weakly developed and
difficult to trace in the crenulated schist.
The Morobe Granodiorite, dated at 14.3Ma
(Lowenstein, 1982), crops out over a 100 x 35km belt
as a medium-grained unaltered granodiorite with local
adamellite, and monzonite, which displays local
foliation (Carswell, 1990). The granodiorite has
hornblende–biotite diorite variants at its apophyses
(Sillitoe et al., 1984).
The term Edie Porphyry (Fisher, 1945) is applied to a
range of mainly dacitic porphyry intrusions that occur
throughout the region as domes, dykes and sills, and
is also commonly found as fragments within the
Namie Breccia. Age dating of biotite and plagioclase
has yielded ages within a 6.4–2.4Ma range, with a
pooled age of 3.5Ma (Page and McDougall, 1972a),
which is within the range of a recent hornblende date
of 4.5±0.4Ma (Cussen et al., 1986). Some domes
display crumble breccia margins indicative of
emplacement at shallow crustal levels. Flow banding
in the domes is common. The estimation of a 4.15Ma
36
age of adularia associated with gold mineralisation at
Hidden Valley is consistent with the common
perception that Morobe Goldfield mineralisation is
related to these intrusions (Nelson et al., 1990).
The Bulolo Volcanics, a poorly bedded mass of felsic
ignimbrite which also contains travertine deposits and
jasper bands, crops out in the northern Wau Basin
overlying Kaindi Schist. Many Edie porphyry
intrusions crop out within the ignimbrite. It is
tentatively interpreted as the extrusive equivalent of
the Edie Porphyry.
The Namie Breccia encompasses a variety of
hydrothermal breccias composed of Edie Porphyry
and fragments of milled basement material (Kaindi
Schist, resistant metamorphic quartz, and Morobe
Granodiorite). In some instances, jagged outlines of
Edie Porphyry in the Namie Breccia are indicative of
emplacement while the porphyry was still molten.
The Namie Breccia occurs in association with several
dome complexes, including diatremes recognised at
Wau (Sillitoe et al., 1984), Kerimenge, Edie Creek,
and between Nauti and Webiak Creeks. Bedded
breccias containing accretionary lapilli at Upper
Ridges (Sillitoe et al., 1984), and in the headwaters of
Webiak Creek, are interpreted to represent diatreme
tuff ring deposits. Linear breccias occur within faults
and Namie Breccia is also common at the contacts of
Edie Porphyry intrusions (Escarpment Fault, Edie
Creek). The relationship between gold mineralisation
and diatreme breccias is most clearly evident at
Kerimenge (see Morobe Goldfield).
The mid-Pliocene Otibanda Formation overlies
Bulolo Volcanics as a 700m thick sequence of poorly
sorted conglomerate, sandstone, siltstone and
intercalated reworked tuff (Carswell, 1990) in the
northern part of the Bulolo Graben. Seams of alluvial
gold-bearing magnetite sands, derived from erosion of
Morobe Granodiorite, are common.
One of PNG’s more exciting prospects in this belt is
the Wafi gold and related Golpu Porphyry. Drilling is
still underway on the prospects. It is uncertain if the
Wafi mineralisation is related to the mineralising
events at Wau or is part of a larger belt that runs
through Wamum, Kainantu and Yandera.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Terranes and Mineralisation (cont.)
The Tolukuma gold mine lies approximately 150km
SSE of Wau on the faulted western margin of the Mt
Davidson Volcanics. The volcanics extend 25km
further south to a major volcanic centre at Mt
Cameron. The volcanics comprise andesitic and
dacitic lahars often with a shoshonitic affinity, and
tuffs with intercalated sedimentary units. The
volcanics unconformably overlie slate of the Kagi
Metamorphic portion of the Owen Stanley
Metamorphic Complex. K-Ar dating indicates
volcanism is about 4.8Ma (Dekba, 1993; Langmead
and McLeod, 1991; Davies and Williamson, 1998).
The Papuan Ultramafic Belt occurs in the hanging
wall of the Owen Stanley Fault System as a 400km
long belt some 25-40km wide, lying along the north
coast of the Papuan Peninsula (Davies, 1971) and into
the Papuan Islands as peridotite noted on Normanby
and Fergusson Islands. Pieters (1978) subdivided a
wedge of original Jurassic to Cretaceous ocean-floor
rocks that have been obducted onto the Owen Stanley
Metamorphic Complex into (roughly from top to
bottom):
•
Lokanu Volcanics basalt (1000m or more);
•
high-level gabbro (1000m);
•
granular gabbro (3000–4000m);
•
cumulate gabbro 1000m);
•
cumulate ultramafics (up to 500m); and
•
tectonite ultramafic (4000–8000m).
The intrusion of Eocene tonalite into Papuan
Ultramafic Belt rocks that are elsewhere
unconformably overlain by Middle Eocene volcanics
(Pigram and Davies, 1987) provide an upper limit of
emplacement as probable Oligocene age (Rogerson et
al., 1987a).
Mineralisation
The Papuan Peninsula contains narrow vein-style gold
mineralisation associated with Eocene tonalite
(50–40Ma), typically located in the northern section
of the Papuan Ultramafic Belt. Significant amounts of
alluvial gold and minor platinum have been worked in
many parts of the Papuan Peninsula for more than a
The Geology and Mineral Potential of
PAPUA NEW GUINEA
hundred years. A number of alluvial gold areas are
still being mined by local people employing small
scale mining methods. These metals are probably
related to potash-rich intrusions that vary to
ultramafic in composition (gabbros). Many of the
intrusions are emplaced along the Owen Stanley Fault
System and extend east to Milne Bay.
The Lokanu Volcanics hosts chalcopyrite within
amygdules, and also chalcopyrite, sphalerite, galena
and silver within shear zones.
High-level epithermal gold mineralisation is
associated with dormant Quaternary stratovolcanoes
in the central northeast of the Peninsula.
Nickel sulphide mineralisation is locally remobilised
into shear zones, possibly in association with MioPliocene porphyry intrusions (Davies and Smith,
1974). Disseminations and veinlets of primary
platinum and chromite, as well as lateritic nickel, have
also been considered as exploration targets in this
region.
The lateritic nickel occurrence at Wowo Gap is less
well defined than that at Ramu, but is estimated to
have resources of about 120 million tonnes averaging
1.2% nickel. It is also similar in metallurgical
characteristics to the Ramu ore, but is still at the
exploration stage. The Mumbare Plateau, Kokoda,
also has the potential to host a significant nickelcobalt lateritic deposit. Other laterites in the area
include the Ibau Plateau and the Keman and
Awariobo Ranges, south of Wowo Gap.
Papuan Islands
The Papuan Islands Terrane represents the eastward
extension of the Papuan Peninsula and includes the
islands of the D’Entrecasteaux Island Group
(Goodenough, Ferguson and Normanby Islands), the
Louisiade Archipelago (Misima, Sudest (Tagula) and
Rossel (Yela)), Woodlark Is. and many other smaller
Islands. The islands lie on two ESE trending oceanic
highs within the Solomon Sea, namely — the
Woodlark Rise to the north and the Pocklington Rise
to the south — that are separated by the Woodlark
oceanic spreading centre which commenced opening
37
Geological Terranes and Mineralisation (cont.)
from 5Ma (Benes et al., 1994). Many of these islands
have been significant gold producers.
On the 40km long Misima Island, Paleogene
basement rocks are the Awaibi Association metaigneous ophiolite rocks in the west, and the Sisa
Association metasedimentary suite in the east, which
is intruded by many small stocks of the 8.1±0.4Ma
Boiou Granodiorite. The two associations are
separated by an original thrust fault with later
extensional activation (Williamson and Rogerson,
1983; Adshead and Appleby, 1996; Adshead, 1997).
Skarn mineralisation is associated with the Boiou
intrusions that are cut by later extensional faults
hosting 4–3.2Ma (Adshead, 1997) epithermal gold
mineralisation (Umuna Lode described in Section 8).
Some detrital gold occurs within an overlying Pliocene
volcano-sedimentary
sequence
dated
from
foraminiferal micritic limestone as 5.1–3.05Ma. This
is further overlain by alkali basalt agglomerate
(Williamson and Rogerson, 1983). Uplift on the
north coast to 400m is noted in raised Quaternary
coral reefs.
Sudest and Rossel Islands are dominated by
monotonous pelitic slate and phyllite of the ?Owen
Stanley Metamorphic Complex of the Papuan
Peninsula, possibly deposited off the rifted margin of
continental northern Australia during the Mesozoic.
Alternatively, an origin as a remnant of the continent
rifted during the opening of the Coral Sea has been
considered by some workers. Scattered Tertiary mafic
porphyritic intrusions throughout the islands are of
unknown provenance.
Most of Woodlark Island is covered by Pleistocene
coralgal limestone which surrounds a 12km wide
‘basement’ horst block. The block consists of Eocene
age, ocean-floor low-K basalts and volcaniclastics of
the Loluai Volcanics, overlain by Miocene Wonai Hill
Beds (16.5–13Ma; Smith and Milson, 1984), volcanic
rocks and high-K comagmatic porphyritic intrusions
(Joseph and Findlay, 1991). The latter is associated
with epithermal gold mineralisation (described in
Section 8) (Corbett et al., 1994).
The Trobriand Islands and a number of smaller groups
are comprised of Pleistocene to Recent coral atolls.
38
Cretaceous to late Palaeozoic layered and domed
gneiss, schist, mylonite and amphibolite, which in
most cases are overthrust by unmetamorphosed
ultramafics, form basement rocks to the islands of the
D’Entrecasteaux Group. These units are intruded by
Pliocene to Holocene granodiorite on Fergusson
Island, resulting in doming and unroofing possibly in
association with the Woodlark Rift spreading centre.
The calc-alkaline to andesitic island arc intrusive rocks
emplaced into the detachment faults date the core
complex activity at 1.2–0.4Ma on Fergusson Island
(Chapple and Ibil, 1997) and 3.2Ma on Normanby
Island (Hill, 1990).
Mineralisation
Alluvial gold was first discovered on Sudest Island in
1888 and subsequently on the adjacent islands.
Alluvial and eluvial gold have since been traced to
their hard-rock sources. The auriferous epithermal
quartz veins at the Umuna open pit mine (now closed)
on Misima, and lodes on Woodlark Island that were
mined pre-World War II, is discussed in detail in
Section 8 of this publication.
Most of the 10,000oz of gold produced from Sudest
was from alluvial and eluvial sources. Hard-rock gold
mineralisation could be associated with saccharoidal
and epithermal quartz noted in the Mt Adelaide and
Cornucopia Mine workings (Corbett et al., 1991),
although Williamson (1984) prefers a metamorphic
origin for most of the gold mineralisation.
At the Wapolu Prospect on Fergusson Island, gold
mining was undertaken on a modest scale in 1996, but
the operation ceased soon after due to low gold grades.
The epithermal gold mineralisation could be described
as very low-temperature, low-sulphidation quartz
vein–breccia localised within detachment fault zones
(Chapple and Ibil, 1997).
Volcanic–exhalative base metal sulphides with gold are
depositing from ‘black smokers’ on the present-day
Franklin Seamount east of Normanby Island.
New Guinea Islands
Island arcs have built up since the Eocene in the
hanging wall of the southwest-dipping Kilinailau
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Terranes and Mineralisation (cont.)
Trench, due to subduction-related magmatism. The
originally linear island arc chain includes Manus, New
Britain, New Hanover, New Ireland, Bougainville,
and then on through the Solomon Islands, Vanuatu
and Fiji along the Pacific Plate margin. The cessation
of magmatism as the subduction zone zipped closed
during the Miocene resulted in deposition of extensive
limestone on the emerging volcanic edifices. This was
overprinted by younger magmatism on New Britain
and Bougainville, while the volcanism on the Tabar
–Feni chain is viewed as a separate entity.
Manus Island, at 100km long, is the largest of the
Admiralty Islands Group in the far north of Papua
New Guinea. Interpreted oceanic basement is
overlain by (47–20Ma) Eocene to Mid-Miocene
island arc andesite, basaltic agglomerate, tuffs and
breccias up to 2000m thick that cover most of the
island (Jaques, 1980). Cessation of the magmatism
saw Early to Mid-Miocene bioclastic limestone
deposited at the fringes of the volcanic edifice grading
to Late Miocene marginal volcanic then calcareous
sediments. The Yirri Intrusive Complex represents a
later event, comprising multiphased quartz diorite to
quartz monzonite intrusions extending from premineralisation (17.6Ma) phase, to intrusions
associated with alteration and porphyry copper
mineralisation (15.0Ma), and finally a post-mineral
andesite porphyry phase (13.1 and 11.2Ma) (Jaques,
1980).
New Britain is typical of the other Melanesian Island
arcs and comprises a thick basal sequence of Late
Eocene basaltic to andesitic lava, breccia and
associated sediments that are overlain by Oligocene
island arc volcanic rocks and 30–22Ma co-magmatic
intrusions (Ryburn, 1975, 1976). The hiatus in
volcanism in the Miocene is represented by extensive,
locally thick shelf limestone with karst topography,
which is in turn overlain by Pliocene volcaniclastic
sedimentary rocks.
Bougainville and Buka Islands are geologically similar
to other islands of Melanesian Arc magmatism (Blake
and Miezitis, 1967; Hilyard and Rogerson, 1989;
Rogerson et al., 1989).
Much of the
Eocene–Oligocene submarine basaltic to andesitic
volcanism and associated sediments are overlain by
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Miocene neritic limestone grading to Pliocene
sediments, containing conglomerates indicative of
active syn-deformational faulting.
Renewed magmatism in the Pliocene resulted in
development of volcanic edifices as two separate
chains and also overprinted existing island arcs. One
arc extends for 1000km from close to the north coast
of Papua New Guinea, eastward as the Schouten
Islands Group (Manam, Karkar, Bagabag, Long and
Umboi or Rooke Islands), and then along the north
coast of New Britain as the Mt Andewa and Mt
Schrader stratovolcanoes to the Willaumez Peninsula,
Mt Pago and Mt Uluwan, through into the recently
active volcanism at Rabaul in East New Britain and
then southwards through Bougainville. Although
many of these volcanoes remain active, some have
been prospected. Mt Andewa and Mt Schrader have
collapsed northwards in a Mt St Helens style
event and contain some mineralised quartz veins.
On Bougainville, Late Miocene to Pliocene intrusives
host the Panguna porphyry Cu–Au mineralisation
that will be described in a later section. Several latest
Pliocene and Quaternary andesitic stratovolcano
complexes overlie the older rocks and locally
interfinger with shallow water marine sediments.
The second arc is represented by the 250km long NW,
trending Tabar–Lihir–Tanga–Feni island chain of
Pliocene to Recent volcanoes that lies offshore to the
northeast of New Ireland, possibly exploiting an
earlier crustal discontinuity. Lihir Island hosts the
giant Ladolam gold deposit (Moyle et al., 1990;
Muller et al., 2002; Corbett et al., 2001). Individual
islands (Lihir) and island groups (Tabar) display a
N–S elongation. The arc is inferred to have been
derived from magmatism associated with subduction
of the Solomon Sea Sub-Plate into the New Britain
Trench, under New Britain and New Ireland (Lindley,
1988; Shatwell, 1987). Many workers have discussed
the relationship between shoshonitic magmatism and
gold mineralisation on these islands (Heming, 1979;
Wallace et al., 1983; Muller et al., 2001).
Mineralisation
Alluvial gold was worked on Simberi and Tatau
Islands, the latter being traced to a hard-rock source
39
Geological Terranes and Mineralisation (cont.)
at Tugi. Four eroded volcanoes were explored
extensively in the 1980s, leading to the drill testing of
many prospects.
On Simberi, resources of 29 million tonnes grading
1.6g/t Au using a 0.5g/t cut-off for 1.48 million
ounces of gold have been estimated (2004) for the
hypogene and oxide mineralisation. Pronounced
leaching is associated with an early event of K-feldspar
flooding followed by introduction of pyrite
grading to arsenean pyrite and arsenopyrite, in which
gold is encapsulated within sulphides. A later
event of high-grade gold is associated with
sphalerite–pyrite–carbonate (Corbett and Leach,
unpub. report, 1995). Auriferous quartz veins occur
peripheral to the main prospect. Alteration and
mineralisation at Simberi are typical of the intrusionrelated low sulphidation style formed peripheral to an
alkaline magmatic source at depth. Consequently,
early alteration is characterised by K-feldspar
flooding and an absence of silica, and the style of
mineralisation grades from (quartz)–sulphide–gold, to
carbonate–base metal–gold and marginal epithermal
quartz–Au–Ag. Some workers have suggested that
a major phreatomagmatic eruption predated
mineralisation (Corbett and Leach, unpub. report,
1995).
sinter deposits located adjacent to the springs assayed
up to 33g/t Au (Licence et al., 1987).
On Manus Island, the Arie and other nearby prospects
were explored from 1968 though the 1970s for
porphyry copper style mineralisation
The
mineralisation mainly occurs as stockwork fracture
veins, and disseminated sulphides within the mid to
late Miocene Yirri Intrusive Complex (Jaques, 1980)
and adjacent volcanic rocks. In the area near
Mt Kren, mineralised intrusions are overlain by an
extensive blanket of cliff-forming silica–alunite–pyrite
alteration typical of the shoulders of barren advanced
argillic alteration that commonly forms in the vicinity
of porphyry Cu–Au deposits at depth (Corbett and
Leach, 1998). Epithermal gold mineralisation
(Metawarei) is known to occur some 12km east of the
porphyry copper mineralisation within epiclastic
phases of the Middle Miocene Tasikim Volcanics.
In central New Britain, several 30–22Ma age
porphyry copper style mineralisation occurrences
(Pleysumi, Kuku, Wala River, Torlu River, Ala River
and Esis-Sai) were prospected during the 1970s and
80s (Hine and Mason, 1978; Hine et al., 1978).
Some of the occurrences are partly obscured by
Pliocene Ania Tuff. Many of the porphyries are the
current focus of exploration activities.
On a seamount 10km south of Lihir Island, sampling
in 1,050m water depth has yielded gold values to
230ppm in association with stockwork pyrite veins
and polymetallic sulphides containing sphalerite,
galena, chalcopyrite and marcasite with anomalous
Sb, As, Ag and Hg. Not surprisingly, Paterson et al.
(2002) interpret the mineralisation to have formed as
magmatic sea-floor deposits.
In East New Britain a 23–22Ma intrusion-related
epithermal gold occurrence, variably named Wild
Dog (Lindley, 1987), Nengmukta or Sinivit (Lindley,
1998), occurs within pre-mineral structures that can
be traced for several kilometres. Early silicification
and high-temperature advanced argillic alteration is
overprinted by sulphide-poor Au–Te epithermal
mineralisation best developed at cross-structures
(Corbett and Leach, 1998), although other
interpretations have also been provided (Lindley,
1987, 1998). Initial exploration results from the
mineralised cross-structures were encouraging, but
despite intensive exploration over many years by
several explorers, the system has proved to be only
marginally economic.
On Ambitle Island in the Feni Island Group, a
summit caldera contains young (0.68±0.1 and
0.49±0.1Ma) domes, a phreatomagmatic eruption
(2300±100 years) and many active hot springs. Silica
East New Britain also hosts a small high-sulphidation
gold occurrence at Maragorik that has developed at a
very high crustal level and demonstrates both
lithological and structural fluid controls to
The giant Ladolam gold deposit on Lihir developed at
the transition from a porphyry to an epithermal
setting, resulting from a Mt St Helens sideways
collapse of the volcanic edifice. Mineralisation at
Ladolam is described in detail in a later section.
40
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Geological Terranes and Mineralisation (cont.)
mineralisation deposition (Corbett and Hayward,
1994). Exploration has yet to identify a commercial
resource at Maragorik (Corbett and Leach, 1998).
black smoker style vents within volcanic ridge
In the Manus Basin, volcanogenic massive sulphides
(Cu–Pb–Zn–Ag–Au) are currently being deposited by
mineralisation is currently being explored by an
spreading centres, dacite lavas and caldera collapse
settings (Parr et al., 1995; Gena et al., 2001). This
Australian based consortium.
The best results are still realised by foot traverses of creeks and streams.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
41
6. Tectonics and Mineralisation
INTRODUCTION
Papua New Guinea has long been cited as a classic
setting of porphyry Cu–Au and epithermal Au–Ag
mineralisation within a subduction-related magmatic
arc, associated with continent–island arc collision.
Analysis of the relationship between tectonism and
mineralisation is best considered in the light of the
protracted timeframe of varying subduction and
collision. The rate of collision convergence is
currently one of the fastest on earth at 100km per
million years.
The geology of jungle-covered Papua New Guinea is
poorly understood, so any models for tectonism and
mineralisation should be treated as constantly
evolving, especially concerning such aspects as the age
of igneous rock formation and associated
mineralisation. There is certainly no firm consensus
among geoscientists on the geological evolution of
PNG. There is considerable disagreement among
geoscientists about the timing of the collision between
the Australian continent and the island arc(s), ranging
from Eocene in the Papuan Peninsula (Davies and
Smith, 1974), to Oligocene–Early Miocene (Hall,
2002), and Mid–Miocene (Rogerson et al., 1987b),
while several workers suggest a history of multiple
collision events with varying docking times in
different parts of Papua New Guinea (Pigram and
Davies, 1987; Davies et al., 1996; Veevers, 2000).
GEOLOGICAL HISTORY
Pre - Paleocene
The Australian continent, which underlies Papua New
Guinea almost to the north coast, may occur as a series
of rifted micro-continents, remnants of which are
exposed as the Permian Kubor Intrusion Complex
(244±0.8 and 239±4.2Ma; Page, 1976), the Amanab
Block (262Ma; Rogerson et al., 1987b) and the Bena
Bena Metamorphics (Rogerson and Hilyard, 1990).
Rifting is thought to have facilitated deposition of
thick sequences of marine clastic sediments containing
1.8 Ga continental-derived detritus that is intercalated
with Late Triassic (221±3Ma) tuffs (Van Wyck and
Williams, 2002) and Cretaceous volcanic-derived
material (Hill and Hall, 2003; Struckmeyer et al.,
1993; Francis, 1990). The Late Paleozoic-Mesozoic
time, therefore, saw the development, at a passive
42
continental margin, of geological material that would
later comprise a large part of Papua New Guinea.
Davies et al. (1997) would have a north dipping, and
also a north-east dipping, subduction zone located off
the north and north-eastern parts of the craton, above
which were developed the Irian Arc and the East
Papua arc respectively. These arcs were accreted onto
the Australian Craton, together with ophiolite
assemblages, in the Late Cretaceous for the Irian Arc
and Paleocene for the East Papua arc.
Paleocene (66-55Ma)
During the Paleocene, the northern margin of the
Australian craton underwent extensive rifting
associated with the opening of the Coral Sea basin
(Weissel & Watts, 1979; Davies, 1997), possibly along
deep basement structures. These structures project
from the basement and are now imposed on the
overlying accretionary wedge and may have
contributed to localising much younger
mineralisation such as Porgera and Ok Tedi (Dekker
et al., 1990; Corbett, 1994).
The Laloki volcanogenic massive sulphide
mineralisation may have developed at this time in a
small ocean basin termed Uyaknji by Davies et al.
(1997).
As mentioned above, the Papuan Ultramafic Belt was
sutured to the Owen Stanley Metamorphic Complex
protolith during the Paleocene, based on dates
of 62-57Ma, the cooling age of hornblende
porphyroblasts in granulite facies metamorphics in the
Complex (Lus et al., 2004).
Eocene (55–34 Ma)
The Pacific Plate tectonic setting changed
dramatically in the Eocene as the Indo-Australian
Plate moved rapidly northward to terminate the
opening of the Coral Sea. There is a paucity of Eocene
clastic sedimentation in eastern Papua New Guinea
compared to adjacent West Papua (Hill and Hall,
2003), and the northern part of the Craton, which
was the site of thick limestone deposition.
A north-dipping subduction zone developed as an
intra-oceanic plate subduction zone to the north of
the emergent landmass. The Sepik Arc developed in
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Tectonics and Mineralisation (cont.)
the Eocene in response to the subduction (Davies et
al., 1997).
subduction zone (Kilinailau Trench) that was initiated
in the Late Eocene, well to the north of the Australian
craton.
Oligocene (34–23.8 Ma)
Davies et al., (1997) would have the Sepik Arc
accreted to the advancing Australian Craton in Late
Eocene to Mid-Late Oligocene.
Collision is
interpreted to have continued through the Oligocene
as evidenced by fault-bounded slivers of ultramafic
rocks present within the thrust sheets of the Western
Orogen. In each case, medium to high-pressure
metamorphism of the accretionary wedge has
accompanied ophiolite emplacement and grades away
from the basal thrust into the underlying older
metamorphic rocks (Rogerson et al., 1987b; Pieters,
1978). The ultramafic rocks have been prospected
for laterite nickel.
On the mainland, collision resulted in substantial
uplift and metamorphism of the Cretaceous marine
sedimentary pile, and contributed towards the
termination of volcanism. The cooling age of the
Alife Blueschist at 23Ma (Rogerson et al., 1987a) in
the Sepik region corresponds well with the cessation of
magmatism associated with the Sepik Event at about
22Ma, as a result of collision (Findlay et al., 1997b).
The accreted Sepik Arc now occurs as a 600km long
zone straddling the Sepik River and extending into the
headwaters in which medium-grade metamorphic
rocks contain Oligocene to Early Miocene intrusions.
Volcaniclastic sediments such as the Wogamush
Formation were deposited on the north side of the
range in the Sepik headwaters.
A south dipping subduction zone developed at the
northern edge of the craton and accreted terrains in
the latest Oligocene (Rogerson et al., 1987) that
resulted in the formation of a continental volcanic arc
from Miocene times, referred to as the Maramuni Arc
(Dow, 1977).
From the Oligocene to Mid-Miocene, thick platform
limestone (Darai Limestone) was deposited on the
landmass in the southwestern mainland Papua New
Guinea (Pigram and Symonds, 1991).
The New Guinea Islands developed at this time
as emerging submarine volcanic edifices and
co-magmatic intrusions above a south west dipping
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Miocene (23.8–5.5Ma)
The New Guinea Islands region underwent significant
tectonic disturbance in the Late Oligocene-Mid
Miocene. From about 22–20Ma the Ontong Java
Plateau, a thick segment of southward-moving
oceanic Pacific crust extending northeast from near
Bougainville Island, jammed the Kilinailau
subduction zone in that region (Bruns et al., 1989).
The subduction zone progressively closed from the
southeast to the northwest over a protracted period.
As a result the Eocene–Oligocene submarine andesitic
volcanism, which accounted for development of the
New Guinea Islands archipelago, ceased in the Late
Miocene and the emerging volcanic edifices became
capped by shallow marine limestone and sediments
for much of the Miocene and Pliocene. In addition,
also in response to the jamming of the Kilinailau
Trench, it is thought that a southeast dipping
subduction zone developed at the Trobriand Trench
located well to the south west of the Solomon Sea
plate, close to the strike continuation of the trench
lying north of the Maramuni Arc (Rogerson et al.,
1987).
Deep erosion has locally exposed porphyry Cu–Au
mineralisation associated with some co-magmatic
intrusions (Plesyumi and Esis in Central New
Britain). At higher crustal levels, structurally
controlled, intrusion-related, low sulphidation
epithermal gold mineralisation, is locally preserved
(Wild Dog, East New Britain), and high sulphidation
mineralisation in the same district may not be of the
same age (Maragorik).
The Trobriand Trench system failed in the Middle
Miocene and northward directed subduction was
initiated along the New Britain Trench.
Both the north dipping New Britain Trench and the
south west dipping trench at the edge of the
Australian craton were consuming oceanic crust of the
Solomon Sea Plate.
43
Tectonics and Mineralisation (cont.)
One of the greatest manifestations of collision-related
shortening on mainland Papua New Guinea was the
development of the foreland thrust deformation,
particularly in the Western Orogen, where Papuan
Basin sediments were extensively deformed and thrust
southwards from the Late Miocene to the present
(Rogerson et al., 1987a).
The Maramuni Event (of the Maramuni Arc; Dow,
1977) was originally cited as Miocene, but may have
extended back to the Oligocene (Rogerson et al.,
1987), was most active from 17–10Ma (Findlay et al.,
1997b) and could be argued to have continued into
the Pliocene (Findlay, 2002). The event represents the
main period of magmatism and related mineralisation
on mainland Papua New Guinea. It occurs as a
40–60km wide linear belt of intrusions stretching for
750km from the Indonesian-PNG border, to the Wau
district south of the Huon Gulf at the western
extremity of the Eastern Orogen, and sporadically into
the offshore Papuan Islands (eg. Woodlark Island).
The Nena–Frieda and Wafi porphyry–epithermal
Cu–Au hydrothermal systems represent some of the
main events related to the Maramuni magmatism.
At Wafi, porphyry Cu–Au mineralisation is capped by
high sulphidation gold mineralisation and surrounded
by low sulphidation gold mineralisation. The
mineralisation is localised by prominent NNEtrending structures, possibly originally developed as
Mesozoic rifts or faults. While some workers place
similar trending structures in the vicinity of the Frieda
porphyry Cu–Au and adjacent Nena Cu–Au deposits,
tectonic controls may have been strike-slip
deformation (Corbett, 1994; Corbett and Leach,
1998). The porphyry intrusions may have been
localised on a splay off the Fiak–Leonard Schultze
Fault. Intrusions began to be emplaced in the
Frieda area at about 17Ma. However, much of the
mineralisation is interpreted to have developed in the
waning stages at about 11.9Ma (Hall et al., 1990).
In the central part of the New Guinea Orogen
substantial uplift of 4.5km and associated 3km
denudation at about 8–5Ma (Crowhurst et al., 1996;
Hill and Raza, 1999) have exposed the batholithic
levels of many intrusions related to the Maramuni
Event. For example, the Morobe Granodiorite
(Wau–Bulolo), the Bismarck Intrusive Complex
44
(Yandera), and the Akuna Intrusive Complex
(Kainantu area), each of which host younger
intrusions with associated Cu–Au mineralisation.
Here, and in many other locations (Misima,
Woodlark Island), it is evident that melts have
exploited earlier structural frameworks.
Much of the Late Miocene to Pliocene magmatism on
mainland PNG occurs as high-level porphyry
intrusions with associated Cu–Au (Yandera,
Kainantu) and low sulphidation epithermal gold
mineralisation (Porgera, Mt Kare, Irumafimpa lode at
Kainantu). Crustal melting at deep levels may be
initiated by rapid unroofing, which promotes a
sudden decrease in pressure (Mason and Heaslip,
1980), and so it is proposed that porphyritic
intrusions have been rapidly emplaced at higher
crustal levels in waning stages of subduction.
Significantly, many of these mineral occurrences (Ok
Tedi, Porgera, Mt Kare) are localised by large fractures
that have allowed the migration of porphyritic
intrusions to relatively shallow crustal levels. Recent
field data at Porgera, where age dates are more
constrained, suggest that the interpreted removal of at
least 600m of crust in only a few thousand years by
thrusting, under conditions of rapid convergence, has
promoted late-stage emplacement of structurally
controlled feldspar porphyry intrusions and the
bonanza-grade epithermal gold mineralisation (see
later section on Porgera). The lack of identifiable
extrusive activity at Porgera is consistent with the
preservation of magmatic volatiles and mineralisation
within the buried magmatic source at depth until later
release (Corbett et al., 1995). Delayed partial melting
has previously been suggested to account for the
development of gold-anomalous shoshonitic Pacific
Rim magmatism (Johnson, 1987). These alkaline
rocks, although present as only a few percent of the
southwest Pacific igneous suite, account for about
20% of the region’s gold mineralisation (Porgera,
Lihir, Emperor; Sillitoe, 1997), and possibly
significantly more, depending on the geological
models employed.
Recent fieldwork by the Geological Survey of Papua
New Guinea (Findlay, 2002) suggests that at least the
Finisterre Ranges, and possibly other parts of the
Torricelli-Finisterre Terrane to the west, formed in
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Tectonics and Mineralisation (cont.)
their present setting as mixtures of continental and
volcanic detritus extending to east New Britain.
However, most previous workers (Davies et al., 1997,
Rogerson et al., 1987) would have this as an
allochthonous terrane docking with the mainland in
the Late Miocene.
Pliocene
On mainland PNG, Late Miocene magmatism and
delayed partial melting due to rapid unroofing of the
Maramuni arc continued into the Early Pliocene.
In the Eastern Orogen, porphyry Cu–Au
mineralisation
developed
at
Mt
Bini.
Adularia–sericite style low sulphidation gold veins at
Tolukuma developed in a possible rifted margin of a
major volcano-plutonic complex.
The Morobe Goldfield at Wau–Bulolo, occurs in
association with the Bulolo Graben (Corbett, 1994),
which is in part overlain by sediments of the Wau
Basin, is currently being studied by the Geological
Survey of Papua New Guinea. Graben-bounding
(Upper Watut Fault) and intra-graben (Escarpment
Fault) faults to the Bulolo Graben have listric
character. These structures localise Edie Porphyry
intrusions, phreatomagmatic breccias, and associated
mineralisation.
In the Western Orogen, the Ok Tedi intrusions were
emplaced into NNE-trending structures, possibly
active since Mesozoic rift development. Many
Quaternary stratovolcanoes, such as Mt Bosavi, also
occur on NNE trending structures.
In the New Guinea Islands, subduction on the
Kilinailau Trench had been inactive since the Early
Miocene, although collision continued. Convergence
was accommodated by arc reversal at around 10Ma
(Bruns et al., 1989). Subduction was then initiated
along a new northward-dipping New Britain Trench,
south of New Britain and extending towards the
Solomon Islands.
Renewed subduction resulted in development of
significant Pliocene magmatism, although individual
volcanic centres may have been active since the Late
Miocene.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Plio-Quaternary
Magmatic activity along the Tabar–Lihir–Feni–Tanga
island chain is thought to be unrelated to subduction
at the New Britain Trench (Rogerson et al., (1989) but
may have been localised by an earlier structural grain
derived from the Kilinailau Trench and so lies adjacent
to New Ireland. Individual volcanic centres (Lihir,
Tabar Island) are aligned along fanned N–S structures
possibly formed as tension fractures as the downwardmoving Solomon Sea Sub-plate was progressively
deformed. Here, gold-rich shoshonitic magmatism is
interpreted to have been derived from the later
remelting of previously partially melted oceanic crust
(Solomon, 1990; Johnson, 1987).
Plio-Quaternary volcanism also developed in another
arc stretching for 1000km from close to the north
coast of Papua New Guinea in the Sepik region,
eastward through the Schouten Islands, Manam,
Karkar, Bagabag, Long and Umboi, and then along
the north coast of New Britain as the Mt Andewa and
Mt Schrader stratovolcanoes, to the Willaumez
Peninsula, Hoskins and East New Britain, again
mimicking an earlier structural grain.
The oblique character of collision facilitated a
Pliocene rearrangement of the New Guinea Islands.
From 6Ma, the opening of the Woodlark Basin
divided the Papuan Islands by rifting at an estimated
rate of 150mm/y over the past 3.5 million years
(Taylor et al., 1999; Benes et al., 1994). Since 3.5Ma,
the Manus Basin has opened by seafloor spreading in
the eastern and sinistral (left lateral) strike-slip
displacement on the associated transform faults and,
has contributed towards repositioning of much of the
New Guinea Islands (Taylor, 1979). Earlier models
(Davies, 1991; Benes et al., 1994 ) have been modified
to suggest that the Kilinailau Trench and the
Melanesian Arc, which once extended as a linear
island arc – subduction zone couple southeast from
west of New Britain, have been substantially
rearranged, such that the trench lies several hundred
kilometres offshore and the island arc has been
dismembered. Massive sulphide mineralisation is
currently being deposited (black smokers) in the
Manus Basin.
45
Tectonics and Mineralisation (cont.)
LATE CRETACEOUS
• IRIAN ARC-CONTINENT COLLISION 68-61Ma
Fig. 6.1 Major Ophiolites were emplaced by collision of the
Irian Jaya volcanic arc with the Australian craton in the Late
Cretaceous and of the East Papua volcanic arc with the Owen
Stanley terrane in the Paleocene (see figure 6.2).
EOCENE
• SEPIK VOLCANIC ARC
• EARLY EOCENE KAMI EVENT
• APPROACH OF EAST PAPUA COMPOSITE
TERRANE - EO-OLIG. OMAURA FORMATION
IN FOREDEEP
Fig. 6.3 The Uyaknji small ocean basin closed in the Late
Paleocene or Early Eocene and the Schrader, Marum, Jimi,
Bena Bena, Kubor and Pale terranes were satured to the
craton. The Salumei or Sepik volcanic arc was active in the
Middle Eocene.
46
PALEOCENE
• OPENING OF CORAL SEA 65-55Ma
Fig. 6.2 The Coral Sea opened in the Paleocene. The Kami
and Uyaknji small ocean basins may have opened at the same
time, as a northwesterly extension of the Coral Sea. The East
Papua Composite terrane formed by collision of the East
Papua volcanic arc and the Owen Stanley terrane. The Border
terrane may have been sutured to the craton at this time or
may be a salient of the Australian craton.
OLIGOCENE
• SEPIK ARC-CONTINENT COLLISION
• EASTERN PAPUA COMPOSITE TERRANE
COLLISION
• APPROACH OF FINISTERRE VOLCANIC ARC
Fig. 6.4 The Salumei volcanic arc collided with the craton in
the Late Eocene and Oligocene to form the Sepik terrane. The
Uyaknji small ocean basin closed and the East Papua
Composite terrane was accreted to the craton towards the end
of the Oligocene. The Finisterre volcanic arc developed in the
mid-Oligocene.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Tectonics and Mineralisation (cont.)
PLIOCENE
MIOCENE
• FINISTERRE ARC-CONTINENT COLLISION
- EVIDENCE IS UNCOMFORMITY
• SUCCESSOR BASINS
• VOLCANISM WITHIN OROGEN
Fig. 6.5 The Finisterre arc collided with the craton in the
Early Miocene. The Ramu-Markham basin developed in the
foreland as collision progressed. Magmatic activity within the
orogen (“Maramuni volcanic arc”) may have been triggered by
collision, rather than subduction.
PRESENT DAY CONFIGURATION
• BISMARK ARC-CONTINENT COLLISION
- EVIDENCE IS SEISMIC & SEISMICITY
• FINISTERRE THRUST BELT
• PAPUAN FORELAND THRUST BELT
• ROTATION OF DOCKED TERRANES
Fig. 6.6 The Bismark volcanic arc collided with the craton in
the Pliocene, resulting in the development of thrust faults
within the Finisterre terrane and in the foreland of the
Australian craton (Papua fold belt). Palaeomagnetic studies
show that most or all terranes were rotated anti-clockwise
during the Pliocene, presumably in response to E-W leftlateral strike-slip movements.
• CONVERGENT MARGIN SINCE LATE
CRETACEOUS
• MUCH OF OROGEN IS ALLOCHTHONOUS
• COLLISIONS OLDER THAN RECOGNISED
KAMI - AURE EO (-OLIG?) SEAWAY
• BISMARK ARC COLLISION
Fig. 6.7 Present-day configuration. The heavy line marks the
edge of autochthonous Australian continental lithosphere. The
collision with the Bismark Volcanic Arc continues and causes
crustal shortening across the Orogen with active thrust faulting
in the Finisterre Range and the Papuan Fold Belt and the
transpressional faulting in-between.
Legend for Figures 6.1-6.7.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
47
Tectonics and Mineralisation (cont.)
SEISMICITY OF THE PNG
REGION
As mentioned above, PNG lies in the collision zone of
two major lithospheric plates, the Pacific Plate to the
northeast and the Indo-Australia Plate to the south
and southwest. The collision is oblique as the
direction of movement of the Pacific Plate is
northwest while the Indo-Australia Plate is moving
towards the north-northeast. Within the collision
zone are several minor plates. The Solomon Plate lies
in the southeastern part of the region while the South
Bismarck, North Bismarck and Caroline Plates
occupy the northern part of the region. Most of the
seismicity of the region occurs at the plate boundaries.
The main concentration of seismicity is at the
northern and northeastern margins of the Solomon
Sea (Fig. 6.8) where the Solomon Plate subducts
beneath the South Bismarck Plate and the Pacific
Plate, respectively. Seismicity in this area has been
described as the most intense in the world (Ripper and
McCue, 1983; Cooper and Taylor, 1989). From this
area the seismicity continues towards the southeast
through the Solomon Islands, and towards the
northwest under the northern part of mainland PNG
and Irian Jaya. The other main belts of seismicity in
the PNG region are across the southern margin of the
Solomon Sea, across the Bismarck Sea, and an arc
north of Manus Island and New Ireland.
Most of the seismicity is at shallow depth, less than
40km, but there is significant deeper seismicity with
some earthquakes at depths of about 600km. In order
to illustrate the relationships between the shallow and
deeper seismicity three vertical cross-sections have
been constructed (Figs 6.9, 6.10, 6.11). These profiles
are oriented approximately orthogonal to the local
trend of the seismicity and represent the seismicity in
vertical zones approximately 100km wide.
Profile 1,
across mainland PNG, shows three main
regions of seismicity. The shallow seismicity in the
southern part of the cross-section represents activity in
the Papuan Fold Belt on high-angle thrust faults. The
deeper seismicity in the central part of the section
reflects the presence of the westward extension of the
Solomon Plate, now deeply buried beneath the IndiaAustralia Plate. The western part of the Solomon
Plate has been shown to have an arch-like
configuration resulting from subduction both to the
north and to the south (Ripper, 1980, 1982; Cooper
and Taylor, 1987; Pegler, Das and Woodhouse, 1995).
The shallow and intermediate depth seismicity of the
northern part of Profile 1 results from oblique
collision of the North Bismarck Plate with the India-
Fig. 6.8 Epicentres of earthquakes in the PNG region.
48
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Tectonics and Mineralisation (cont.)
Australia Plate. The transpressional nature of the
collision is reflected in the intense high angle strikeslip and thrust faulting in the coastal area of northern
mainland PNG. Deep subduction of the North
Bismarck Plate probably is inhibited by the
underlying presence of the Solomon Plate (Cooper
and Taylor, 1987).
Profile 2,
Fig. 6.9 cross section through northern PNG mainland.
from the northern part of the Solomon Sea
to the northern part of the Bismarck Sea, shows the
seismicity at two plate boundaries. The zone of
seismicity that dips northward beneath New Britain is
a classical representation of subduction. In this case
the Solomon Plate is subducting beneath the South
Bismarck Plate. The Solomon Plate supports
seismicity at depths as great as about 600km. The
main concentration of shallow seismicity under the
Bismarck Sea reflects predominantly strike-slip
dynamics (and some normal fault activity) at the
boundary between the South Bismarck and North
Bismarck Plates.
Profile 3, from the eastern Solomon Sea to the Pacific
Ocean, shows the seismicity at the boundary between
the Solomon Plate and the Pacific Plate. The
northeast dipping zone of seismicity signifies the
subduction of the Solomon Plate beneath the Pacific
Plate. There is an apparent discontinuity between the
dipping zone of seismicity and the deep seismicity.
Adjacent seismicity indicates that the Solomon Plate is
splintered and contorted at depth (Cooper and Taylor,
1989).
Fig. 6.10 cross section through New Britain-Manus basin.
Fig. 6.11 cross section through Bougainville.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
49
7. Mineralisation Styles
INTRODUCTION
Papua New Guinea possesses mineralisation styles
characteristic of its setting at a convergent plate
margin. Mineralisation is dominantly porphyry
Cu–Au and epithermal Au–Ag styles as illustrated in
Fig. 7.1.
Volcanogenic massive sulphide, exhalative manganese
and limestone are also present. Weathering of select
bedrock has resulted in the formation of lateritic
nickel-cobalt-chromite and bauxite occurrences. In
addition, erosion of hard rock deposits has generated
placer deposits of gold, platinum, titaniferous
magnetite and chromite. In this broad discussion of
mineralisation styles, individual deposits are cited as
examples and expanded upon with references later in
this volume.
GOLD, COPPER AND SILVER
Porphyry Cu–Au deposits
Porphyry Cu–Au deposits, which although of low
metal tenure (commonly <1% Cu and 1g/t Au), form
large bulk-mineable resources, and so potentially
represent high-value ore systems (eg. Panguna,
Ok Tedi, Frieda River, Yandera, and Golpu (formerly
Rafferty’s, at Wafi). While the porphyry Cu–Au
deposits in Papua New Guinea are broadly similar to
examples of this mineralisation style in other
magmatic arcs (eg. western USA and Chile), the high
gold content of PNG porphyry Cu–Au occurrences,
relative to copper and molybdenum, is a characteristic
typical of many southwest Pacific deposits and is
consistent with the influence of oceanic crust.
Porphyry Cu–Au deposits are developed at relatively
deep (1–2km) crustal levels and display a
close relationship with intrusion source rocks.
Mineralisation commonly occurs at the apophyses
of large magmatic masses and is commonly
associated with the repeated emplacement of
porphyryitic intrusions of intermediate composition.
The polyphasal event commonly includes
post-mineral intrusions, which may constitute
internal waste in mineable reserves. Initial
prograde hydrothermal alteration, associated with
Fig. 7.1 Conceptual model for styles of magmatic arc Cu–Au mineralisation (after Corbett, 2002).
50
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineralisation Styles (cont.)
intrusion emplacement, ranges from proximal
potassic
(K-feldspar–biotite–magnetite)
and
local albite alteration, to distal propylitic
(inner propylitic actinolite–epidote to outer
propylitic
chlorite–carbonate)
alteration.
Phyllic
(silica–sericite–pyrite)
and
argillic
(clay–chlorite–carbonate–pyrite) alteration are
commonly imposed upon the prograde alteration.
Retrograde alteration destroys prograde mineral
assemblages and can downgrade magnetic anomalies,
such that Cu–Au ore in any one deposit may be
associated with regions of elevated or depressed
magnetic intensity (eg. Kainantu).
Barren advanced argillic alteration may develop
subjacent to porphyry Cu–Au deposits, due to the
alteration of host rocks by high-temperature
magmatic volatiles emanating from the intrusion
source early in the porphyry-related hydrothermal
process. Analogies are recognised in active magmatic
arc geothermal systems (Reyes et al., 1993). These
advanced argillic alteration zones comprise resistant
massive silica, alunite, pyrophyllite, pyrite (locally
with high-temperature andalusite and corundum),
and may form prominent topographic highs in the
vicinity of more deeply eroded, less-resistant porphyry
mineralisation eg. (Ekwai-Debom at Frieda River,
Oro Prospect at Kainantu, Mt Kren on Manus
Island). These alteration zones are termed ‘barren
shoulders’ by Corbett and Leach (1998) and occur
within the root zones of the ‘lithocaps’ described by
Sillitoe (1995).
The chalcopyrite–bornite–pyrite mineral assemblages
typical of porphyry systems may be hosted within
stockworks and sheeted quartz veins (Frieda River), or
as fracture coatings (Panguna), and less commonly as
breccia fill (Ok Tedi). The highest ore grades are
commonly close to the intrusion margin and often
extend into the country rocks. Weathering of pyriterich porphyries (most commonly present in phyllic
alteration) generates acidic groundwaters that leach
copper from upper levels of the system to
subsequently replace sulphides near the base of
oxidation, to form underlying chalcocite blankets of
higher (1–2% Cu) metal grades (Ok Tedi, Frieda
River). Copper and gold concentrate separately during
supergene processes. Supergene gold enrichment
occurs at near-surface settings, close to the base of
The Geology and Mineral Potential of
PAPUA NEW GUINEA
oxidation, and is often structurally controlled.
The specific gravity of host rocks is lowered during the
oxidation/supergene enrichment process.
Skarn deposits
Skarn deposits are formed by the replacement of
country rocks when they are intruded by an igneous
body. Proximal skarns in Papua New Guinea can
contain Cu–Au where limestone is in contact with the
intrusion (Ok Tedi, Frieda River), or gold where
favourable host rocks have been replaced in more
distal settings (Mt Victor). Magnetite skarns are
usually well preserved in the rivers downstream from
the area in which they were developed. These boulder
trains led to identification of the porphyry Cu–Au
deposits at Frieda River and Ok Tedi.
Epithermal gold
Epithermal gold mineralisation develops at shallower
crustal levels than porphyry systems, most commonly
<1km. Younger gold mineralisation may be
superimposed upon older porphyry systems that
originally formed at deeper crustal levels and have
been subsequently uplifted and eroded (Kainantu
district; Rogerson and Williamson, 1986). Most
classifications of epithermal Au–Ag–Cu deposits
(Corbett and Leach, 1998; Corbett, 2002 and
references therein) distinguish high and low
sulphidation epithermal deposits on the basis of
gangue and ore minerals reflecting ore deposition
from dramatically different fluids (Fig. 7.2). Low
sulphidation deposits can be further categorised into
four types of mineralisation (Fig. 7.3), which develop
at decreasing crustal depths. They are:
• quartz–sulphide Au±Cu (QS);
• carbonate-base metal–Au (CBM);
• epithermal quartz–Au–Ag (EQ); and
• adularia–sericite epithermal Au–Ag banded quartz
vein deposits.
Low sulphidation deposits
Low sulphidation deposits are derived from
circulating dilute meteoric-dominated fluids which
entrain metals, possibly derived from the inferred
51
Mineralisation Styles (cont.)
Fig. 7.2 The derivation of high versus low sulphidation Au–Ag–Cu deposits from differing fluid styles (after Corbett, 2002).
Fig. 7.3 Vertical zonation in low sulphidation epithermal gold deposits (after Corbett and Leach, 1998).
52
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineralisation Styles (cont.)
Fig. 7.4 Vertical zonation in mineralisation styles in the Wau district, showing the gradation in style from deeper to higher
crustal levels.
intrusive heat source(s) responsible for development
of the circulating hydrothermal cells (Fig. 7.4). These
deposits tend (although not always, e.g. Ladolam) to
display low sulphide contents, and are dominated by
pyrite, sphalerite, galena, chalcopyrite, with quartz
and occasionally carbonate gangue minerals. The
near-neutral hydrothermal fluids promote wall-rock
alteration dominated by illite group clays. However,
during late-stage collapse of the hydrothermal system,
acidic condensate waters promote the development of
advanced argillic and argillic alteration.
Quartz–sulphide Au±Cu (QS) mineralisation
QS type mineralisation represents the earliest type of
intrusion-related low sulphidation gold deposits to
form. QS type mineralisation commonly, although
The Geology and Mineral Potential of
PAPUA NEW GUINEA
not always (Ladolam), develops at deeper crustal levels
in the vicinity of porphyry intrusions (Kainantu),
where they exploit pre-mineral structures (Irumifimpa
and Kora at Kainantu). QS type mineralisation is
transitional to the "D" veins of the early porphyry
copper literature (Gustafson and Hunt, 1975), which
occur marginal to porphyry Cu–Au deposits (Frieda
River). At Hamata in the Morobe Goldfield, veins of
the QS type mineralisation are associated with
potassic (magnetite–K-feldspar) alteration at the
deepest level exposed by erosion.
QS type
mineralisation is overprinted by later CBM type gold
veins at Porgera and Mt Kare. The Kerimenge
Prospect, also in the Morobe Goldfield, grades from
QS type mineralisation at depth to CBM and EQ
styles at higher levels.
53
Mineralisation Styles (cont.)
A quartz and pyrite assemblage typifies QS type
mineralisation, although other sulphides include
chalcopyrite–pyrrhotite at deeper levels and marcasite
and arsenean pyrite at higher levels.
The gold of QS type mineralisation commonly occurs
in fractures and on the margins of sulphide crystals
(typically pyrite but also chalcopyrite and locally
tellurobismuthinite), and the ores may possess good
metallurgical recovery characteristics (Hamata).
However, in some deposits where the fluids have been
quenched, gold may be encapsulated within very fine
grained sulphides and therefore possess poor
metallurgical recovery characteristics (Kerimenge,
Ladolam). Metal and mineral zonation are apparent
as some deeper formed veins may carry copper (Frieda
River) or pyrrhotite (Jez Lode at Porgera), and higher
level deposits may contain gold encapsulated within
arsenean pyrite (Ladolam, Simberi).
The gold grades within QS type mineralisation vary
from sub-economic in many veins marginal to
porphyry systems, to zones of high grade (>10g/t Au)
eg. Ladolam and bonanza gold grades (Irumifimpa).
QS type mineralisation is susceptible to supergene
enrichment in surficial tropical environments, and is
often worked by artisanal miners (Kainantu and Wau
districts), especially within deeply weathered
structures.
In some QS type mineralisation developed within
silica-poor K-rich shoshonitic host rocks (eg. Porgera,
Mt Kare, Tanga–Feni–Lihir–Tabar Island chain)
K-feldspar (typically as adularia and other
low-temperature forms) dominates over quartz.
At the giant Ladolam deposit on Lihir Island, fluid
control is provided by structure and lithology
(ie. permeable lithologies, breccia including
phreatomagmatic diatremes) and the ores are
associated with fine-grained arsenean pyrite within
zones of K-feldspar alteration.
Carbonate–base metal–Au (CBM) deposits
The CBM deposits typically develop later, and occupy
higher crustal level settings, than the QS deposits.
They represent some of the most prolific gold
producers in the southwest Pacific (Kelian, Mt Muro,
Indonesia; Antamok, Acupan, Victoria, Philippines;
Penjom Malaysia; Kidston, Mt Leyshon, northeast
54
Australia), and are well developed in Papua New
Guinea (Porgera, Mt Kare, Wooklark Island, Misima,
Hidden Valley, Kerimenge, Wau). These deposits are
characterised by a gangue rich in carbonate and
sulphides comprising early pyrite followed by pyrite,
sphalerite (in greater quantities than galena) and rare
chalcopyrite. Both carbonate and sphalerite may
display a zonation within the deposit. Carbonate is
deposited from collapsing bicarbonate waters that are
neutralised by reaction with wallrocks and so at higher
levels siderite forms, while manganese and magnesium
species (rhodochrosite, dolomite, kutnahorite) occur
at intermediate levels, and calcite forms at depth.
Low-temperature sphalerite displays Zn>Fe
compositions and is typically pale coloured and
changes with increasing depth and temperature of
formation through yellow, red and brown colours, to
the black Fe>Zn compositions formed at greater
depth and temperature conditions (Corbett and
Leach, 1998).
CBM deposits typically overprint earlier formed QS
deposits (Mt Kare, Porgera), and pass to QS deposits
with increasing depth (Kerimenge) or in proximity to
porphyry source rocks (Maniape).
They are
commonly overprinted by EQ deposits of higher gold
grade, and this overprinting event contributes towards
the irregular gold distribution recognised within
CBM deposits (Porgera, Mt Kare, Busai). CBM
deposits have many forms, including fissure veins
(Edie Creek), lodes (Kulumadau), and stockwork vein
systems are most common (Mt Kare and Porgera), and
may be locally transitional to quartz vein systems
(Misima). Many CBM deposits are associated with
high-level intrusions (Porgera), or phreatomagmatic
breccias (Upper Ridges, Kerimenge). Some develop at
the margins of high sulphidation systems either from
separate fluids (Frieda River district), or by
neutralisation of acid fluids through wall rock reaction
(Link Zone at Wafi), and so display higher gold grades
and possess better metallurgical recovery
characteristics than the subjacent high sulphidation
systems.
The gold within CBM systems is deposited by the
mixing of rising ore fluids with bicarbonate waters,
and so is commonly found at the carbonate-base metal
sulphide interface (Corbett and Leach, 1998). The
gold grades display highly irregular distribution and
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineralisation Styles (cont.)
tenor, and the metallurgical
characteristics may also vary
significantly within individual
deposits (Porgera). There is
usually a positive correlation
between gold grade and base
metal content. Manganese wad
formed from the weathering of
rhodochrosite
is
easily
recognisable
in
weathered
exposures and is an indicator of
this mineralisation type. The wad
may scavenge gold and silver,
resulting in surficial enrichment
of those metals.
Epithermal quartz Au–Ag
(EQ) systems
Epithermal quartz Au–Ag systems
generally occur in association
with other types of intrusionrelated low sulphidation gold
mineralisation and are often
localised in fault zones (Porgera
Zone VII, Busai and Kulamadau
on Woodlark Island). They
typically form in the latter stages
of mineralisation so that they
overprint other styles (QS at
Ladolam, and CBM at Porgera
and Mt Kare) or crop out in
marginal settings (Kerimenge),
and are commonly preserved at
the highest topographic levels
(Edie Creek). QS, CBM and
Fig. 7.5 Cross-section through the high sulphidation alteration zones at Nena
EQ low sulphidation Au–Ag
produced by laterally flowing hydrothermal fluid (after Bainbridge et al., 1993).
deposits are all known to occur in
the mountainous terrain of
contributes towards the highly irregular gold
Morobe Goldfield at different topographic levels.
distribution within carbonate–base metal–gold
This type of mineralisation is characterised by
deposits (Porgera, Mt Kare). Weathering and erosion
bonanza gold grades (Porgera Zone VII, Mt Kare,
may result in substantial alluvial deposits (Mt Kare,
Busai), and is interpreted to have been deposited by
Morobe Goldfield).
the mixing of rising ore fluids with oxygenated, low
pH or bicarbonate waters, and commonly contains
Adularia–sericite epithermal Au–Ag banded
very little associated gangue (Porgera, Edie Creek).
quartz vein deposits
Free gold may include crystalline and wire gold
Adularia–sericite epithermal Au–Ag deposits
shapes, thus epithermal quartz Au–Ag mineralisation
overprinting earlier formed mineralisation types,
commonly occur as veins with a banded texture. The
The Geology and Mineral Potential of
PAPUA NEW GUINEA
55
Mineralisation Styles (cont.)
texture comes from successive layers of fine-grained
quartz (chalcedony), quartz replacing platy calcite,
adularia, and black sulphidic "ginguro" layers (named
from the Japanese mining term; Corbett, 2002 and
references therein). Circulating dilute meteoricdominated waters can deposit minerals by rapid
cooling (fine-grained chalcedonic silica), and by
boiling (quartz replacing platy calcite, adularia). The
mixing of ore fluids with oxygenated, low pH and
bicarbonate waters may also account for the
deposition of most bonanza-grade Au–Ag (electrum
and free gold) which commonly occurs within the
black sulphidic ginguro bands (Corbett and Leach,
1998).
The banded veins typically develop within a dilational
structural setting, which permits access for circulating
hydrothermal fluids and enables repeated rapid
mineral deposition. Although common elsewhere in
Pacific Rim back arc settings (Patagonia of Argentina,
western USA, Taupo Volcanic Zone of New Zealand,
Japan), deposits of this style also occur in magmatic
arcs characterised by andesitic–felsic volcanism
(Coromandel Peninsula of New Zealand, Kamchatka
of eastern Russia, Japan), especially within dilational
structural settings (Tolukuma, Papua New Guinea).
Sediment-hosted replacement gold deposits
The best known sediment-hosted replacement gold
deposits are in Nevada, western USA. They have also
been recognised in southwest Pacific magmatic arcs
(Mesel, Indonesia; Bau, Malaysia) and in China.
These deposits are thought to develop from the
reaction of quartz–sulphide-style low sulphidation
fluids with reactive impure limestone host rocks at
shallow crustal settings. The major controls on ore
deposition tend to be lithological at shallower crustal
levels and structural at depth. Although not currently
recognised within Papua New Guinea, this style of
mineralisation presents a viable exploration target
where the essential requirements of formation, namely
a magmatic source, dilational structures and impure
limestone host rocks are present.
High sulphidation epithermal gold (HSE)
deposits
High Sulphidation Epithermal deposits develop from
volatile-rich magmatic-derived fluids which rapidly
56
migrate from deeper to shallower crustal levels,
without significant interaction with either the host
rocks or dilution by groundwater (Corbett and Leach,
1998; Corbett, 2002). Depressurising fluids exsolve
volatiles (dominantly SO2), which in turn oxidise to
become hot, strongly acidic (pH <2) solutions. At
shallow crustal levels these fluids react with the host
rocks to create the zoned hydrothermal alteration
characteristic of these deposits (Fig. 7.5). In the core
portions of HSE alteration, the most strongly acidic
fluids (pH 1–2) leach the host rocks to form a
mappable alteration type termed vughy silica
(reflecting the texture) or residual silica (indicating the
remaining composition). As the fluids are
progressively cooled and neutralised by rock reaction,
alteration zones are developed outwards from the
core to result in mineral assemblages dominated by
alunite, pyrophyllite–diaspore, dickite, kaolin and
illite–smectite–chlorite. The detailed mineralogy and
thickness of alteration zones will vary according to the
crustal level, proximity to fluid upflow and the nature
of the permeability control.
Sulphide minerals which overprint the alteration are
dominated by pyrite and enargite (Nena, Wafi), and
locally with the low-temperature polymorph luzonite
(Maragorik). They may vary from gold-rich at
higher crustal levels to copper-rich at depth, where
covellite and chalcocite may host ore (Nena). High
sulphidation gold deposits in Papua New Guinea are
gold-rich and silver-poor (similar to elsewhere in the
southwest Pacific), as distinct from the silver-rich
Andean systems (Yanacocha, Pierina, Alto Chicama in
Peru; La Coipa, Pascua in Chile). High sulphidation
gold deposits are commonly associated with felsic
domes and display permeability controls dominated
by host-rock lithology, structure or breccias (typically
as phreatomagmatic breccia diatremes), and
interactions of these controls. The mineralisation at
Wafi occurs in association with a diatreme - dome
complex, while that at Nena is localised by the
intersection of a major structure and a permeable
lapilli tuff, and mineralisation at Maragorik occurs
within a permeable rock unit and a deeper "feeder"
structure.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineralisation Styles (cont.)
COPPER, ZINC, LEAD, GOLD and
SILVER
Volcanogenic massive sulphide
(VMS) deposits
VMS deposits hosting Cu, Pb, Zn, Au and Ag occur
as stratiform lenses within volcaniclastic and volcanic
rocks at Laloki near Port Moresby and Ofuo. Black
smokers in the offshore Manus Basin, the Woodlark
Basin and offshore from the Tanga–Feni–Lihir–Tabar
Island chain, are currently depositing gold with
sulphides.
channelled porewater expulsion.
Syntaphral
deformation of host cherts indicates that exhalation
probably occurred during synsedimentary faulting on
a submarine palaeoslope.
NICKEL AND COBALT
Weathering of ultramafic rocks
Weathering of ultramafic rock units has produced
laterite nickel–cobalt–chromite deposits at Ramu,
Safia and Ioma.
SECONDARY GOLD, BEACH
No stratabound deposits (Mississippi Valley type)
have been recognised within PNG but the northern
margin of the Fly Platform and the Papuan Fold Belt
may be likely settings for this type of mineralisation.
MANGANESE
Manganese concentrations occur within radiolarites
and carbonate turbidities of the Eocene hemipelagic
Port Moresby Beds as cm-size ferromanganese
concretions and locally as manganese-dominated
stratiform bodies of up to 103t in size. The largest
known stratiform occurrence, the Pandora deposit
80km SE of Port Moresby, was mined and probably
largely worked out between 1938 and 1964,
producing some 2400t of high grade manganese oxide
for battery manufacture. Previous discovery of
siliceous microfossils within Pandora ore strongly
suggested a sedimentary genesis for this deposit. This
was confirmed by demonstrating seawater-inherited
rare earth element patterns for the ore (Finlayson &
Cussen, 1984). Negative Ce anomalies, high Mn/Fe
and only slightly enriched hydrogenous element
chemistry indicate an exhalative mechanism for ore
emplacement. Mn oxide and chalcedony-veining of
the chert breccia host, coupled with absence of
significant associated footwall Fe enrichment, imply
that Mn oxidation and exsolution began sub-seafloor
through mixing of an Fe-deficient ore fluid with oxic
porewater; ore precipitation occurred at the site of
exhalation. An origin within the Port Moresby Beds
for the manganese of both stratiform deposits and
concretions is supported by evidence of substantial
planktonic test dissolution, organic combustion and
metal movement during diagenesis. Stratiform
deposits were formed locally and transiently by
The Geology and Mineral Potential of
PAPUA NEW GUINEA
SAND AND PLATINUM.
Placer gold deposits
Placer deposits have historically been important
sources of gold for PNG and have been worked as
major operations (Bulolo produced 3.5 million
ounces) or by artisanal miners (Wau, Kainantu, Mt
Kare) who produce about 70,000 ounces per year.
The placers are a useful exploration tool because they
provide guidance to nearby hard-rock ores (Porgera,
Mt Kare). The Mt Kare case is most interesting in
that as much as 1 million ounces of consistently high
fineness gold is thought to have been chemically
remobilised and redeposited down slope from the
hard-rock source of variable fineness.
Beach Sands
Papua New Guinea has significant heavy mineral
beach deposits consisting of titaniferous magnetite
sands, chromite sands and olivine sands. The largest
being the titaniferous magnetite sands containing
accessory rutile, ilmenite and zircon. The lighter heavy
minerals rutile, zircon, ilmenite and monazite are
sparsely distributed, mainly within the beach sand
deposits.
Several types of placers are known in PNG, including
strandline (shoreline or beach) placers, coastal Aeolian
placers, marine placers, and fossil strandline and
dunes. In general, the richest heavy mineral
accumulation occurs along the base of frontal dunes
on open beaches, and in natural traps formed by
headlands and other barriers impeding longshore
currents. This is also the case for PNG beach sand
deposits.
57
Mineralisation Styles (cont.)
The major beach sands areas on the south coast are in
the Gulf of Papua between Daru and Kerema, on the
southeast coast between Beagle Bay and Mullins
Harbour, on the Papuan peninsula’s north coast near
Popondetta, and on Bougainville Island. Most of the
sands have been derived from the erosion of volcanics.
Chromite-bearing sands derived from ultramafics also
occur in significant concentrations, but are not as
widespread as the iron sands. Major occurrences have
been noted along the north coast of the Papuan
Peninsula between Salamaua and Morobe.
Occurrences have also been reported northwest of
Vanimo, on Fergusson and Normanby Islands in
Milne Bay, and in the delta of the Purari River in the
Gulf of Papua.
deposit model could be the source for some of the
PGM, it would not account for all the PGM
mineralisation. A possible source model for the high
platinum alloy found in some of the occurrences
would be the zoned ultramafics of the Alaskan/Ural
type. Smith and Davies (1976) noted several high
alkali intrusive suites within the area. These intrusives
are usually thought to reflect deep crustal fracturing
and lithospheric rifting.
The most common deposit type applicable to PNG is
the ophiolite-related deposit type and derived placers.
Significant examples of the latter include the
osmiridium-rich alluvials along the Lakekamu, Gria
and Aikora Rivers, and McLaughlins Creek. PGM
associated with olivine-rich and chromite-rich rocks
are enriched in osmium, iridium and ruthenium.
Another PGM deposit type that one would expect to
occur in Papua New Guinea are the hydrothermal and
skarn related deposits. High concentrations of
palladium and platinum, averaging 75ppm and 4ppm
respectively, are associated with the copper ores of the
New Rambler deposit, Wyoming USA (McCalum
et al., 1976). There the metals are thought to have
been concentrated by hydrothermal leaching of the
gabbroic rocks and redeposited along shear zones as
palladium and platinum-rich copper ores. The PGM
occur as discrete minerals in the main stage
chalcopyrite-pyrrhotite assemblage, and in the early
hypogene pyrite (up to 60ppm Pd in solid solution).
Platinum, palladium and osmium have also been
reported in Mo-Cu hydrothermal and skarn deposits
in China with Os Pt Pd (Cabri, 1981a).
Hydrothermal/skarn PGM deposits (eg. New
Rambler, Wyoming, USA) have yet to be identified in
Papua New Guinea.
PGM mineralisation is also associated with sulphide
mineralisation. Sulphide mineralisation within
ophiolites is commonly associated with volcanics,
though minor sulphide concentrations can occur
within cumulus gabbros. The Doriri Creek nickel
prospect sampled by INSEL in 1968 and by Nord in
1978 was found to be anomalous in platinum and
palladium. Platinum and osmiridium were also
produced from the Astrolabe Mineral Field, just east
of Port Moresby.
It is interesting to note that porphyry Cu-Mo deposits
may also contain PGM.
In the western USA,
approximately 1g/t platinum is recovered as a byproduct during copper refining (Cabri, 1981b). It is
estimated that Arizona produces between 55-70% of
the US platinum recovered from copper concentrates.
Relatively little is known about the PGM mineralogy
or geochemistry of these deposits. Porphyry Cu-Mo
deposits in the little Caucasus, Amenia, also contain
PGM.
The Milne Bay area was the focus for a short-lived
gold and platinum rush during the mid 1930s. It has
been recorded that PGM occurrences in the Dowa
Dowa River headwaters were not associated with
alluvial gold. It is thought that although an ophiolite
PGM-bearing placers have been reported throughout
Papua New Guinea from north of the Sepik, throughout
the Highlands and down through the Papuan Peninsula
to Milne Bay and the D’Entrecasteaux Islands. PGMbearing placers are also auriferous and have been worked
Foundry grade olivine sands have been reported along
the coast between Salamaua and Morobe, and on
Fergusson Island.
Platinum Group Metals
58
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineralisation Styles (cont.)
in Papua New Guinea since the turn of the century.
Most production was prior to World War II, and since
then it has been sporadic. Major PGM production has
come from the Lakekamu, Gira, Aikora and Milne Bay
areas. During the 1930s, the Milne Bay area was the
focus of a short-lived gold and platinum rush, but the
source of the PGM in this area is yet to be identified.
Work by the Geological Survey of PNG in the north
Sepik area has identified extensive ophiolite
sequences, from the Prince Alexander Mountains to
the Bewani Range. The area may be considered to
have potential for PGM mineralisation. In most of
these areas no assessment of PGM potential has been
conducted.
Ground magnetics is an exploration tool that is widely used in PNG.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
59
8. Mineral Projects and Mines
Fig. 8.1 Location of mining operations and prospects discussed in this section.
INTRODUCTION
Despite its relatively short period of exposure to
western influences and the mining industry, Papua
New Guinea has attained credibility as a mineral
producer based on the presence of several major
mines, beginning with the Bulolo gold dredging
operation in the 1930s. There are currently three
world class mines in operation, two smaller mines in
production, and several projects about to come into
development. The important mining operations and
some of the prospects are discussed below in
alphabetical order (Fig. 8.1).
FRIEDA RIVER
Location and ownership
The Frieda River Project is one of the largest
undeveloped Cu–Au resources in PNG, containing in
excess of 600Mt of copper and 12 million ounces of
gold. This resource is substantially higher than
Ok Tedi’s contained metals (313Mt of copper and
9.7 million ounces of gold) in the pre-mining proven
ore reserves.
60
Fig. 8.2 Location of the Frieda River Project.
The prospects are located in the Sandaun Province,
about 800km NW of Port Moresby, 175km NW of
Porgera gold mine, and 90km NE of the Ok Tedi
copper mine. The nearest coastal town of Wewak is
250km to the NE (Fig. 8.2). The area is remote and
sparsely populated, with very little government
infrastructure.
The Frieda River prospects are centered on latitude
4o42’9’’S and longitude 141o45’55’’E at altitudes
between 300 and 1200m asl and are covered by
EL 58. The area is serviced by fixed-wing aircraft to
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
the Frieda airstrip, thence by a 12km helicopter flight
to the main base camp. Barges from Madang or
Wewak ply the Sepik River up to the Iniok Depot
located near the mouth of the Frieda River, where
cargo is transferred to small motorised canoes for the
remainder of the journey to Frieda airstrip.
Ownership is currently shared between Highlands
Pacific Ltd (HPL: 87.9%) and OMRD Frieda Ltd
(12.1%). OMRD is a Japanese consortium headed by
Sumitomo Metals Mining. In 2002, Noranda reached
an agreement with HPL and OMRD wherein
Noranda will take up to 72% interest in the Frieda
River property on meeting certain obligations. This
arrangement is still in place.
Exploration history
Mineralisation at Frieda River was discovered in 1967
by an Australian Bureau of Mineral Resources
geological mapping party. BMR geologist J.A. Smit
traversed Ok Uwaii, a creek draining the Frieda River
deposit, and recognised ferruginous float
(Lord, 1979) and collected stream
sediment samples, which subsequently
yielded anomalous copper values. In
1968, PA 58, subsequently EL 58, was
granted to Mount Isa Mines Ltd (MIM)
and in the years to 1973 several porphyry
copper deposits, including Horse and
Koki, were identified. From 1974 to
1978, OMRD Frieda Ltd earned 40%
equity, during which the Ivaal porphyry
and the Nena high sulphidation
mineralisation (follow-up of earlier stream
sediment anomaly) were discovered.
recommended that bankable feasibility studies based
on their pre-feasibility studies be undertaken for the
combined Nena and Ivaal porphyry resources.
However, the advancement of the project was affected
by the unsolicited takeover of Highlands Gold by
Placer Dome. Highlands Pacific Ltd was restructured
to acquire non-Porgera assets including Frieda River at
a cost of A$108 million. In 1998, Cyprus Amax
(USA) entered the Frieda River project through a joint
venture agreement and focused on re-evaluating the
porphyry deposits, until Phelphs Dodge took over
Cyprus Amax in late 1999. Phelps Dodge elected not
to have any interest in PNG and the property reverted
to HPL. In 2002, Noranda farmed in and drilling of
the Trukai prospect commenced, with several drill
intercepts of 100m grading >1% Cu. Drilling is
continuing.
Geological setting
The Frieda River area is located on the southern
margin of the New Guinea Mobile Belt, a zone
In 1979, CRA and Norddeutshe Affinerie
joined the venture, and two separate
feasibility studies were completed in 1981
and 1985. MIM transferred its equity in
Frieda River to Highlands Gold Properties
Pty Ltd (an MIM subsidiary) in 1992,
whilst CRA and Norddeutshe Affinerie
exited the venture in 1993 and 1994,
respectively. Total exploration expenditure
to 1997 was US$50.8 million.
By 1997, Highlands Gold announced a
measured resource for Nena and
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Fig. 8.3 Geology of the Frieda River Intrusive Complex.
61
Mineral Projects and Mines (cont.)
characterised by faulting and intense folding caused by
the oblique collision of the Pacific and IndoAustralian Plates since Miocene times. Consequently,
major structural trends are WNW (arc parallel) and
ENE (arc normal). The interaction of the WNWtrending arc-parallel structures (Fiak–Leonard Shultz
Fault Zone and Frieda Fault) and a NE-trending
transfer structure (Ok Tedi Structure), provided the
focus for intrusion of the multi-phased Frieda River
Intrusive Complex.
The Frieda River Intrusive Complex (15Ma) and
associated volcanism (Debom Volcanics) intruded two
basement rock units (Fig. 8.3). The oldest and more
widespread basement rock is Upper Cretaceous –
Eocene-aged Ok Binai Phyllites (Takaoka, 1984),
which are overlain by sedimentary sequences of MidMiocene Wogamush Formation.
At intrusive
contacts, these sediments are hornfelsed, brecciated,
and in places host skarn and porphyry mineralisation.
Britten (1981) defined five distinct phases for the
Frieda Intrusive Complex. The oldest is the Koki
Diorite Porphyry (Kdp), with the Frieda Diorite
Porphyry (FdP), Horse Microdiorite (Hmd), Knob
Diorite and Flimtem Trachyandesite (Fta)
representing the younger intrusives. Mineralisation
and alteration are spatially and genetically related to
west-northwest-trending stocks and dykes of Horse
Microdiorite bodies. These narrow apophyses (dykes)
apparently coalesce at depth.
Three sets of structures are dominant: NNE–NE
(post-mineral), WNW (syn-mineral) and E–W
structures. The E–W-trending structures are steeply
dipping normal faults and often delimit the northern
and southern margins of Horse Microdiorite and Koki
Diorite Porphyry bodies.
The syn-mineral WNW structures dip steeply north
(Hawkins, 2001). These faults are recognised within
the high-grade part of the main mineralised zones and
often have hydrothermal and tectonic breccia roots.
The NNE-trending structures dissect all the
porphyries, and displace mineralisation. For example,
the NNE-trending Ivaal Fault truncates the northern
limits of the Trukai mineralisation and juxtaposes it to
the northwestern part of the Ivaal mineralisation. At
the Horse, Koki and Ekwai prospects, these structures
62
are often intruded by Flimtem Trachyandesite (Fta).
Porphyry mineralisation
Work by Morrison et al. (1999) and Hawkins (2001)
on the paragenesis and controls of high-grade
hypogene porphyry mineralisation significantly
improved the understanding of the mineralisation at
Frieda River.
For brevity, five distinct alteration assemblages
associated with the porphyry mineralisation can be
recognised (Fig 8.4). From earliest to youngest these
alteration assemblages are:
•
•
•
•
potassic (biotite–magnetite); POB phase;
propylitic;
potassic (K-feldspar–quartz; K-feldspar); POK phase;
SCC (sericite–clay–chlorite±carbonate
–rutile–quartz); and
• phyllic–argillic.
The above assemblages are overprinted by an
advanced argillic alteration (AAA) assemblage which is
unrelated to the porphyry intrusion.
The two phases of potassic alteration were
accompanied by primary Cu–Au mineralisation, as
was the SCC alteration. The relative proportion of
copper mineralisation contributed by each phase
varies within a deposit, but each contributes equal
amounts. However, almost all the gold mineralisation
was entirely introduced during the potassic alteration
phase (Hawkins, 2001).
Alteration and mineralisation accompanying the
Porphyry intrusions at Frieda River, (Fig. 8.5) can be
summarised as follows:
1) Early Potassic (POB) alteration is variably
developed and widespread.
Characteristic
minerals are secondary biotite, magnetite, quartz
and rutile. Mineralisation associated with this
phase is relatively widespread within the Horse
Microdiorite, typically as fine-grained and
fracture-filled chalcopyrite–bornite, granular
quartz–biotite–magnetite–anhydrite
veins,
and less commonly as extremely finegrained chalcopyrite–bornite intermixed with
biotite–magnetite grains, which pseudomorph
hornblende. Typical grades associated with this
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Fig. 8.4 Frieda River porphyry mineralising events.
2) Transitional and peripheral to the early potassic
alteration is propylitic alteration. This assemblage
is characterised by sericite, chlorite and epidote,
with lesser but variable amounts of quartz,
carbonate, clay (usually smectite) and hematite.
Unusually, no significant mineralisation has been
identified within this alteration assemblage.
difficult to gauge because the mineralisation rarely
occurs in isolation, and is typically found
overprinting the earlier mineralised phase
associated with POB alteration. However, visual
estimation of the mineralisation that is associated
with POK alteration indicates an additional
0.3–0.5% Cu may be contributed to the host.
Chalcopyrite, bornite, molybdenite, pyrite and
gold are typically associated with granular and
seamed quartz veins, with K-feldspar, anhydrite,
and haematite–magnetite gangue.
3) The POK alteration overprints the earlier POB
and propylitic alteration assemblages. The POK
alteration is characterised by K-feldspar, quartz,
muscovite, anhydrite and magnetite. This
alteration assemblage is more restricted and
confined to the WNW-structures or hydrothermal
breccias where highest grades are generally
associated with veining. Metal grades that may be
considered typical of this mineralising event are
4) Overprinting both potassic alteration assemblages
is the SCC alteration assemblage characterised by
sericite–clay–chlorite±carbonate–rutile–quartz.
Copper mineralisation associated with SCC
alteration is primarily chalcopyrite with comb
quartz veins plus chlorite–pyrite. Veins and
fractures typically develop wide alteration selvages
of sericite–chlorite–clay–carbonate–rutile–quartz.
Mineralisation associated with SCC alteration
phase of mineralisation range between 0.3 and
0.4% Cu, and in exceptional cases 0.5% Cu
(Hawkins, 2001).
The Geology and Mineral Potential of
PAPUA NEW GUINEA
63
Mineral Projects and Mines (cont.)
Fig. 8.5 Alteration pattern at Frieda.
often contributes 0.5% Cu or more to the overall
tenor of the porphyry prospects. The copper
grades in these instances are typically >0.80%.
Nena High Sulphidation
Mineralisation
5) Phyllic–argillic alteration is poorly developed due
to the nature of the hosts and overprints all earlier
alteration events.
The alteration suite is
illite–kaolinite–anhydrite–carbonate. No hypogene
copper mineralisation was introduced during
phyllic–argillic alteration.
Follow up work of stream sediment samples collected
in 1975 identified high sulphidation mineralisation at
the Nena prospect located some 6km to the northwest
of the Horse and Ivaal porphyry deposits. Drilling
commenced in 1976 and by 1996 approximately 42,
909m had been drilled, achieving a nominal grid of
50m x 50m; 80% of the Nena resource now falls in the
measured category (Table 8.1).
Advanced argillic alteration (AAA) overprints earlier
porphyry related alteration–mineralisation and this is
often established by careful examination of rocks
(Hawkins, 2001). Fluids associated with the AAA
overprint, stripped copper and, to a lesser extent, gold
from the system.
The Nena prospect mineralisation and alteration are
hosted within the Debom Volcanics, and located on
the prominent NW–SE-striking Nena structure.
The prospect lies within an extensive advanced
argillic-altered zone covering about 13km x 4km.
Intense pervasive acid leaching and clay alteration of
the host rocks often precludes the identification of
64
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Nena Deposit - Identified Mineral Resource
Measured
Indicated
Inferred
Total
Resource Style
Mt Cu% Au g/t
Mt
Cu%
Au g/t
Gold resource
13.8
0.1
1.4
3.4
0.1
1.4
0.8
0.1
1.5
18.0
0.1
1.4
Copper resource 42.2
2.3
0.6
7.6
1.7
0.6
1.2
1.8
0.4
51.0
2.2
0.6
Notes:
Mt Cu% Au g/t
Mt
Cu% Au g/t
1. Copper – lower cut off grade 0.5% copper, Gold – lower cut off grade 0.6g/t gold.
2. Measured and indicated mineral resources are inconclusive of proved and probable reserves.
3. This estimate is based on 166 diamond drill holes or 38,441m of drilling.
Table 8.1 Nena deposit ore reserves (HPL, 2002).
Prospect
Horse–Ivaal
Category
Mt
Cu (%)
Au (g/t)
Source
Indicated Resources
109
0.6
0.3
(HPL, 2004)
Inferred Resources
895
0.5
0.3
(HPL, 2004)
1005
0.5
0.3
(HPL, 2004)
Total
Koki
Inferred Resources
274
0.4
0.3
(HPL, 2004)
Ok Nerenere
Inferred Resources
60
0.4
0.3
(DMP, 1999)
Ekwai
Inferred Resources
60
0.4
0.3
(DMP, 1999)
Horse–Ivaal and Koki were calculated at a lower cut-off of 0.2% Cu. Resource calculations for Horse–Ivaal were based on
221 diamond and percussion holes. Drill spacings were at 100m N–S and 150m N–S for Ivaal and Horse deposits, respectively.
Resource for Koki was based on 30 drillholes on a nominal 150m x 300m grid (HPL = Highlands Pacific Limited).
Table 8.2 Identified Mineral Resources — Porphyry Deposits, Frieda River (HPL, 2002).
original rock types. However, the Debom Volcanics
are typically porphyritic andesite, lavas and
pyroclastics.
The alteration pattern at Nena is symmetrical about
the NW-striking Nena structure and displays a lateral
zonation outwards from a core of residual silica,
through a broad replacement silica–alunite halo,
grading out into a clay zone. The vertical zonation is
represented by (from deeper to shallow levels)
andalusite,
Na-sericite,
quartz–illite–pyrite,
illite–smectite–dickite–pyrophyllite, alunite–silica,
and vuggy massive silica (Bainbridge et al., 1996).
Primary mineralisation in the main Nena zone
consists of polymorphs of luzonite and enargite and
less commonly chalcocite.
The polymorphs
commonly occur in fractures and are accompanied by
barite, and less commonly occur as small isolated
The Geology and Mineral Potential of
PAPUA NEW GUINEA
grains filling interstitial sites between pyrite, quartz
and alunite. The chalcocite zone is hypogene and
contains less arsenic and silver than the
luzonite/enargite zone. The supergene profile is
erratically developed and extends to a maximum
depth of 130m, forming leached, oxidised and
supergene enrichment zones. Cuprite, malachite,
azurite and scorodite occur in the oxidised zone.
Five stages of paragenesis recognised from earliest and
progressively younger are:
(1) epithermal quartz veining;
(2) acid leaching by magmatic volatiles, with the
formation of inner vuggy to massive
silica–alunite+clay zone;
(3) poly-phase sulphide event, with
pyrite–marcasite–melnacorite±amorphous silica;
65
Mineral Projects and Mines (cont.)
(4) brecciation and fracturing with deposition of
barite–luzonite–enargite±pyrite±native sulfur; and
(5) development of supergene enrichment of copper
and gold (Espi et al., 2001).
Identified mineral resources for the Frieda River
Project are contained within five mineralised
porphyry bodies (that apparently coalesce at depth)
and a high sulphidation epithermal Cu–Au deposit at
Nena (Tables 8.1, 8.2). The corresponding contained
metal resource amounts to over 600Mt of copper and
12 million ounces of gold, excluding mining and
metallurgical recovery factors. The contained metals
for Frieda River exceed that of Ok Tedi’s 313Mt of
copper and 9.7 million ounces of gold in the premining proven ore reserves by about 50%. The Ok
Tedi operation faced similar development constraints
but is currently one of the more successful operating
mines in PNG.
Two unsolicited takeover bids for Highlands Gold and
Cyprus Amax between 1997 and 1999, respectively,
adversely affected the exploration and advancement of
the property towards development.
Successful resolution of the controls on high-grade
hypogene porphyry mineralisation (Hawkins, 2001)
has confidently led to identification of several
prospects with the potential to host an additional
200–300Mt grading >1% Cu or better. Trukai
prospect, the only target drill tested in 2002–03,
confirmed intercepts of >100m at 1% Cu or better.
Recent work (2002) by the HPL–Noranda JV also
indicated a very high probability of discovering
another significant high sulphidation epithermal
system at Otnepal (Ekwai–Debom), where upper
portions of high-grade Trukai were stripped and
re-deposited at slightly higher elevations within
advanced argillically altered host rocks.
66
Further work and drilling, and evaluation of these and
several other prospects in the area in light of the
current understanding at Frieda River, will improve
the overall tenor of the project’s Cu–Au resource.
Resources and Potential of the
Frieda and Nena Prospects
The successful discovery of higher grade hypogene
copper mineralisation (>1% Cu or Cu equivalent) at
the Trukai prospect exemplifies a highly remarkable
achievement through diligent logging, mapping and
direct application of current scientific understanding
of South West Pacific porphyries on evaluating the
paragenesis and structures in a project explored for
over three decades.
Even in the lithocaps where the surface rockmass was
completely obliterated by post-mineral AAA,
underlying porphyry-style mineralisation was
confidently recognised through the identification of
stable quartz veins associated with porphyry Cu-Au
mineralisation.
Previously, quartz–illite–pyrite
assemblage of the AAA was mistakenly identified as an
SCC alteration suite associated with the mineralised
porphyry intrusive. This mis-identification had
significant ramifications for the prospectivity of the
porphyry copper system as the AAA destroys copper
mineralisation and is therefore usually represented by
low copper grades.
The advance in understanding gained from
exploration at the Frieda prospect not only improved
the probability of discovering higher Cu-Au grades in
the area, but also provided invaluable knowledge on
the methodology required to evaluate mineralised
porphyries underlying advanced argillic alteration in
other parts of PNG such as that at Mt Kren (Manus
Island).
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
KAINANTU
History
Location and status
In 1927, Ned Rowlands found payable gold near
Kainantu town. In the 1960s, Noel Stagg worked
alluvial gold on the divide overlooking the Markham
Valley. In the 1960–70 period, Ken Rehder
established a five-head stamp battery, access and adit
levels at the Kora Mine (southern end of the
Irumafimpa lode), and produced several tons of handpicked copper ore and gold from two separate veins.
The Kainantu Gold Project (lat. 6 o07’S, long.
145o53’E), 12km north of Kainantu town in the
Eastern Highlands Province, extends for 7km north
from the town as a series of individual prospects,
including the old Kora Mine, Irumafimpa (on the
same structure), Maniape and Arakompa. The project
lies between 800 and 1900m elevation on the north
eastern fall of the Eastern Highlands overlooking the
Markham Valley.
While the broader Kainantu Gold Project is covered
by Exploration Licences, underground development
of the main lodes of Irumafimpa is being undertaken
by Highlands Kainantu Ltd, a subsidiary of Highlands
Pacific Ltd on a Mining Lease granted in 2004.
Production should commence in 2005.
In 1982, RGC Exploration (a subsidiary of Renison
Goldfields Consolidated) took up PA 470 (PA refers
to a Prospecting Area – a term used to describe a
tenement under old legislation, that is now largely
equivalent to an Exploration Licence under the
current Mining Act). The tenement originally
covered about 4000sqkm in the Kainantu district, and
was progressively reduced in size. In 1983, gold was
panned at Baupa Creek and enargite–sulphur-bearing
840 Portal of the Highlands Pacific mine at the Irumafimpa lode, Kainantu.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
67
Mineral Projects and Mines (cont.)
advanced argillic-altered breccias were identified in
float. Follow-up auger sampling focused on the Oro
Prospect and attempted to locate the source of the
altered float but met with no success. Geological
mapping at that time identified mesothermal veins
grading to 15g/t Au in an area that later became
known as the Kainantu mineralisation. These veins
were not considered a viable target at that time. The
Arakompa lodes were identified during a 1985
trenching program of a ridge-top soil anomaly (max
0.63ppm Au) designed to follow up an earlier panned
concentrate sample of 132g/t Au collected from
drainages some 700m downstream. The lodes at
Maniape were identified at about the same time. The
veins at Arakompa and Maniape were
rapidly further exposed by local gold miners who were
also exploiting the Kora structure at the time.
The prospects were initially drilled by RGC.
Remains of Ken Rehder’s stamp battery at the old Kora Mine
in 1984.
RGC joint ventured the project to Highlands Gold in
1989, who later commenced an intensive exploration
program of detailed geological mapping and
trenching. Southern extensions to Maniape were
identified during stream-traverse follow up of
gossanous float during a 1991 field program.
Highlands Gold drill tested the prospects in 1993
(Corbett, Leach, Thirnbeck et al., 1994).
Local villagers working the Kora structure in 1984.
68
Irumafimpa was identified
by a geologist sent to
the area to follow up
on anomalou drainages
samples downstream and an
isolated 0.35ppm Au soil
auger
anomaly
along
structural strike from the
Kora Mine. The drainage
anomalies (pan concentrates
were 12.2g/t grading to
6.78g/t close to the
prospect,
and
further
downstream a -80 mesh
stream sediment sample
yielded 0.39g/t Au (Corbett,
1992)). Irumafimpa soon
displayed the most likely
potential for development
and Highlands Pacific
(HPL) moved to detailed
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
drilling in joint venture with Nippon
Mining and Metals (NMM).
In 2001, following the departure of NMM,
HPL drove an adit to allow bulk sampling
of the lodes and carried out additional,
mostly
underground,
drilling
at
Irumafimpa. Continued exploration has
exposed additional veins and extended the
strike length of the mineralised structural
corridor to several kilometres southeast to
Yar Tree Hill and northwest to the Kesar
Creek porphyry–vein system.
Geological setting
The Basement host rocks in the area
comprise Triassic Bena Bena Formation
phyllite intruded by the syntectonic
quartzofeldspathic Karmantina Granite
Gneiss (Tingey and Grainger, 1976;
Rogerson et al., 1982; Van Wyck
and Williams, 2002). Basement is
unconformably overlain by the Omaura
Formation (Tingey and Grainger, 1976,
Hawkins and Akiro, 2001), (Fig. 8.6). The
mid-Miocene Akuna Intrusive Complex is
well exposed as tonalite, granodiorite, local
monzonite, and occasionally more mafic to
ultramafic variants, all in intrusive contact
with the basement. Parts of the Akuna
Intrusive Complex may be equivalent to the Morobe
Granodiorite. The Elandora Porphyry intrudes the
Akuna Intrusive Complex as hornblende–biotite
porphyry, and occurs as fragments in diatreme
breccias at Irumafimpa and Maniape. A diatreme
breccia with associated Elandora-style porphyry
intrusions crops out south of Maniape and a premineral andesitic stock is present within the
Irumafimpa adit. Although the Akuna Intrusive
Complex has provided a 17–13Ma age and Elandora
Porphyry have been dated at 9–7Ma, the relationships
remain unresolved (Page, 1976; Rogerson and
Williamson, 1985).
Fig. 8.6 Geological relationships in the Kainantu area.
The Kora–Irumafimpa lode system has a strike length
of over 2.5km and has been traced down dip for
several hundred metres. The slaty cleavage of the
Quartz–sulphide,
low-sulphidation
type
mineralisation at Irumafimpa occurs within a
corridor of steeply dipping northwest-trending
structures, parallel to the Ramu–Markham Fault.
Kainantu Mine area looking north.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
69
Mineral Projects and Mines (cont.)
Bena Bena Formation changes to a crenulation
cleavage in the vicinity of these structures, suggesting
they have been formed at depths in the order of 5km,
and exhumed prior to reactivation and mineralisation
(Corbett, Leach, Thirnbeck et al., 1994). These
structures therefore display a protracted pre-, syn-,
and post-mineral history of activity. Early geological
mapping suggested that ore shoots are localised at the
intersection with north–south structures, possibly in
association with dextral strike-slip movement on the
northwest-trending host structures. A set of northeast
fractures, formed normal to the northwest structures,
hosts veins at Maniape and Arakompa, and at
Maniape develop tension gash veins by dextral strikeslip movement (Corbett, Leach, Thirnbeck et al.,
1994). These kinematics are consistent with
development during northeast–southwest collisionrelated compression, although a Riedel fracture
analysis was used to establish a (most recent) sinistral
sense of movement on the Kora–Irumafimpa lode
system and suggests that mineralisation here and at
Maniape formed in separate kinematic regimes
(Findlay, 2002). Earlier workers (Corbett, Leach,
Thirnbeck et al., 1994) related the Kora–Irumafimpa
and Maniape–Arakompa prospects to the separate
Oro and Taneka porphyry intrusions, respectively.
Kora–Irumafimpa lodes
Detailed geological mapping of trench and outcrop
exposures in 1991–92 delineated mineralisation
within the structural corridor between the old Kora
Mine and Irumafimpa. The corridor consists of
several parallel, northwest-trending, steeply dipping
structures with slickensided fault faces, puggy fault
gouge and breccia with a vertical exposure of more
than 800m (Corbett, Leach, Thirnbeck et al., 1994).
At lower topographic levels, the structures contain
minor breccias of phreatomagmatic affinities
exhibiting epidote-altered Elandora Porphyry
The Irumafimpa lode exposed in the adit workings showing fuchsite at left, and pyrite clasts with later stage quartz, with which
the high-grade ore is associated.
70
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
syn- and post-mineral activity,
and occur in the vicinity of
porphyry Cu–Au alteration
and mineralisation. Initial
quartz deposition from dilute
(<2wt% NaCl), rapidly cooling
(210–33oC) fluids was followed
by
sulphide
deposition.
Sulphides occur as massive
chalcopyrite lodes at Kora, but
elsewhere throughout the lodes
are typically an assemblage of
coarse crystalline pyrite with
minor
sphalerite,
galena,
chalcopyrite and tennantite.
Coarse, bright green fuchsite
and mariposite (chromium
micas) occurs in the margins of
High-grade mineralisation associated with late-stage quartz at Irumafimpa.
the lodes.
High fineness
fragments and specular haematite. Chiastolite crystals
(834–922) gold occurs as inclusions in chalcopyrite
within the phyllites are also indicative of intrusive
and coarse pyrite or associated with ferberite (Corbett,
activity, probably of Akuna or Karmantina affinity.
Leach, Thirnbeck et al., 1994; Corbett and Leach,
1998) but the bonanza gold occurs within tellurides
Recent detailed geological mapping of Kora and
associated with late stage banded and comb quartz,
Irumafimpa adit exposures, together with information
and is commonly associated with bismuth minerals.
from underground and surface drilling, has facilitated
The Kora–Irumafimpa lodes are representative of low
further definition of the mineralised structural
sulphidation
quartz–sulphide
style
Au±Cu
corridor. Several individual lodes ranging from a few
mineralisation,
which
is
interpreted
to
have
locally
centimetres to several metres in width occur within a
evolved to epithermal quartz Au–Ag style
300m wide zone as remarkably continuous features.
mineralisation where bonanza gold occurs with
The western lode (Robinson) is more copper rich
tellurides. A vertical zonation is evident from gold
while the eastern lode (Mill) is more gold rich.
associated with ferberite (Fe wolframite) and banded
opaline silica at the Kora Mine, grading to tellurides
Veins anastomose slightly and are locally cut by postwith cockade quartz at Irumafimpa. There is a lateral
mineral faults. Higher gold grades occur at the
zonation from Cu-rich in the south close to the
intersection of cross structures with main structures
inferred Oro Cu–Au porphyry source to gold-rich in
(A. Bainbridge, Highlands Pacific Ltd, pers. comm.,
the north.
2003). There is a continuity of lodes from those
worked by Rehder at Kora (1860–1900m asl) in the
southeast and the current Irumafimpa (1300m asl)
workings located about 1500m NW along the strike.
Such lateral continuations, although typical of
mesothermal veins, are surprising in the light of the
protracted deformation history of the host structures.
Mineralogical and textural variations of vein
assemblages within the lodes suggest a progression
from early mesothermal to a more epithermal style of
mineralisation. Structures exhibit protracted pre-,
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Assays from multiple bulk samples of the lodes
exposed in the adit, revealed very irregular gold
distribution, grading from a few grams to many tens
of grams per tonne in the same face. Assays from
consecutive 1m drill core intervals yielded gold grades
ranging from <1 to >100g/t, with mineralisation
contained within only a few centimetres of the
mineralised 1m interval. A. Bainbridge (Highlands
Pacific Ltd, pers comm., 2003) reported that a
resource estimate determined from uncut assays
71
Mineral Projects and Mines (cont.)
provided a close correlation with underground bulk
sampling.
galena. Higher grade gold is associated with a range
of Bi–Ag–Pb telluride and sulphide minerals.
Highlands Pacific Ltd estimates a resource for the
Kora–Irumafimpa system of 1.7Mt at 22g/t Au for
1.240 million ounces gold at a 5g/t Au cut off.
Mining is expected to commence early in 2005 with
an anticipated annual production of 115,000oz
of gold.
The Arakompa prospect displays features indicative of
an intrusive association (initial quartz–magnetite
followed by pebble dykes). The high fineness (723995) auriferous quartz–pyrite lodes locally hosting
higher gold grades in association with bismuth and
telluride minerals is possibly indicative of
quartz–sulphide–gold style mineralisation evolving to
an epithermal quartz–Au–Ag style. The lodes have
also undergone supergene enrichment.
Arakompa lodes
At Arakompa, northeast-trending steep-dipping
structures within Akuna Intrusive
Complex granodiorite are host to
quartz–sulphide lodes (Fig. 8.7). The
lodes can be traced for several hundred
metres adjacent to polished or puggy
faults, and vary in width from a few
centimetres to a maximum of 3m.
Although elevated gold grades are
recognised, Bainbridge (Highlands Pacific
Ltd, pers. comm., 2003) reported that
gold is more evenly distributed at
Arakompa than Irumafimpa. Arakompa
lies a few hundred metres north of the
outcropping Taneka porphyry Cu–Au
alteration and mineralisation.
A sequence of lode forming events is
discernible at Arakompa that is similar to
that at Kora–Irumafimpa (Corbett, Leach,
Thirnbeck et al., 1994; Corbett and
Leach, 1998). Pebble dykes, exploiting
pre-existing structures, represent a
manifestation of a porphyry system at
depth. The pebble dykes host fragments
of quartz-veined Akuna Intrusive
Complex (Rogerson et al., 1982),
and shale fragments derived from
deeper crustal levels. The pebble dykes
are
cut
by
quartz
associated
with pyrite–sericite–magnetite–epidote–
carbonate. Fluid inclusions of gangue
minerals indicate deposition from a
dilute (<2wt% NaCl), rapidly cooling
(245–315oC) hydrothermal fluid. Most
gold mineralisation occurs as inclusions
within the coarse cubic pyrite associated
with chalcopyrite, bornite, sphalerite and
72
Fig. 8.7 Plan of the Arakompa lodes.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Maniape
At Maniape (1.5km southwest of Arakompa), a
regionally significant north-northeast-trending
structure, cutting Akuna Intrusive Complex
granodiorite intruded by andesitic porphyry
(Elandora Porphyry) breaks into a series of imbricate
puggy faults over an 800m strike length
(Figs 8.8, 8.9). Quartz-sulphide lodes occur within the
imbricate structures, and also as intervening,
northeast-trending lodes interpreted to have
developed in response to dextral strike-slip movement
on the imbricate faults (Corbett, Leach, Thirnbeck et
al., 1994).
Mineralisation commenced with the initial deposition
of quartz–pyrite lodes from a dilute (<2wt% NaCl),
variable temperature (250–350oC) fluid and
associated sericite alteration. This was followed by the
deposition of Fe-poor (low temperature) sphalerite,
galena and minor chalcopyrite associated with lower
temperature quartz–chlorite–illite–carbonate wall
rock alteration (Corbett and Leach, 1998). Massive to
banded carbonate fills open spaces with minor
interbanded base metal sulphides. The carbonates
display a vertical zonation from shallow level Mn–Fe
rhodochrosite–siderite, through Mn–Mg kutnahorite
and Mg-calcite with increasing depth, to calcite and
low-Mg calcite at depth. Manganese content of the
Artisan miner working possibly supergene-enriched gold from
an oxidised lode adjacent to a polished fault surface.
A pebble dyke at Arakompa.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
73
Mineral Projects and Mines (cont.)
of the ore fluid with bicarbonate waters. In
Bonki Creek at the southern part of the
lode, epithermal quartz Au–Ag style
mineralisation (with quartz–chlorite–
illite), may have formed by mixing of ore
fluid with oxidising waters, resulting in the
deposition of higher grade but lower
fineness gold (ie. higher silver). Based on
the carbonate mineral zonation, ore fluids
are interpreted to have flowed from north
to south, leading the above mentioned
authors to suggest that Maniape was
mineralisation derived from the same
Taneka porphyry source as that at
Arakompa, but deposited in a more distal
setting.
Porphyry Cu–Au and
metamorphic associations
Fig. 8.8 Plan of the Maniape vein system.
carbonates increases laterally from north to south
along the lode. Mineral deposition is interpreted to
have occurred by mixing of a hot (>250-300oC),
saline (>6-7wt% NaCl) fluid (determined from
sphalerite fluid inclusion data) with a cool, dilute
fluid.
Gold displays a wide fineness range (551–845), with
the higher fineness gold associated with
carbonate–base metal style gold mineralisation in the
central portion of the prospect possibly due to mixing
74
Early
reconnaissance
mapping
identified potassic alteration defined
by
fracture-controlled
secondary
biotite–magnetite–silica–pyrite within
Akuna Intrusive Complex granodiorite at
Taneka near Arakompa, and secondary
copper staining on fracture faces at the Oro
Prospect. The latter prospect occurs as part
of a much larger porphyry Cu–Au
manifestation,
including
pervasive
barren
silica–alunite
grading
to
silica–sericite–dickite alteration, which
crops out at several hundred metres higher
elevation and contains enargite-bearing
vughy silica breccia within a fault (the
Headwaters Prospect). The two porphyry
occurrences at Taneka and Oro are
represented on the aeromagnetic data as an intense
high and diffuse low, respectively, as the former
contains secondary biotite–magnetite, and some parts
of the latter have been overprinted by intense
magnetite-destructive sericite–clay alteration. These
porphyry Cu–Au intrusions are interpreted source
rocks for the low sulphidation Au±Cu mineralisation.
Initial studies (Rogerson and Williamson, 1986)
suggested that Au–Cu mineralisation in the Kainantu
region is associated with Elandora Porphyry
intrusions, yet differentiates of the Akuna Intrusive
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Complex, which are often
similar in hand specimen to the
Elandora Porphyry, could also
have been source rocks for
Cu–Au mineralisation.
At Yar Tree Hill, 5km southeast
of Kainantu Village along the
Kora–Irumafimpa
structural
trend, local people work gold
from soils, sand and gravels
hosting boulders of buck quartz,
typical of metamorphic quartz in
the region. However, the quartz
at Yar Tree Hill hosts coarse
boxworks after variably oxidised
pyrite. Petrological assessment
of the buck quartz revealed that
it had been brecciated and filled
with chalcedony and coarse
pyrite (P. Ashley, University of
New England, pers. comm.,
1991). Thus, the metamorphic
quartz is interpreted to have
acted as a competent host rock
for later low sulphidation
quartz–sulphide-style
gold
mineralisation of a high fineness
(860–940).
W
E
6 m at 12.8 g/t Au
4 m at
13 g/t Au
107 g/t Ag
49 m at
4.08 g/t Au
27.8 g/t Ag
0.6 m at 44.0 g/t Au
2.0 m at 42.8 g/t Au
0
50 m
Further evidence of an intrusive
Bena Bena Formation
Carbonate veins
association is provided by
diatreme breccias with associated
Akuna Granodiorite
Fault or shear
Elandora-style
porphyry
Fig. 8.9 Cross-section through the Maniape vein system.
intrusions near Yar Tree Hill. In
the former case, alluvial gold is
exposed intrusions, and are interpreted to have
worked in the vicinity of the clay-altered, poorly
sequentially generated low sulphidation gold
exposed diatreme where boulders of vughy or residual
mineralisation of quartz–sulphide–Au±Cu type,
silica, typical of high sulphidation mineralisation,
carbonate–base metal–gold type and epithermal
occur. It remains uncertain whether the boulders are
quartz–Au–Ag type. The highest gold grades occur in
in situ or were transported to the area.
the latest event, while the corresponding gold to silver
Discussion
ratio decreases. (Corbett, Leach and Thirnbeck et al.,
1994).
In the Kainantu district, two low grade porphyry
Cu–Au occurrences are associated with partially
The Geology and Mineral Potential of
PAPUA NEW GUINEA
75
Mineral Projects and Mines (cont.)
LADOLAM (LIHIR ISLAND)
Location and status
Ladolam gold mine lies within the Luise Caldera,
located on the eastern side of Lihir Island
(lat. 152o38’E, long. 3o08’S) within the
Tabar–Lihir–Feni–Tanga chain of islands northeast of
New Ireland. The Ladolam gold mine is operated by
Rio Tinto on behalf of Lihir Gold Ltd.
Discovery history
In 1981, when the Niugini Mining – Kennecott
Exploration Australia (NMKEA) joint venture began
prospecting in Papua New Guinea, there was a
moratorium on the granting of new prospecting
licences, and the joint venture was restricted to
potential acquisitions of existing tenements. In 1983,
while NMKEA was engaged in an evaluation of the
Tabar Island group in joint venture with Nord
Resources, models of hot spring style gold
mineralisation were becoming more widely
disseminated, and published data (Wallace et al.,
1983) emphasised the alkaline geochemical
correlation within the island group. Consequently,
Peter Macnab and Ken Rehder (Niugini Mining)
visited the adjacent Lihir Island where hot springs and
red-stained cliffs are shown on the published
1:100,000 topographic map. During this inspection
they panned gold and obtained 20 chip samples over
a 450m length of pyritic coastal exposure and
boulders, which yielded assays in the 0.53–4.36g/t Au
range, averaging 1.79g/t Au (Moyle et al., 1990).
After the moratorium was lifted in November 1982,
the NMKEA joint venture acquired tenure over the
island and initiated a program of geological mapping
combined with rock chip and soil sampling. In the
Coastal Zone (Fig. 8.10), a >1ppm Au soil and rock
chip anomaly measured 450m x 250m and contained
a hand-dug trench assaying 219m at 4.58g/t Au. By
September 1983, the first drillhole (DDH L1) in the
Coastal Zone had been sunk and assayed 180m at
3.07g/t Au, including 35m at 6.52g/t Au, below the
coastal bluffs. During 1984, the first trench in the
Lienetz Zone soil anomaly was completed and
channel samples returned an assay of 3.38g/t Au over
216m. Drill testing (DDH L13) of the surface
mineralisation intersected 53m at 2.19g/t Au of oxide
mineralisation overlying 70m at 5.16g/t Au of breccia
hosted sulphide mineralisation.
76
Discovery outcrops on Lihir Island.
Luise Caldera in about 1985. The camp is by the beach, an
alunite bluff crops out at the Coastal Zone and drill roads
define the Lienetz Zone. The yet-to-be-discovered Minifie
Zone lies in the left of the photo, left of the grassy area on the
ridge between the two valleys. Note also the hot springs in a
possible ring fracture; the Kapit Zone is in the vicinity of the
hot springs to the right.
Initial feasibility studies of the geothermally hot, high
stripping ratio, and metallurgically difficult
mineralisation of the Coastal and Lienetz Zones were
not promising. However, prospecting in late 1985
evaluated a 0.1ppm Au anomaly from a hand-dug
trench located on the caldera rim, and identified the
Minifie Zone. Prospecting during 1986 defined a soil
auger and shallow Reverse Circulation anomaly,
which was drill tested (DDH L88) in November 1986
yielding 197m at 5.86g/t Au and was subsequently
deepened to provide a total intercept of 272m at
5.01g/t Au. The Minifie mineralisation possessed
higher gold grade, better metallurgy, lower geothermal
temperatures, and lower strip ratio than that of the
Coastal and Lienetz Zones, and thus the zone
underwent definition drilling during the late 1980s.
By the end of 1989, 57,000m of diamond and
19,000m of RC drilling had been completed on Lihir
Island. A new feasibility study was completed in
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
1992, a Special Mining Lease granted in 1995,
construction began in 1996 and the first gold was
poured in 1997.
The geological interpretation of the mineralisation
distribution at Lihir improved following the exposure
of mineralisation in the Minifie Pit, and this was
applied to a new resource estimate at Lienetz.
Following successful resource modelling and mining
of other geothermally active parts of the Luise
Caldera, drilling is currently focusing on the Kapit
Zone. Some results obtained in mid 2003 include
148m at 14.8g/t Au from 214m (DDH 963), 86m at
11.44g/t Au from 224m (DDH 979), and 276m at
5.39g/t Au from 200m (DDH 995).
Fig. 8.11 Lihir Island geology (after Wallace et al., 1983).
Geological setting
that has allowed melts to vent to the surface from
relatively great depths. The Luise Caldera occupies
the youngest (3-1Ma) of several Miocene to
Quaternary alkaline volcanoes developed on the island
(Wallace et al., 1983; Fig. 8.11). Intrusion-related
potassic alteration occurred in the period
0.917–0.342Ma, while the epithermal gold
mineralisation is dated at 0.336Ma, possibly
continuing to 0.1Ma, although the geothermal system
is presently active (Moyle et al., 1990; Davies and
Ballantyne, 1987).
Lihir Island is about 20km x 12km and elongates N–S
within a possible deep fracture in the oceanic plate
While the Luise Caldera trends elongate NNE, it is
cut by N–S structures associated with the deep
Fig. 8.10 Luise Caldera showing important locations and
original surface alteration.
Since April 2003, 6 MW of geothermal power has
been produced from wells in the north Lienetz area. A
further 30 MW powerstation under construction is
expected to be commissioned in early 2005. The
combined 36 MW will meet 60% of the total power
needs of the mine.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
77
Geothermal steam venting in the open pit.
remaining underlying part of the edifice. This failure
removed about 1km (compared to 400m at Mt St
Helens) from above an active porphyry Cu–Au
deposit and initiated development of the epithermal
mineralisation.
Geology and mineralisation
The process of mineralisation at Ladolam is
interpreted to have evolved over time. It is thought
that the following events took place:
Onshore, side-looking radar image, and offshore, sea-bed
topographic data, show the form of the Luise Caldera on the
eastern side of Lihir Island, and debris associated with
sideways failure of the volcanic edifice.
fractures that are interpreted to localise the
magmatism of Lihir Island. NW-trending fractures
are also evident and host the NE-dipping Minifie
mineralisation. The Luise volcano is interpreted to
have collapsed sideways in a Mt St Helens style failure
at about 0.34Ma, with debris evident from recent seabed bathymetric data. NE-dipping listric-style faults
are interpreted to have developed within the
78
Porphyry gold mineralisation associated with fracturecontrolled silica–sericite–pyrite alteration.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Milled matrix breccia typical of phreatomagmatic eruptions.
1) Porphyry-style alteration and mineralisation is
evident as overprinting events of potassic grading
to propylitic alteration and anhydrite matrix
breccias, which are in turn overprinted by
phyllic–argillic alteration. A deep geothermal
drillhole intersected anomalous copper values at a
depth of 600m. Vertically attenuated intrusions at
Lihir are known to contain gold, which occurs
as disseminations within pyrite replacing
mafic
minerals,
or
within
stockwork
silica–anhydrite–pyrite fractures. This porphyry
gold mineralisation, combined within anomalous
gold that is known to occur within the anhydrite
breccias, provides potential for additional lowgrade gold mineralisation, of unknown
metallurgical characteristics, below the level of the
planned mining of the epithermal
mineralisation.
Lithologically controlled ore from the Minifie Pit, with intense K-feldspar–pyrite
alteration, which assayed 13.1g/t Au.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
2) Collapse of the volcanic
edifice
resulted
in
a
depressurisation of the active
intrusion related, hydrothermal
system
and
promoted
development of phreatomagmatic
(diatreme) breccia pipes, which
overprint the porphyry and
anhydrite breccia ores at Minifie
and Kapit. While the clay–pyritealtered phreatomagmatic breccias
tend to be incompetent and
barren, these breccia pipes
79
Mineral Projects and Mines (cont.)
Fluidised breccia from the Coastal Zone, assaying 15g/t Au.
Free gold with anhydrite from DDH 714.
fractured the adjacent more competent host rocks
and tapped the magmatic source to the
mineralisation. Consequently, the breccia margins
and nearby structures became sites of later gold
deposition. Current drilling in the Kapit Zone has
defined a funnel-shaped breccia system of possible
phreatomagmatic association.
3) The K-feldspar–pyrite–gold event is characterised
by fluids derived from magmatic source rocks at
depth, rapidly entering the system following
80
Sulphide-rich feeder structure assaying >20g/t Au.
depressurisation. In the alkaline K-rich shoshonitic
host rocks, K-feldspar dominates over silica.
Mineralisation was localised in a variety of settings,
including: moderate to steep-dipping feeder
structures (Minifie Zone), sub-horizontal listric
fault segments (Lienetz Zone; Fig. 8.12); breccia
pipe margins where fluidised breccias grade to
marginal crackle breccias; and in permeable altered
lithologies (Corbett et al., 2001). Mineral
deposition occurred by the rapid quenching of the
ore fluid and flooding of the host rocks with finegrained arsenean pyrite, in which gold is
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
encapsulated. The original horizontal ‘boiling
zone’ ores at Lienetz are now recognised to occur
within listric faults and were deposited as a result
of fluid cooling.
4) The cooling ore fluids grade from epithermal low
sulphidation quartz–sulphide–gold to minor
carbonate–base metal–gold and epithermal
quartz–Au–Ag ores, with the latter hosting free
gold and being responsible for localised bonanza
gold grades. Although minor, the epithermal
quartz–Au–Ag ore types contribute towards the
variable metallurgy evident at Ladolam. This
evolution to epithermal ores provides potential for
additional structurally controlled epithermal
mineralisation to occur in marginal settings,
similar to the Emperor gold mine in Fiji, or
possibly Porgera.
5) Cooling of the very youthful magmatic ore system
has resulted in current geothermal activity
comprising both acid and neutral hot springs, with
extensive argillic (silica–kaolin–pyrite) and
advanced argillic (argillic with additional alunite)
alteration.
Fig. 8.12 Conceptual cross-section through the Lienetz Zone.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Resources and potential
By 1990, a resource had been estimated comprising
4.7Mt of oxide ore grading 1.96g/t Au, and 168.2Mt
of sulphide ore at 3.48g/t Au, for a total of 19.12
million ounces of gold. By the start of mining in
1996, this had been revised to a gold content of 14.6
million ounces. To the end of 2003, the mine had
produced 3.55 million ounces of gold and 20.4
million ounces remained, including 2.9 million
ounces in stockpiles.
Most recent exploration has focused on infill drilling
in the Lienetz-Minifie Zones and substantial
exploration at Kapit, which was neglected in the
original evaluation until the technology to exploit the
geothermally active pit was tested.
Current (Dec 2003) proved and probable ore reserves
at US$340/oz are 163.5Mt averaging 3.88g/t Au for
20.4 million ounces of gold.
Active thermal area Feni Project.
81
Mineral Projects and Mines (cont.)
MOROBE GOLDFIELD
Location and tenure
The Morobe Goldfield lies mostly within the Wau
Basin, a volcanotectonic feature close to the western
margin of the Eastern Fold Belt of Papua New
Guinea, near the border with the Aure Deformation
Zone. It is centred on the town of Wau (lat. 7o20’S,
long. 146o43’E), some 75km SSW of Lae (well over
100km by road). The Wau Basin is described
separately as a geological framework element of Papua
New Guinea in this publication. The mountainous
country varies from <1,000m elevation at Bulolo on
valley floor to over 2,500m at the Hidden Valley gold
project area.
The main prospects in the Goldfield were originally
(in the 1980s) covered by tenements held by Renison
Goldfields Ltd (RGC) and Rio
Tinto (formerly CRA). After
several changes of ownership,
titles are now held by wholly
owned subsidiaries of Harmony
Ltd from South Africa. Alluvial,
eluvial and saprolite gold
is currently being mined
throughout the Goldfield, using
mechanised means, by several
small private concerns and
several hundred local miners.
The Morobe Goldfield covers
several discrete gold projects
(Fig. 8.13), including:
• Hidden Valley
• Wau, encompassing Upper
Ridges and Golden Ridges
• Kerimenge
• Edie Creek
• Hamata
• Ribroaster
• Bulolo gold dredging
operation.
Production
Fig. 8.13 Geology and mineral deposits of the Wau Basin, including the Bulolo Graben.
82
An estimated 100 tonnes (3.2
million ounces) of gold had
been produced from the alluvial
workings to 1977 (Lowenstein,
1982), mostly from the Bulolo
gold
dredging
operation.
An additional 0.77 million
ounces of gold and 1.1 million
ounces of silver had been
produced from hard rock
sources to 1993 (Denwer et al.,
1995). Production continues
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
from alluvial and local saprolite sources, and the
estimated resources are cited for each deposit herein.
Discovery history
The former German administration that governed
New Guinea showed little interest in mineral
prospecting and gold production, despite the fact that
mining provided a major source of income for Papua,
which was then being administered by the Australian
State of Queensland. The German expeditions tended
to be large scientific affairs, although some Australian
prospectors were invited to explore for gold with
German prospectors in the Wau district. Arthur
Darling, an Australian, and two German prospectors,
Dammkohler and Oldorpis, identified gold at
Koranga Creek in 1910. After Darling’s death in
1921, William ‘shark eye’ Park relocated the find in
1922 and began to work the gold in secret, but by
1923 miners had begun to flock to the area.
In 1926, William Royal and Dick Glasson climbed
past the substantial waterfalls to discover the incredibly
rich alluvial gold deposits in Upper Edie Creek. By the
middle of that year, native labourers were working the
field, which to this day, supports both mechanised
operations and several hundred local miners. The
grades obtained by miners from the workings are
estimated to have exceeded 100g/t Au in places.
In those days it took carriers eight days to transport
supplies along the torturous Gadugadu Track from the
coastal town of Salamaua to Wau, even though it was
only 30km in a straight line. Allowing for feeding the
carriers en route, miners needed to make at least one
ounce of gold per day just to break even.
After Cecil Levien, the Government District Officer,
inspected Park’s Koranga claim, he resigned his post
and set about prospecting, eventually taking up
licences covering the alluvial flats at Bulolo (see
below).
Modern exploration only began in the early 1980s.
RGC purchased New Guinea Goldfields, took out EL
497 in 1983 and began detailed evaluation of the Wau
District tenements as well as undertaking regional
geological reconnaissance. At the same time, CRA
began evaluation of the region to the south of RGC’s
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Loading a Junkers aircraft at Lae.
Junkers aircraft at Lae.
holdings where the Hidden Valley deposit was
discovered.
Bulolo gold dredging
Levien reconnoitred the district and concluded that
the rich alluvial gold worked at Koranga and Edie
Creeks could have continued downriver through the
narrow Bulolo Gorge, to be deposited in the wide
alluvial flats below. He took up leases at Bulolo and
Koranga and formed Guinea Gold NL, which in 1929
began test pitting the Bulolo alluvial deposits
(Dunkin, 1950). The leases passed to Placer
Development Ltd, which was registered in Vancouver
in 1926 but consisted mainly of Australian capital. In
1930, as alluvial testing of the leases progressed with
encouraging results, Bulolo Gold Dredging Ltd
(BGD) was formed as the operating company.
The Morobe Goldfield was only accessible at that time
to hand-carried stores and even road and light rail
transport would have been prohibitively expensive in
83
Mineral Projects and Mines (cont.)
Bulolo dredge in operation.
the mountainous terrain. The breakthrough came
with the introduction in 1927 of the first air transport
from Lae to Wau by Guinea Airways Ltd, a company
owned by Guinea Gold NL. Consequently, BGD
embarked upon the innovative plan to fly dredges in
pieces into Bulolo using three German Junkers G31
aircraft. These aircraft, which began service in 1931,
carried a normal load of 5,700lb, although a
maximum load of 8,290lb is recorded, and until
destroyed by the Japanese military aircraft in 1942,
carried almost 39,417 short tons of cargo (Dunkin,
1950). Everything to enter Bulolo, including the cow
to provide milk for the managing director’s tea, arrived
in this airlift, which rivalled the Berlin blockade.
A decision to mine the alluvials was made on the basis
of a resource of 40 million cubic yards of material to
an average depth of 22ft and grade of 1.2ppm Au
equivalent (allowing for the gold fineness).
Hydroelectric power was developed, and the first
two dredges commenced
operation in 1932 and
by November 1939,
eight dredges were in
operation. Two of these
(No’s 5 and 7) were built
to dredge to 125 feet
using a 40ft boom, while
smaller dredges (No‘s
6 and 8, respectively),
worked the alluvials
below Koranga Creek and
in the Watut River.
A view of the Escarpment Fault in the vicinity of the two plumes of smoke, and hosting the
Upper Ridges open pit behind the Wau maar–diatreme shown as a depression with marginal
high points occurring as endogenous domes.
84
The operation produced
1.297 million ounces of
gold and 0.575 million
ounces of silver to 1942,
at an overall grade of 5.26
grains/cu yd, with a high
of 6.9 grains/cu yd in
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Upper Ridges Namie Breccia cut by a vein comprising
quartz–pyrite followed by carbonate–base metal sulphide and
then additional quartz.
Wau
In the earlier discussion of the Wau Basin geology, the
Escarpment Fault and adjacent Wau Maar emerge as
the dominant geological features, although their
relationship to mineralisation remains equivocal.
Fig. 8.14 Geological map of the Wau district (after Sillitoe et
al., 1984).
The Escarpment Fault occurs as a NNW (340o)trending moderate (45o) east-dipping normal fault
that can be traced for at least 10km along strike and
several hundred metres down dip (Figs 8.14, 8.15).
Exposures of the fault in the Edie Creek road display
fault gouge, silicification, pyritisation, as well as
hosting intrusions of Edie Porphyry and hydrothermal
breccias similar to those termed Namie Breccia found
at Upper Ridges.
The Wau maar–diatreme is a 2km wide crater rimmed
by Edie Porphyry domes (Sillitoe et al., 1984),
occurring in the immediate hanging wall of the
Fig. 8.15 Conceptual cross-section through the Escarpment
Fault and Wau maar–diatreme–dome complex (after
Carswell, 1990).
1933 (Dunkin, 1950). Total production by BGD to
1965, including other sources, is estimated to be 2.1
million ounces at a grade of 0.15g/t Au from 220
million cubic metres of material (Lowenstein, 1982).
Only No. 5 dredge was sunk during World War II, but
the facilities which were not destroyed by Japanese
bombing suffered in a scorched earth policy. The
operation recommenced after the war and closed in
1966.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Namie-style breccia formed at a brecciated dome margin and
characterised by fragments of Edie Porphyry, Kiandi
Metamorphics and metamorphic quartz.
85
Mineral Projects and Mines (cont.)
an additional 130,000oz from ores grading <2g/t Au.
Bedded Namie Breccia from Upper Nauti Creek.
Escarpment Fault. The diatreme fill displays acid
sulphate (cristobalite, alunite, kaolinite, pyrite)
alteration.
The Namie Breccia (see discussion of Bulolo Graben
and Wau Basin) hosts gold and silver mineralisation
and is thought to partly occur as allochthonous blocks
derived from higher levels that have collapsed along
the Escarpment Fault face (Upper Ridges) into the
maar–diatreme (Golden Peaks, Davidsons). The term
‘Davidsons Breccia’ has been used to describe Namie
Breccia which is further brecciated by later
mineralising events. Individual mineral occurrences
in the area are described below.
Namie Breccia from the Upper Ridges pit showing Edie
Porphyry fragments within a milled Kiandi Metamorphics
matrix.
At Upper Ridges, seven Namie Breccia-hosted lodes
strike NW (300–320o) and dip 30–50o SW are 150m
long and up to 15m thick. Underground mining until
1962 of ore from these lodes grading 13.8g/t Au
produced 111,000oz Au, while open-pit mining in
the 1962–89 period (from 1982 by RGC) produced
86
Petrological studies of gold mineralisation from the
Upper Ridges area indicate a two-stage process of
mineralisation (Syka, 1985; Denwer et al., 1995;
Corbett and Leach, 1998). Stage I comprises
quartz–pyrite deposition with associated sericite
alteration, followed by dark (Fe-rich, hightemperature) sphalerite and galena deposited from a
fluid in the 210–390oC range with a salinity of 125% NaCl. Gold of 613 fineness and Bi-tellurides
occur as inclusions in pyrite. Stage II consists of
rhodochrosite-dominated carbonate–base metal veins,
which give way to banded carbonate and
quartz–carbonate with minor sulphides. Low fineness
gold (468) was deposited as inclusions in sulphides
from a much cooler fluid (198–220oC).
At Golden Ridges, the Homestead Lode averaged
21.2g/t Au and produced 148,000oz Au up until
1960. The Demitrius and Davidson open cuts mined
ore grading 1.9g/t and produced a further 35,000oz.
In 1989, reserves were estimated at 138,000t grading
2.14g/t Au (Carswell, 1990).
Golden Peaks produced 232,000oz Au between 1953
and 1977 from open-cut workings of lode and
stockwork veins, and the Anderson’s Creek workings
produced 9,600oz from underground lode workings
to 1957. Recent artisanal mining is continuing within
the immediate footwall of the Escarpment Fault in the
Ribroaster workings, but historical production is
unknown.
Many aspects of the Wau mineralisation remain
unresolved. The suggestion by some workers (Sillitoe
et al., 1984) that the Upper Ridges mineralisation
occurs within tuff ring material derived from the Wau
maar–diatreme, where alteration formed at nearsurficial levels crops out, is not consistent with
probable 500–1000m depth of formation evident for
the high-temperature Stage I mineralisation. Studies
of post-mining exposures have led some workers to
suggest that the tuff ring deposits unconformably
overlie the Namie Breccia. Nevertheless, it seems
reasonable to correlate the Stage I Upper Ridges
mineralisation with the quartz–sulphide-style gold
mineralisation at Ribroaster (Syka, 1985), and so a
spatial association with the Escarpment Fault is likely.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Edie Creek
Alluvial gold has been mined continuously at Edie
Creek (Fig. 8.16) since its discovery in 1926. After
lodes were identified by the early prospectors and
miners, an access road was constructed to link the
Wau airstrip in 1927, and this became part of the
famous Bulldog Track which linked Wau to Port
Moresby during World War II.
High-grade gold won from a ‘line’ or crosscutting structure at
Edie Creek.
Trommel and jigs which form part of the plant in use by
Edie Creek Mining Ltd.
Lode mines at Edie Creek produced about , 88,000oz
of Au during 1931–62, from ores grading
between 11 and 40g/t Au (Lowenstein, 1982).
Quartz–pyrite–arsenopyrite mineralisation found at
Fig. 8.16 Geological map of Edie Creek (after Neale and
Corbett, 1997).
The Geology and Mineral Potential of
PAPUA NEW GUINEA
deeper levels in the Enterprise workings display an
increase in the amount of carbonate at shallower
levels. This, and the paragenetic sequence described
by Lowenstein from the Karuka Mine of
quartz–pyrite>base metal, sulphides>carbonate+silver
sulphosalts and banded manganese carbonate veins,
are typical of magmatic arc intrusion-related low
sulphidation style of Au–Ag mineralisation (Corbett,
2002). The association of gold with manganese wad
is typical of carbonate–base metal–gold deposits.
Most lodes at Edie Creek occur as a set of en echelon
veins within a 3km long NW-trending structural
corridor, and are most mineralised in the vicinity of an
Edie Porphyry intrusion. In the north-western part of
the ‘corridor’ they are overlain by tuff ring breccias
associated with the Nauti diatreme interpreted to lie
Trench at the old Edie Creek lode workings. The trench on
the left follows the old lode, while the trench on the right cut
across strike yielded 7m at 42g/t Au from a bulk sample.
87
Mineral Projects and Mines (cont.)
west of the ‘corridor’ (Neale and Corbett, 1997).
Mineralisation also occurs outside the main lodes and
is mined as ‘lines’, which vary from auriferous
manganese oxide-lined fractures to gold within quartz
veins. The potential for open-cut mining of some of
the mineralisation at Edie Creek warrants further
consideration.
Edie Creek Mining Ltd, a joint venture between
Melanesian Resources Ltd and local landowners, is
currently mining a mix of alluvial and saprolite
material using an excavator, bulldozer and treating the
material with a trommel, back-end jigs and
Knudsen bowls. Gold production is approximately
3,000oz/year. Recent work by Edie Creek Mining has
identified gold mineralisation in stockworks (7m at
42g/t Au from a bulk sample) adjacent to Edie No 1
Lode. Other stockworks within Edie Porphyry
intrusives await sampling.
Kerimenge
The Kerimenge Prospect was discovered in 1983
during regional reconnaissance prospecting by RGC
staff. A -80 mesh stream sediment sample collected in
Kerimenge Creek well downstream from the prospect
yielded 0.27g/t Au. Follow-up rock chip sampling of
a silicified pyritic dacite porphyry outcrop from
within a 300m thick sill of competent Edie Porphyry
yielded 4.7g/t Au. The gold mineralisation at
Kerimenge is localised within sheeted veins, within
the north to N-NE trending Kerimenge Fault and its
hanging wall, and adjacent to a diatreme - breccia pipe
The Kerimenge Prospect, discernible as vegetation regrowth.
The Kerimenge Fault lies in the valley to the left, while the
skyline is dominated by the diatreme breccia on the left and
Lemenge Prospect on the right. The camp is in the lower
portion of the photo.
88
Fig. 8.17 Geological map of Kerimenge showing location of
cross-sections used in the conceptual cross-section (after
Denwer, 1997).
(Figs 8.17, 8.18). The mineralisation continues from
Kerimenge north to the Lemenge prospect area, at
about 300m higher elevation, where it is hosted by
NW trending, NE dipping structures. Exploration of
Kerimenge was discontinued through the 1990s
because the fine grained pyritic ore posed substantial
metallurgical problems. However, evaluation of the
adjacent Lemenge Prospect continued through the
1990s (Akiro, 1986; Syka and Bloom, 1990; Hutton
et al., 1990; Denwer, 1997; Corbett and Leach,
1998). The current resource at Kerimenge-Lemenge
is estimated at 15.1Mt at 1.6g/t Au containing 0.78
million ounces of gold.
The mineralisation at Kerimenge has a well-developed
vertical zonation and paragenetic sequence typical of
magmatic arc intrusion-related low sulphidation gold
deposits. The gold mineralisation changes from early
fine-grained quartz–sulphide-style mineralisation
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
tennantite and chalcopyrite, which are interpreted to
have been deposited by the mixing of a hot dilute ore
fluid with cool (165oC) CO2-rich waters.
Hamata
Hamata, which crops out at the lowest altitude
in the Morobe Goldfield (1,900m - 2,000m), displays
a transition from alteration associated with intrusive
related mineralisation to quartz–sulphide–gold-style
mineralisation, with a later post-mineral
carbonate–base metal overprint.
The outcropping lodes at Hamata (Fig. 8.19) were
identified from the follow up of reconnaissance -80
mesh stream sediment samples collected in July 1987
that yielded grades of 28.5 and 5.5g/t Au (Denwer
and Mowat, 1997; Denwer et al., 1995; Wells and
Young, 1991; Corbett and Leach, 1998).
The current resource is estimated as 3.9Mt at 3.9g/t
Au containing 0.41 million ounces of gold.
Fig. 8.18 Conceptual cross-section through Kerimenge (after
Hutton et al., 1990).
hosting refractory gold (occurring at deeper levels), to
younger non-refractory gold deposited at shallower
crustal levels (Lemenge). Two stages of ore
formation can be recognised.
The host rock at Hamata is the Morobe Granodiorite
and two phases of andesitic dykes. Dacitic porphyry
occurs as a large body in the footwall of the
mineralised lodes and as small dykes throughout the
Stage I mineralisation occurs at depth as
quartz–arsenopyrite–pyrite–marcasite
vein/breccias overprinting earlier zoned
biotite–potassic
propylitic
alteration.
Quartz–sulphide veins were deposited from
relatively dilute (<3.3 wt% NaCl) fluids in
the 145–240oC range. Refractory gold is
encapsulated within the lattice of early fine
pyrite and arsenopyrite that were deposited
from a rapidly cooling fluid.
The
quartz–sulphide mineralisation at depth
passes to carbonate–base metal veins manifest
as thick, barren, banded manganocarbonate
lodes that occur at higher levels and are
believed to have formed late in this stage.
Stage II quartz–manganocarbonate–sulphide
breccia fill contains non-refractory high
fineness (837) gold associated with hessite,
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Fig. 8.19 Geological map of Hamata lodes (after Denwer and Mowat,
1997).
89
Mineral Projects and Mines (cont.)
View of Hamata located in a hill within the Upper Watut
Valley.
deposit. Hamata prospect appears to occur in the
vicinity of a WNW splay in the Upper Watut Fault,
which can be traced along strike to the Hidden Valley
prospect (see below). The lodes at Hamata dip about
45o SE and are constrained between 45o NW-dipping
structures which overlie a basal fault that dips more
shallowly to the NW. One structural interpretation is
that during extension on the Upper Watut Fault,
represented by the NW splay, the steep faults have
taken on a reverse component and the lodes developed
as intervening tension gash features.
Morobe Granodiorite at Hamata with porphyry-style
alteration characterised by magnetite–specular haematite–Kfeldspar.
haematite, and pyrite veins with K-feldspar selvages
deposited from a hot (270–340oC) saline (up to
35wt% NaCl) fluid.
The Stage II mineralising event is represented by
coarse pyrite lodes, with lesser chalcopyrite and minor
tetradymite and ferberite, that contain native gold of
average 911 fineness and are associated with Bitellurides (similar to the Arakompa lode at Kainantu).
Stage III mineralisation occurs as local carbonate–base
metal overprint, followed by and extending into Stage
IV quartz–barite–arsenopyrite deposition.
Hidden Valley
Hamata quartz–pyrite lode.
Denwer and Mowat (1997) describe the lodes as
varying in thickness from tens of centimetres
(30–50cm for Eastern Reef ) to several metres (3–4m
for Masi and Lower Reefs). The mineralisation at
Hamata appears to have developed by a polyphase
process as outlined below (Corbett and Leach, 1998).
Stage I pre-mineral porphyry-related potassic
alteration is characterised by magnetite, specular
90
Hidden Valley is the highest (2,600m) known
prospect in the Morobe Goldfield, and lies on the
southern continuation of the Upper Watut Fault from
Hamata.
Four-wheel-drive access is currently
provided from near the prospect area to Wau via Edie
Creek along the upgraded Bulldog Track. The climate
is described by most workers as cold, wet and
uncomfortable.
When CRA began to prospect the area, alluvial gold
was known to have been worked at Hidden Valley by
W.H. Chapman in 1928, and local miners in the
1960s (Lowenstein, 1982). Regional reconnaissance
exploration by CRA staff in April 1984 identified
abundant pyritic granodiorite float in the Upper
Watut River and a -80 mesh stream sediment sample
returned 8.4g/t Au, with visible gold noted in the pan
concentrate sample collected from the same location
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
(Nelson et al., 1990). Follow-up channel chip
sampling of a mineralised landslip exposure yielded
100m at 3g/t Au and 45g/t Ag.
Ownership has changed hands frequently since 1997
from CRA, to Australian Goldfields, then Aurora
Gold Ltd, and currently Harmony Ltd. The latter
two companies compiled geological interpretations
based on detailed structural mapping and the use of
orientated drillcore (Hoppe and Korowa, 2001), to
produce the most recent economic evaluation that was
used for the current proposal for development. The
broad approach on current perceptions of resources is
to mine Hamata, then the oxide and transition ores at
Hidden Valley, then the Hidden Valley main ore zone,
and lastly possibly access Kaveroi from underground.
The Hidden Valley prospect lies mostly within the
Morobe Granodiorite close to the contact with the
Kaindi Metamorphics, which here display effects of
both regional and contact metamorphism (Figs 8.20,
8.21). Although andesite dykes, associated with the
Morobe Granodiorite by some workers (Denwer and
Mowat, 1997), and Edie Porphyry occur within the
mineralised zones, most workers associate the
mineralisation with the Edie Porphyry (Hoppe and
Korowa, 2001) due to adularia age dates of 4.0-4.2Ma
(Nelson et al., 1990).
Fig. 8.20 Hidden Valley geological relationships (after Hoppe
and Korowa, 2001).
The
gold
mineralisation
occurs
within
carbonate±adularia±quartz±sulphide fracture veins
which are generally <100mm wide, but occur
in vein packages up to several metres wide.
The paragenetic sequence discernible from hand
specimen and supported by petrology is:
adularia>pyrite> Fe-rich (dark, high-temperature)
sphalerite–galena>carbonate> quartz (commonly as
chalcedony). The carbonate comprises calcite,
rhodochrosite and kutnahorite. The gold ranges in
fineness from 600–750 and occurs as electrum and
rare gold tellurides within pyrite and base metal
sulphides.
Stereoplot data (Hoppe and Korowa, 2001)
demonstrated that while veins exposed in a road
cutting through the deposit display an almost random
distribution of orientations, orientated drillcore has
identified two different vein orientations within each
of the two ore zones as:
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Hidden Valley auriferous carbonate–base metal vein.
• In the NE-dipping Hidden Valley Zone, veins
form a sheeted stockwork array dipping generally
91
Mineral Projects and Mines (cont.)
extensional tectonic feature
(Bulolo Graben – Wau Basin)
which hosts a significant number
of gold mines and occurrences that
often display a spatial association
with the Pliocene Edie Porphyry.
Regionally continuous extensional
structures and the numerous faults
and structures that host the
mineralisation in the goldfield
have a history of re-activation.
Extrusive
and
intrusive
phreatomagmatic breccias also
appear to pre-date and localise
gold
mineralisation
(Wau,
Kerimenge).
The extensive vertical relief exposes
varying styles of intrusion-related
low sulphidation epithermal
gold mineralisation. At the deepest
Fig. 8.21 Conceptual cross-section through Hidden Valley mineralisation (after
levels
(Hamata),
massive
Hoppe and Korowa, 2001).
quartz–sulphide–gold
style
mineralisation is associated with
NE, constrained between the Hidden Valley and
magnetite–K-feldspar
porphyry style alteration, at
Upper Boundary Faults.
mid-levels (Wau and Hidden Valley) carbonate–base
metal–gold mineralisation occurs, while bonanza• In the Kaveroi Creek Zone, most veins dip steeply
gold-grade epithermal quartz–Au–Ag mineralisation
to the WSW, and are constrained between the
(Edie Creek) crops out at the highest topographic
steep ENE-dipping Darby and Levien Faults.
level. Most deposits display overprinting relationships
Thus, mineralisation may be interpreted to be
(Edie Creek), while the Kerimenge prospect hosts
associated with a listric Upper Watut Fault and related
three low sulphidation mineralisation styles types
hanging wall splays. In the Kaveroi Zone, the
variably telescoped over 250m vertical extent.
abundance of carbonate over base metal sulphides,
Most gold production to date has come from the rich
which feature yellow Fe-poor sphalerite, is indicative
alluvial gold deposits in the Bulolo Valley, which have
of a lower temperature and higher crustal level of
great historical significance to Papua New Guinea and
formation. Hidden Valley demonstrates many
represented the start of Placer Dome. Although smallfeatures typical of the carbonate–base metal style of
scale and artisan mining is currently being undertaken
intrusion-related gold deposits.
throughout the goldfield, the major economic
advance for the area and PNG will come from the
The current measured, indicated and inferred resource
development of the Hidden Valley and Hamata
at Hidden Valley is estimated at 35.27Mt at 3.13g/t
Projects. Furthermore, many anomalies and sites of
Au and 47.1g/t Ag in two distinct structural zones
artisan mining within the Morobe Goldfield warrant
(Hidden Valley and Kaveroi Creek), representing a
continued prospecting and could contribute to
resource of over 3.5 million ounces of gold.
significant new discoveries.
Conclusions
The Morobe Goldfield encompasses a structurally
complex area that has been described as a major
92
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
MT BINI
Location and status
The Mt Bini porphyry Cu–Au prospect lies 50km
ENE of Port Moresby close to the historic Kokoda
Trail (lat. 9o18’S, long. 147o35’E), at an altitude of
about 1,200m, in steep tropical rainforest covered
terrain.
Discovery history
BHP identified and drill tested the Mt Bini Prospect
relatively late in the discovery history of porphyry
Cu–Au mineralisation in Papua New Guinea. In May
1992, during a helicopter-supported stream sediment
sampling at a site in Ofi Creek, 2km downstream
from Mt Bini, an anomalous pan concentrate sample
(157ppm Au) and a pyritic silicified float sample were
collected (20.7g/t Au, 463g/t Ag, 0.15% Cu and
0.6% Pb) (Dugmore et al., 1996; Dugmore and
Leaman, 1998). Effective delineation of the
prospective area quickly followed through further
drainage sampling and a subsequent ridge and spur
soil sampling survey that resulted in the definition of
a 2,000m x 200m >0.2g/t Au anomaly, largely
encompassing a 650m x 350m zone at >150ppm Cu
and >18ppm Mo, coincident with a mineralised stock.
A peripheral lead anomaly,
associated with marginal latestage low sulphidation epithermal
carbonate–base metal veins
extends for 1,800m x 1,300m.
Metamorphic Complex (Fig. 8.22), which, although
of Cretaceous to Eocene age, were subjected to
deformation and greenschist facies metamorphism
during the Middle Miocene (Rogerson and McKee,
1990). BHP geologists suggested that the Mt Bini
porphyry system occurs within a 15–20km wide
extensional zone, localised by NNE regional
structures, that hosts subaerial volcanic rocks and
high-level porphyry stocks (Leaman, 1996). The
extensional zone parallels transfer structures, which,
localise porphyry Cu–Au and epithermal gold
deposits elsewhere in Papua New Guinea (Corbett,
1994).
Mt Bini is on the SE margin of the 10km x 4km
Pliocene Bavu Igneous Complex, which may have
acted as a coeval intrusive source for the Astroblabe
Agglomerate to the south, while vesicular basalt,
andesite lava and Pleistocene eruptive centres are
described from further north (Dugmore and Leaman,
1998). The Tolukuma Au–Ag vein system lies within
the Mt Cameron Volcanic Complex to the west.
The Bavu Igneous Complex contains many intrusions
such as the Track Diorite, an unaltered pyroxene
diorite that crops out 500m north of the Bini stock.
Geological mapping defined the
style and extent of porphyry
Cu–Au mineralisation leading to
initial drill testing. The first
drillhole (BDD 001) intersected
235m assaying 0.31% Cu and
0.47g/t Au from 104m. This was
followed by an additional six
diamond-drillholes
totalling
2,082m
in
two
drilling
campaigns
(Dugmore
and
Leaman, 1998).
Geological setting
Mt Bini, in the Eastern Orogen
of Papua New Guinea, lies with
in Kagi Metamorphic country
rocks of the Owen Stanley
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Fig. 8.22 Geology of Mt Bini area (after Dugmore and Leaman, 1998).
93
Mineral Projects and Mines (cont.)
Fig. 8.23 Cross-section through Mt Bini (after Dugmore and Leaman, 1998).
Two phases of potassic alteration
have been described by Leaman
(1996) and Dugmore and Leaman
(1998). They are an initial phase of
pervasive and fracture fill secondary
biotite and a later phase of
orthoclase, which is also developed
as margins to later crosscutting
quartz–sulphide±magnetite veinlets,
typical of A or M veins described in
porphyry copper literature (Sillitoe,
2000).
Primary
magnetite
contributes towards an anomalous
aeromagnetic signature for these
intrusions. Chalcopyrite dominates
over
bornite,
mostly
as
disseminations and on fractures, and
within the quartz–magnetite veins,
while gold occurs as inclusions
within chalcopyrite. Minor amounts
of anhydrite have been recognised
but its relationship to other
prograde alteration phases is
inadequately described in the
literature (Dugmore and Leaman,
1998).
Geology and mineralisation
Slate and phyllite country rocks are intruded
by the Bini porphyry, a 650m x 275m Pliocene
(4.42±0.04Ma)
potassium-rich
calc-alkaline
composite intrusive stock emplaced at the intersection
of NNE and ENE structures (Dugmore and Leaman,
1998). BHP geologists described the intrusions as:
P1 - a porphyritic quartz diorite, containing
plagioclase phenocrysts in a microcrystalline
groundmass of quartz, orthoclase and biotite; and
P2 - a quartz diorite characterised by plagioclase–
biotite–clinoamphibole phenocrysts, both of which
are cut by later dykes of similar composition but with
weaker alteration and mineralisation. A barren dyke
cuts mineralisation and, while the extent of
hydrothermal alteration clearly declines in the
younger intrusions, overall relationships may not be
fully resolved (Fig. 8.23).
94
Extensive retrograde phyllic alteration overprints the
prograde potassic alteration, mainly as a fracturecontrolled sericite–quartz–pyrite assemblage, and is
best developed at the stock margins and extending
into the host phyllite. Later stage retrograde alteration
also includes chlorite–clay (illite) alteration and
replacement of magnetite by hematite (and associated
demagnetisation). This latter assemblage overprints
the sericite–quartz–pyrite assemblage.
Cu–Au mineralisation is concentrated about the
margin of the stock, both within the stock and
also the host phyllite.
While fracture and
disseminated
sulphides
(pyrite–chalcopyrite)
are
associated
with
potassic
alteration,
later
quartz
stockwork
veins
containing
pyrite–chalcopyrite–molybdenite–magnetite are cut
by orthoclase–biotite–quartz±chlortite±carbonate
veins, which are well developed within the marginal
phyllic alteration. These quartz±chlortite±carbonate
veins contain gold as inclusions within chalcopyrite
(Dugmore and Leaman, 1998). Some workers
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
(Corbett and Leach, 1998) have suggested that ore
deposition (pyrite–chalcopyrite–gold) is enhanced by
the mixing of mineralised fluids with collapsing low
pH fluids responsible for phyllic alteration.
lower 50m. Copper carbonates have been identified
on rock exposures in watercourses and extending
down fractures in drill core.
Discussion
An event of propylitic alteration is described as
overprinting the potassic and phyllic alteration
(Dugmore and Leaman, 1998). This alteration grades
from inner propylitic alteration characterised by
magnetite–chlorite–tremolite–actinolite–quartz–sulp
hide, close to the centre of the system,
outward
as
chlorite
replacing
earlier
secondary
biotite,
to
more
marginal
quartz–carbonate–epidote±magnetite±chalcopyrite
veins
with
minor
tennantite–tetrahedrite.
Galena–sphalerite veinlets immediately adjacent to
the Mt Bini intrusions and more distal
galena–sphalerite–carbonate veins are associated with
propylitic alteration. Although a post-mineral
intrusion is shown in the published literature cutting
alteration and mineralisation, no causal association
has yet been made with the propylitic alteration and
late mineralisation. More work is needed to resolve
the sequence of intrusive related alteration and
mineralisation.
Epithermal veins, having widths up to one metre wide
and traceable for 1,400m, overprint the porphyry
Cu–Au mineralisation. The veins are localised along
the ENE–NE structures, and are characterised by
crustiform banded quartz, stibnite, and rhodochrosite,
that have yielded assays of 8m grading 19g/t Ag.
Chalcedony veins and stockworks west of the Bini
stock have returned assay values over 20m of 0.56g/t
Au (Dugmore and Leaman, 1998). The later
marginal epithermal mineralisation is typical of low
sulphidation epithermal carbonate–base metal gold
mineralisation that commonly occurs marginal to
porphyry Cu–Au intrusions.
Supergene covellite and chalcocite replaces and rims
chalcopyrite (Dugmore and Leaman, 1998).
Supergene enrichment results in an increase in copper
and gold with depth from 500ppm Cu, 0.2g/t Au in
the upper 15–20m to 0.18% Cu, 0.5g/t Au in the
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Although the Mt Bini Prospect displays many features
typical of SW Pacific rim magmatic arc Cu–Au
porphyry deposits, results of the early exploration
deemed it to have been of insufficient size to maintain
the interest of BHP. It was identified during a classical
exploration program ranging from initial stream
sediment geochemistry, to ridge and spur soil
sampling and geological mapping, culminating in drill
testing. While two programs of diamond drilling,
amounting to seven diamond-drillholes recovering
2,421m of drill core, might have been sufficient to
delineate the size of the composite intrusion and
nature of porphyry mineralisation, the opportunity
still exists to identify additional higher grade Cu–Au
mineralisation. As is typical of many such intrusions,
the best Cu–Au occurs in association with stockwork
quartz veins and phyllic alteration about the margin of
a cylindrical composite stock.
Mt Bini displays a history of overprinting alteration
events possibly associated with the multiple
emplacement of intrusions. The BHP geologists
(Dugmore and Leaman, 1998) record a zoned
propylitic alteration overprinting the main event of
alteration and mineralisation, and this might be
associated with an as yet unrecognised intrusion
which may locally provide another phase of ore fluids
and deposition. Cu–Au–Ag mineralisation typical of
marginal settings to porphyry intrusions is associated
with this alteration. Thus, further exploration of the
Mt Bini system may yield either additional porphyry
Cu–Au mineralisation or marginal epithermal Au–Ag
mineralisation.
Resources and potential
In citing a poorly defined resource estimate of 85Mt
at 0.4% Cu and 0.6g/t Au, BHP geologists stressed
that the porphyry Cu–Au system remained open at
depth (Dugmore and Leaman, 1998).
95
Mineral Projects and Mines (cont.)
MT KARE
Location and status
The Mt Kare prospect in Enga Province, lying about
17km SW of the Porgera Mine (lat. 5o30’S, long.
142o58’E) at an altitude of 3000m is one of the
highest mineral prospects in Papua New Guinea. It is
accessible only by foot track or helicopter, although
the Porgera access road passes within 5km of the
Exploration Licence (EL 1093) boundary.
Exploration of the Mt Kare Prospect is currently being
carried out by Madison Enterprises Corp. of Canada
who has reserved a future 10% of the project for the
legitimate landowners.
began regional helicopter supported first-pass
reconnaissance exploration. Sampling in March 1986
of the Ere River, which drains to the SW of Mt Kare,
resulted in the identification of two gold anomalies
(Brunker and Caithness, 1990). Sampling 7km
downstream from the current main prospect area
revealed an anomalous sample of 0.6ppm Au in -80
mesh stream sediment, but with no other anomalous
elements, a pan concentrate sample that assayed 8ppm
Au, and a rock float sample assaying 1.45g/t Au.
A site 4km downstream from the main prospect
yielded only a 3.5ppm Au pan concentrate anomaly in
which one speck of gold was noted. No anomalies
were identified in sampling of the drainage to the NE
of Mt Kare. During 1987, more detailed stream
sediment, soil, and rock sampling continued to
evaluate an area of about 1sq km, and resulted in the
recognition of silicified pyritic sediments as an
indicator to a hard-rock source.
By the time CRA staff returned from Christmas break
in February 1988, local landowners, who were avidly
following the entire exercise, had found alluvial gold
and a major gold rush had begun. During 1988, more
than 5,000 miners are estimated to have extracted
0.25 million ounces from Mt Kare (Welsh, 1990), but
the Mt Kare gold rush probably produced in the order
of 1 million ounces up to 1991 (Ryan, 1991), despite
a rapid decline in the number of miners after the
initial 1988 rush. At the height of activity, Mt Kare
displayed all the features of a modern-day gold rush,
with several helicopters employed to ferry goods in
and successful miners out, and many spectacular
nuggets entered collections worldwide. After working
the original rich alluvial deposits, miners progressed to
the eluvial gold, and have most recently begun smallscale hard-rock mining on a very minor scale
(one man), although the Department of Mining is
monitoring the situation very closely.
View of Mt Kare taken in 1989 showing the hard rock
prospect to the left and creek worked for much of the alluvial
gold, with blue sails (tents) visible.
Discovery history
Following an inspection of exploration at Porgera in
the early 1980s, CRA took up PA 591 ‘Mount York’
in the hitherto poorly prospected Mt Kare region, and
96
CRA continued exploration during the gold rush, and
reached an agreement with the landowners to initiate
a helicopter-supported mechanical alluvial gold
mining operation. However, in 1991, a group of
armed men entered the CRA operation at night and
burnt the camp and other facilities, including the core
shed. Not surprisingly, CRA ceased their activities,
and by 1993 CRA had relinquished its rights to the
exploration and alluvial mining tenements.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Hard-rock exploration by CRA included airborne
magnetics, soil geochemistry, rock geochemistry on
trench samples, and geological mapping culminating
in the drilling of 30 diamond-drillholes. Significant
aspects to emerge during this program were the
extensive manganese oxide deposits formed by
weathering
of
rhodochrosite
within
the
carbonate–base metal mineralisation, and the presence
of quartz–roscoelite in outcrop assaying 197g/t Au. The
best result from the CRA drilling program was 38m at
11.2g/t Au from a downhole depth of 44.0m in DDH
88-15.
Several new companies simultaneously applied for the
relinquished area. Competition for the Exploration
Licence over Mt Kare was resolved by a ballot won by
Carpenters Pacific Resources. However, Ramsgate
Resources laid claim as a result of equity in the former
alluvial licences. The matter was finally settled in the
Supreme Court of Papua New Guinea and the Matu
Mining joint venture was formed (2/3 Carpenters and
1/3 Ramsgate). Matu Mining funded exploration
from January to June 1996, after which Madison
entered the joint venture with a right to earn 65% by
spending $US8 million over five years.
Fig. 8.24 Geological map of the Mt Kare region.
scattered spot aeromagnetic anomalies indicative of
shallow mafic intrusions (Laudrum, 1997). As at
Porgera, fault-controlled fluidised breccias with a
milled matrix have been emplaced into a NNE
trending structure and are overprinted by
mineralisation. The ductility of the host rocks at
By June 1997, Matu and Madison had drilled an
additional 37 diamond-drillholes in an area of a little
over 1sq km.
Geology and mineralisation
Mt Kare lies 17km SW of Porgera on the NNEtrending lineament, which Corbett (pers comm.)
refers to as the Porgera Transfer Structure (PTS). The
lineament may represent a major crustal break, which
is interpreted by some workers to project from the
basement, possibly initiated in the Palaeozoic, through
into the younger cover rocks (Fig. 8.24). Host rocks
to mineralisation comprise a sedimentary sequence of
mudstone, calcareous mudstone, sandstone and
limestone, which, at Mt Kare, have been folded with
an axis parallel to the abovementioned lineament.
The Darai Limestone immediately overlies the
sediments.
Thrust deformation is well developed throughout the
region. Intrusive stocks and dykes responsible for
induration of the sediments are similar to those at
Porgera (Richards and Ledlie, 1993), and result in
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Grey, indurated shale-hosted
pyrite–sphalerite–galena–carbonate vein.
97
Mineral Projects and Mines (cont.)
Mt Kare governs their ability to fracture and thus their
amenability to enable mineralisation localisation. The
competent altered sediments and intrusions make the
best hosts due to their brittleness, followed by the
black shale and sandstone, with the least receptive unit
being the incompetent fissile brown mudstone.
Intrusion-related low sulphidation gold mineralisation
at Mt Kare has been divided into three stages (Corbett
and Leach, 1998). Stage I consists of pyrite veins
similar to the Porgera B veins with variable quartz and
carbonate contents, halos of sericite–illite alteration,
and correspond closely to quartz–sulphide–gold-style
mineralisation.
Stage II carbonate–base metal–gold mineralisation
cuts the earlier formed pyrite veins and comprises a
pyrite–galena–sphalerite assemblage followed by
minor chalcopyrite, tennantite and arsenopyrite
mineralisation. The sphalerite varies from early red to
later colourless Fe-poor variety, and is indicative of a
much lower temperature of formation than the
mineralisation at Porgera, and therefore possibly at a
higher crustal setting. Carbonate displays a zonation
similar to other Pacific rim carbonate–base metal
deposits from upper to lower crustal levels as:
siderite>rhodochrosite>kutnahorite>dolomite>calcite.
The gold at Mt Kare is present as inclusions in pyrite
and sphalerite, and displays a highly variable fineness
ranging from 720-930. The rhodochrosite within the
carbonate–base metal mineralisation weathers to black
manganese oxide that defines a series of NE-trending
zones parallel to the main structural trend, but a
thrust relationship cannot be ruled out. Drillhole M
97-17 that was targeted to test one such manganese
oxide zone yielded 100.5m at 7.63g/t Au.
Crystalline gold extracted from a manganese-oxide-filled
structure.
Stage III epithermal gold mineralisation at Mt Kare is
typified by free gold with quartz or roscoelite and is
noted for its bonanza grades. The highest grade
assay to date from drillcore intersecting
quartz–roscoelite–gold mineralisation has been 4.5m
at 2,141g/t Au in drillhole M 97-5, and was enveloped
by a wider intercept of carbonate–base metal
mineralisation assaying 308.4g/t Au over 31.8m.
Quartz–roscoelite mineralisation has been identified
in a number of drillholes at Mt Kare (Laudrum, 1997)
and tends to be intimately associated with the earlier
carbonate–base metal event, rather than within
discrete structures as has been recognised at Porgera,
and so may represent an evolving hydrothermal
system at Mt Kare. The gold within the
quartz–roscoelite breccias at Mt Kare occurs as
electrum (fineness 400–870) with a wide range of
silver sulphosalts, silver sulphides and tellurides, and
may also be intergrown with colourless (lowtemperature) sphalerite and Ag–Sb-rich chalcopyrite
(Corbett and Leach, 1998). These workers also
suggested that the mineral deposition at Mt Kare
occurred during the rapid cooling of an ore fluid from
temperatures in the order of 270oC to as low as
120oC, under oxidising conditions as indicated by the
deposition of marcasite. Late fractures that host
native gold in association with specular haematite and
framboidal pyrite in the presence of opal, may be
further indicative of a rapidly quenched ore fluid in
oxidising conditions.
Discussion
Quartz–gold breccia from the high-grade intersection in
DDH M 97-5.
98
Crystalline gold associated with manganese wad is
commonly found as rounded plates in the eluvial
deposits, and is also abundant within the alluvial
deposits several hundred metres downstream from the
hard-rock source. The consistent high fineness (800)
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The extent of artisan workings is apparent in the late 1990s by an earlier CRA drill casing protruding from the ground.
of the crystalline gold within the alluvial deposits
contrasts with the highly variable fineness in the hardrock source, suggesting possible leaching of silver, a
degree of recrystallisation, or chemical transport and
deposition. The high fineness of the gold in alluvial
occurrences at Mt Kare has led some workers to
suggest that much of the gold may have travelled
chemically, rather than mechanically, several hundred
metres from the variable fineness primary source.
Corbett (pers comm., 2003) speculated that Mt Kare
represents the thrust-off top of the mineralisation at
Porgera, citing the following evidence: The two
deposits are separated by about 17km along a major
lineament. This concept is supported by similarity, in
separation distance and direction between the
deposits, to the 15km of shortening apparent from the
reconstruction of marker units in cross-section data
(Hill, 1991; Standing, 1994) and the similarity of age
dates at 6.0±0.3Ma for Porgera and 6.0±0.1 for Mt
Kare (Richards and Ledlie, 1993). The thrust at the
base of the Mt Kare Main Zone mineralisation and
the Western Boundary Fault at Porgera each separate
overlying mineralisation from underlying barren fissile
brown shale. The contact between shale and Darai
Limestone lies close to the Mt Kare Prospect,
suggesting it could have formed in a higher portion of
the extensively thrusted stratigraphy than occurs at
Porgera. Magnetic anomalies at Mt Kare are indicative
of discrete shallow mafic intrusions, not a large
intrusion complex like that at Porgera.
To counter the thrust model, the mineralised
manganese oxide zones at Mt Kare parallel the transfer
The Geology and Mineral Potential of
PAPUA NEW GUINEA
structures, and so have undergone rotation that might
be expected during thrusting. Furthermore, it is
fortuitous that Mt Kare lies within the same lineament
as Porgera. While gold mineralisation at Mt Kare is
similar to Porgera, these two deposits are typical of
other Pacific rim alkaline-intrusion-related
quartz–sulphide–carbonate–base metal epithermal
quartz–gold deposits (Corbett and Leach, 1998), and
so could easily display many similarities without
having formed as the same deposit. However, if
Porgera underwent thrusting between Stages I and II,
the higher temperature Stage I quartz–sulphide and
carbonate–base metal–gold mineralisation at Porgera
contrasts with the lower temperature for the same
event at Mt Kare. However, the presence (or lack of )
mineralisation may be due to the receptive nature of
the host as discussed previously.
It is still interesting to follow Corbett’s speculation
that the epithermal event at Mt Kare corresponds to
the missing lower temperature portion of Porgera
Stage I.
Resources and potential
The most recent (March 2004) resource estimate
provided by Madison Enterprises Corp. suggests that
Mt Kare contains 25.5 million tonnes grading 2.2g/t
Au and 29g/t Ag using a 1g/t Au cut off and cutting
high grade assays to 30g/t Au. This represents 1.8
million ounces Au and 23.9 million ounces Ag. More
extensive artisan gold workings at Mt Kare over recent
years suggest considerable potential remains to
identify additional Au mineralisation at Mt Kare.
99
Mineral Projects and Mines (cont.)
OK TEDI
Location and ownership
The Ok Tedi porphyry Cu–Au deposit lies in the Star
Mountains close to the border with West Papua and is
centered on Mt Fubilan (lat.5o12’S, long.141o8’E) at
an altitude that was originally 2,053m asl (Fig. 8.25).
The mill at Folomian is at an altitude of 1,630m and
the town of Tabubil is at an elevation of 520m.
Tabubil is connected by 17km of road to the mine and
137km of road to the river port at Kiunga (Rush and
Seegers, 1990). Tabubil is accessible from other
centres, including Port Moresby, by fixed-wing
aircraft. Copper concentrate is pumped via pipeline
from Folomian to Kiunga Wharf, ferried by barges
from the wharf, down the Fly River, to a large silo (the
vessel Erawan) for temporary storage prior to loading
into international vessels for export.
Current ownership is shared between PNG
Sustainable Development Program Ltd (52%), the
Papua New Guinea Government (30%) and Inmet
Mining Corp of Canada (18%). PNG Sustainable
Development Program Ltd is a trust company
Fig. 8.25 Ok Tedi location map.
100
registered in Singapore to hold the 52% equity that
was ‘gifted’ by BHP Billiton when it exited the
Ok Tedi Mine on 8th February 2002.
Exploration and development history
Mineralisation in the Ok Tedi region was first
reported in 1875 when the D’Albertis and Hargraves
expedition examined river sand and found ‘a speck of
gold and also a specimen of copper’ about 50km
downstream of the Mt Fubilan deposit (Goode, 1977
in Davies et al., 1978). In 1963, government patrol
officer Des Fitzer led the first government patrol into
the Star Mountains and brought back samples of
copper mineralisation from Mt Ian, 6km north of Mt
Fubilan (Pratt, 1977 in Davies et al., 1978). Three
years later, patrol officer G.C. Young found yellow and
green encrustation along an entire length of a creek in
the Kavorabip area near Mt Ian. He presumed the
encrustations were sulphur (most likely iron oxides
and copper hydroxide) and copper minerals (Davies et
al., 1978).
Despite the encouraging reports, it was not until June
1968 when Kennecott conducted helicoptersupported exploration that geologists D. Fishburn and
J. Felderhoff recognised oxidised massive
sulphide–magnetite float at Ok Gilor (Ok Tedi – Ok
Menga confluence) and traced it back to its outcrop
source at Sulphide Creek. Initial detailed work with
drilling was focused on skarn mineralisation.
However, significant porphyry mineralisation was
discovered when DDH 17 was collared on a
weathered intrusive at Hong Kong pad, on the top of
Mt Fubilan. By October 1971, 32,800m had been
drilled and a substantial tonnage of copper and gold
had been estimated.
In 1978, Kennecott withdrew when negotiations with
the Papua New Guinea Government failed to reach an
amicable development agreement. Consequently, the
government assumed ownership and appointed
Behre-Dolbear as project managers. During that year,
an additional 4,000m were drilled and mineable
reserves were increased from 130 to 250Mt after
significant gold ore was identified. In 1978,
agreement was reached between the government and
The Broken Hill Propriety Ltd (BHP) of Australia,
and a two-year feasibility study concluded in 1979.
Mine construction commenced in 1981 and gold
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
An aerial view of the mine pit at Ok Tedi
production by cyanide solution and carbon-in-pulp
commenced in May 1984; Cu–Au recovery by
flotation commenced in 1987. The cost of mine
development was K1,340 million (Davies, 1992).
Geological setting
The Ok Tedi deposit lies within the Western Fold Belt
mineral province (which incorporates New Guinea
Fold Belt and the Papuan Fold Belt). The region is
characterised by weakly to moderately folded and
thrust faulted Mesozoic and Cainozoic continental
marine sedimentary units, intruded by stocks of
Middle Miocene to Pleistocene age. The deformation
occurred as a result of accretionary tectonics and
orogenesis during the WSW-trending collision of the
Pacific Plate and Indo-Australian Plate in the Late
Oligocene – Early Miocene (Jenkins, 1974; Jaques
and Robinson, 1977; Pigram and Davies, 1987).
The geology, alteration and mineralisation at Ok Tedi
have been adequately described by Bamford (1972),
Page and McDougall (1972), Arnold and Griffin
The Geology and Mineral Potential of
PAPUA NEW GUINEA
(1978), Arnold et al. (1979a,b) and Hewitt et al.
(1980).
The oldest exposed unit in the region is the Ieru
Formation, comprising marine mudstone and
glauconitic sandstone that is 1,300–1,500m thick.
Disconformably overlying the Ieru Formation is the
Late Oligocene to Early Miocene Darai Limestone.
At Mt Fubilan, this unit is 300–600m thick, thrust
faulted, and hosts many of the skarn bodies. Pnyang
Formation overlies Darai Limestone and consists
mainly of calcareous mudstone and siltstone with
some prominent limestone horizons. The first
volcanic activity occurred in the Mid-Miocene with
deposition of minor tuffaceous sandstone in the Birim
Formation. Overlying the Birim Formation are
volcanoclastic sediments, the Awin Formation, which
represent an eroding stratovolcano of Late Miocene to
Pliocene age.
In the mine area, only Ieru Formation and Darai
Limestone have been intruded by the Ok Tedi
101
Mineral Projects and Mines (cont.)
Intrusive
Complex.
The
Sydney
Monzodiorite stock (2.6Ma) is the oldest
intrusive in the Complex and covers an area
of 1.5km x 2.5km, essentially to the south of
Mt Fubilan (Fig. 8.26). The Sydney
Monzodiorite is generally porphyritic to subporphyritic to equigranular, and contains
andesine,
clinopyroxene,
orthoclase,
hornblende and biotite in decreasing
abundance. Accessory sphene, apatite and
magnetite are common in the Monzodiorite.
The Fubilan Monzonite Porphyry is the main
source of mineralisation, and it intrudes the
Sydney Monzodiorite at Mt Fubilan where it
measures 1km x 0.8km with a downward
tapering cylindrical geometry. The porphyry
also forms a smaller stock with a 400m
diameter within the Sydney Monzodiorite to
the south of Mt Fubilan. The Fubilan
Monzonite Porphyry has a 1.2Ma age date
(Page and McDougall, 1972). The porphyry
consists of phenocrysts of oligoclase,
orthoclase, quartz, and hydrothermal biotite
which replace hornblende and biotite,
accessory minerals are apatite, sphene, rutile
after sphene, and magnetite in a felsic glassy
matrix.
Late-phase dykes transgress all rock types, and
are usually dark coloured, basic dykes up to
3m wide, which volumetrically constitute
about 0.1% of the overall intrusive complex.
Hydrothermal or intrusive breccia dykes also
occur in the Fubilan Intrusive Complex.
Fig. 8.26 Geology of the Ok Tedi Mine area (after Rush and Seegers,
1990).
Hydrothermal alteration and mineralisation
The primary Cu–Au mineralisation at Ok Tedi occurs
as skarn and disseminated sulphide mineralisation.
Gold-rich porphyry copper is hosted principally by
the Fubilan Monzonite Porphyry and, to a lesser
extent, hornfelsed sediments and Sydney
Monzodiorite.
Hydrothermal alteration zoning at Ok Tedi is
telescoped and truncated compared to other porphyry
copper deposits (Rush et al., 1990). Hydrothermal
alteration types recognised at Ok Tedi include two
phases of partially overlapping potassic events,
102
propylitic, phyllic and argillic alteration, subsequently
affected by supergene alteration.
The potassic alteration events (Phase I and II) in the
main stock are centered around the quartz stockwork.
The central quartz stockwork comprises silica flooded
and quartz stockwork veining ±sericite–clay that
formed a carrot-shaped body. The Phase I potassic
event is characterised by dark brown to green-brown
primary igneous mica, K-feldspar, rutile after sphene,
and is commonly associated with chalcopyrite and
martitised magnetite.
Overprinting the Phase I potassic event is the more
intense Phase II potassic event. The alteration
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
assemblage is characterised by phlogopitic mica, redbrown mica, K-feldspar and rutile after sphene.
Chalcopyrite, bornite, molybdenite and gold are
commonly associated with the phlogopitic and redbrown micas.
chalcocite, native copper and covellite. In the
sulphide zone, oxidation has given rise to goethite,
cuprite, malachite and azurite. Cu–Au grades in the
skarns are relatively higher, and contribute
significantly to the overall tenor of the resource.
Immediately surrounding the potassic zones is argillic
alteration typified by an assemblage of hydrothermal
kaolinite and montmorillonite impregnated with
martitised magnetite, iron oxides, minor secondary
sphene and rare sulphide minerals. The propylitic
alteration zone is generally poorly developed because
of the sedimentary host-rock geochemistry.
Hydrothermal alteration in the surrounding
sediments shows bleaching, potassic feldspars along
with pyrite, minor chalcopyrite and chalcocite.
Supergene alteration and mineralisation
Sulphide mineralisation in the protore (hypogene)
zone is characterised by chalcopyrite, bornite, pyrite
and molybdenite, and appears to be associated with
the potassic alteration events. The drilled thickness is
greater than 400m and generally underlies the leached
or supergene enriched zones. Copper grade generally
decreases with depth, from 0.30 to 0.4% near surface
to 0.1–0.2% at depth, along with 0.011–0.02% Mo
and 0.2g/t Au (Fig. 8.27).
Skarns
Several economically significant Cu–Au skarns are hosted
within the thrust faults adjacent to the contacts between
intrusives and limestone (Fig. 8.26). Duncan (1972) and
Katchan (1982) have classified skarn based on
mineralogy as follows - Periskarns (Endoskarns), calcsilicate, massive magnetite and massive sulphide skarns.
Strong empirical observations suggest temporal and
spatial relationships between the various skarns.
Prograde garnet–pyroxene assemblages with
retrograde epidote–actinolite–tremolite were replaced
by magnetite and overprinted by sulphides. In the
Edinburgh and Sulphide skarns, there is a lateral
transition from massive magnetite to massive sulphide
away from the intrusive. Sulphide mineralisation
postdates the earlier calc-silicate alteration and
magnetite replacement as exemplified by the
crosscutting sulphide veins. Pyrite, chalcopyrite,
bornite and marcasite often replace chalcopyrite,
pyrite and pyrrhotite.
In places, the skarn
mineralisation is weathered to supergene digenite,
The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Ok Tedi deposit had optimal conditions for rapid
Cu–Au enrichment. Extremely high rainfall of 10m/y
(Pickup, 1984), coupled with rapid plate uplift of
2mm/y for the mountains of Papua New Guinea
(Chapple, 1974), produces extremely high erosional
rates of 3mm/y. Extensive oxidation of the hypogene
disseminated copper mineralisation has resulted in a
highly differentiated weathering system characterised
by metal redistribution that reflects a pattern common
to weathered porphyry copper deposits that are found
worldwide. Supergene alteration includes partial
replacement of silicates to clays, and almost complete
oxidation of sulphides to goethite, which may be
cupriferous. Copper precipitated at greater depths
forming chalcocite, and lesser digenite and covellite
on the pre-existing hypogene minerals chalcopyrite,
bornite, pyrite and marcasite. The rate for copper
enrichment at Ok Tedi by solute transport calculated by
Danti (1991) is 1.0 x 10-7 g/cm3/y or 4.0 x 10-2ppm/y.
The weathering profile consists of a copper-leached or
Gold Cap, Oxide Copper Zone and Supergene
Enriched Copper Zone, which overlie a Protore Zone
(Fig. 8.27).
The Leached Cap (Gold Cap) was confined to the
upper portions of the deposit, with variable depths
ranging from 40m at top of Mt Fubilan to 290m thick
in the southern portion (Fig. 8.27). Average copper
grade in the Leached Cap was 0.05%, mainly as
cupriferous hydroxides. This zone co-existed with the
Gold Cap that hosted significantly higher gold grades
and formed an annulus about the quartz stockwork.
The gold mineralisation extended downwards through
the enriched copper zone to the top of protore; gold
grade decreased sympathetically with depth, from
>5g/t near surface to about 0.5g/t.
Using high-precision modal mineralogical studies,
electron microprobe analyses, and scanning electron
microscopy with mass balance analysis and generalised
inverse methods, Danti (1991) investigated
103
Mineral Projects and Mines (cont.)
Fig. 8.27 Variation of copper and gold grades with depth at Ok Tedi.
104
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
mineralogical changes in gold at Ok Tedi during
supergene copper enrichment, to determine the source
of secondary gold. He concluded that 80% of gold in
the protore is invisible and occurs principally as
auriferous hypogene pyrite–marcasite, bornite–idaite,
and chalcopyrite. On the other hand, 80% of the
gold is visible in the copper-leached capping as
microscopically identifiable electrum, either as
cupriferous subhedral inclusions within secondary
copper sulphides or copper-free euhedral grains within
secondary iron oxides. Danti (1991) concluded that
gold in the leached cap was enriched by a factor of 50
times at a rate of 2.9 x 10–11 g/cm3 (11.0 x 10–6ppm/y),
principally by regolith reduction and solute transport.
The Oxide Copper Zone has been defined as where
25% of the total copper oxide present is acid soluble
(Seegers et al., 1990). Copper minerals are principally
cupriferous goethite, copper oxide, copper sulphide
with lesser copper phosphates or carbonates,
cupriferous clays and native copper. This zone at Ok
Tedi was irregularly shaped and reached depths of
>180m along structures. The average copper grade in
the oxide zone was about 0.5%; the lower boundary of
this zone marks the oxide–sulphide interface.
The Enriched Copper Zone at Ok Tedi mainly
contains chalcocite and digenite, which occur
predominantly on fractures and in veinlets, commonly
associated with quartz stockwork zone. Secondary
sulphides occur as overgrowths on, and replacement
Prospect
of, pre-existing sulphides such as chalcopyrite and
pyrite. The thickness of this zone is variable but
ranges between 150–400m, with copper grades
ranging from 1 to 4%.
Resources and potential
The initial proven ore reserve for Ok Tedi was 410Mt
comprising 34Mt at 2.87g/t Au in the gold cap
(surficial), underlain by 351 Mt grading 0.7% Cu,
0.6g/t Au and 0.11% Mo, with a further 25 Mt
averaging 1.17% Cu (Francis et al., 1984). Breakdown
of the pre-mining proven ore by Davies (1992) is
shown in Table 8.3, while current resource estimates
are given in Table 8.4. Total contained metals translate
to 36,000kg of gold and 3.13Mt of copper.
After 20 years of mining (to the end of 2003), Ok Tedi
had produced 2.875Mt of copper, 7.136 million
ounces of gold and 15.599 million ounces of silver.
Ok Tedi produces an average 200,000t/y of copper
and the mine is projected to close in 2014. The
significant increase in the volume of metals produced
over the years has been the result of ongoing
exploration and lower cut-off grades based on the
increased performance of the mill.
In spite of large logistical costs and difficulties
associated with mining in Papua New Guinea, the
mine has relatively low average production costs of
around US$0.45cents/pound copper.
Category
Mt
Cu
(%)
Au
(g/t)
Cut-off grade
Cu (%)
Au (g/t)
Leached cap
Reserves
18
0.05
2.00
Sulphide
Reserves
348.9
0.7
0.56
0.4
1.00
Skarn
Reserves
28.8
1.25
1.58
0.4
1.00
Oxide copper
Reserves
19
1.74
1.34
0.6
1.00
1.09
Table 8.3 Initial identified mineral resources at Ok Tedi (Davies, 1992).
Resource
Ore Reserve
Category
Mt
Cu%
Au g/t
Category
Mt
Cu%
Au g/t
Measured
432
0.87
0.97
Proven
215
0.91
0.97
Indicated
216
0.55
0.65
Probable
31
0.57
0.66
Inferred
15
0.45
0.46
663
0.76
0.85
Total
246
0.87
0.93
Total
Table 8.4 Total reserves and resources for Ok Tedi as at 31 Dec 2003.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
105
Mineral Projects and Mines (cont.)
PANGUNA
Location and status
The Panguna porphyry Cu–Au deposit is located in
the Crown Prince Range in central south Bougainville
Island (lat. 6o19’S, long. 155o30’E) at an altitude of
500-1200m. It is owned by the publically listed
company, Bougainville Copper Ltd, a subsidiary of
Rio Tinto (formerly CRA Ltd).
Discovery history
Gold was won at Kupei near Panguna from Cu–Aubearing quartz veins that were mined in a small way
from the 1930s until 1941. As part of the early 1960s
CRA porphyry Cu–Au search, Government Geologist
Jack Thompson directed Ken Phillips to government
reports (Fisher, 1936; Thompson, 1962) of Cu–Au
associated with porphyry intrusions and agglomerates
in the Crown Prince Range of Bougainville Island. In
the initial May 1964 inspection, Phillips recognised
porphyry Cu–Au style mineralisation and drew
analogies with the Atlas and Toledo porphyry copper
deposits in the Philippines. By the following July,
Phillips and his team had defined a 13sq km copper
anomaly focusing on a 300m diameter core, and by
1969 over 80,000m of diamond drilling had been
completed, enabling production to begin in April
1972 (Knight et al., 1973; Baumer and Fraser, 1975;
Clark, 1990). These workers noted that the assay
results from bulk samples collected from 4.7km of
underground workings driven through the
mineralisation, undervalued the medium to lower
grade ore as determined from assays of core from
vertical drillholes (average 94.3% recovery).
Therefore, an additional 75,000m of drilling was
completed on a 122m grid between 1972 and 1986
(Clark and Eyall, 1986).
Geological setting
Geological mapping of the island by the BMR (Blake
and Miezitis, 1967) was updated by the Geological
Survey of Papua New Guinea (Hilyard and Rogerson,
1989). Bougainville represents part of the archipelago
of Papua New Guinea Melanesian arc, built up as calcalkaline island arc subduction-related magmatism
since the Eocene, and interrupted by Miocene
limestone deposition (Keriaka Limestone at
Bougainville). Volcanics, such as the Mid-Miocene
106
ore-hosting Panguna Andesite, include mostly
andesite with lesser basalt and dacite, and extend to
Pliocene time (Kieta Volcanics) and are all associated
with volcanogenic sediments. A wide variety of
intrusions including diorite, granodiorite, monzonite
and locally more alkaline compositions were emplaced
into the volcanic pile over a protracted period of time.
Active volcanism continues at the Bagana Volcano
32km north of the mine and a blanket of recent ash to
several metres thick obscures much of the island,
including the Panguna area.
Geology and mineralisation
The geology of Panguna is well exposed by mining
from 1200m to 380m asl, and from drilling that
extends to below sea level (Clarke, 1990; Baumer and
Fraser, 1975). The ore system occurs at the margin of
a large body of Kawerong Quartz Diorite and its
derivatives, in contact with Panguna Andesite
(Fig. 8.28).
The Panguna Andesite host rock occurs as shallow SEdipping hornblende microdiorite lava, agglomerate,
lapilli tuff and local pyroclastic bands from 1220m to
about 450m asl. As a result of intrusion by the
Kawerong Quartz Diorite, it displays hornblende
hornfels up to 500m from the contact, grading to
epidote–chlorite–albite–K-feldspar–calcite–pyrite
extending to the limit of exposure at 1200m.
The Kawerong Quartz Diorite (5–4Ma; Page and
McDougall, 1972b) at the Panguna Mine occurs as a
complex
intrusion
featuring
differentiates
characterised by gradational, crosscutting and
overprinting relationships. Mineralisation is associated
with the potassic-altered margin termed ‘biotite
diorite’ (3.4Ma; Page and McDougall, 1972b),
which contains biotite, and secondary K-feldspar.
The potassic alteration grades to propylitic
(chlorite–epidote–carbonate) alteration away from the
main body of mineralisation. The intrusive becoming
brecciated at depth and is locally termed leucocratic
quartz diorite, which hosts numerous quartz veins
with orthoclase selvages in association with copper
mineralisation. Higher grade mineralisation forms a
halo of >0.5% Cu associated with potassic alteration,
typically as chalcopyrite with gold, grading to 0.3%
Cu in propylitic alteration. Quartz veins, which cut
the potassic alteration, contain additional chalcopyrite
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
angular andesite fragments in a matrix
of biotite, chalcopyrite, bornite and
local free gold, and are therefore
associated with higher metal grades,
especially at the apophyses to the
intrusion–breccia system.
Metal
grades decline at depth within the
breccia bodies, and they are cut by the
Biuro Granodiorite.
Pebble dykes, including one body
traced laterally in the open pit for
1900m x 50m, display characteristic
fragment milling and consistent
fragment and matrix compositions
over considerable distances.
Phyllic alteration (silica–sericite–pyrite)
irregularly collapses down fractures
and is associated with erratic high
Au–Ag grades, and also dominates in
the late-stage pebble dykes. Argillic
alteration defined by clay minerals
and disseminated pyrite is widespread
but weakly developed.
The fracture pattern exhibits a
concentric form, modified by the
Fig. 8.28 Geological map of Panguna (after Clark, 1990).
regional NE structural grain, which is
and bornite. The Kawerong Quartz Diorite becomes
exploited by intrusive features such as pebble dykes
progressively less quartz veined and displays lower
(Clark, 1990).
Cu–Au values as it grades to the biotite granodiorite at
While copper occurs within chalcopyrite and bornite,
greater depth, and towards the core of the intrusion
chalcocite and other secondary copper minerals are
away from the margins. Central barren portions of
recognised but were not of economic importance.
the intrusion resemble biotite granodiorite. Small,
The gold recovery during processing was about 75%
weakly mineralised dykes of the Biuro Granodiorite
using flotation. The gold was reported (unpubl. data
(3.4Ma; Page and McDougall, 1972b) dilute ore to a
in Clark, 1990) to have been mostly held in
greater extent than was recognised in the initial
chalcopyrite and bornite (40%), to a lesser extent
drilling.
within pyrite (4%), and the remainder (56%) in
silicates and other non-sulphide minerals.
Feldspar porphyries occur as post-mineral intrusions,
Consequently, villagers are now working alluvial gold
as is common in many porphyry Cu–Au deposits.
from tailings, which for 20 years were directly
Another post-ore intrusion includes the barren
disposed of into the Kawerong River, west
Nautango Andesite (1.6Ma; Page and McDougall,
Bougainville. Many of the mines in PNG
1972b).
failed to originally recognise the value of
incorporating gravity recovery for gold at the front
Intrusive breccias formed by emplacement of the
end of the metallurgical circuit.
biotite diorite into the Panguna Andesite contain
The Geology and Mineral Potential of
PAPUA NEW GUINEA
107
Northerly view of Panguna open pit in 1984.
Discussion
Approximately 30% of the ore occurs within Panguna
Andesite and most of the remainder within breccias
and contact or differentiate phases of the Kawerong
Quartz Diorite (biotite diorite). Furthermore, about
80% of the mineralisation is hosted within steeply
dipping fractures and veins and adjacent wall-rock
alteration (Baumer and Fraser, 1975). The higher
Cu–Au grades occur as an annular
shape
encompassing the biotite diorite and breccias and
rimming the biotite granodiorite; the grades decline
with depth (Clark, 1990).
The Panguna ore system displays similarities to recent
models for porphyry Cu-Au deposits, in that
mineralisation appears to have been bled from a major
magma source at depth and deposited in an apophysis
to a brecciated and altered late-stage intrusion on the
margin of a much larger intrusion. Consequently,
erosion has exposed but not removed the best part of
the system, which declines in metal content with
depth.
While Panguna would be apparent to an experienced
eye in our current understanding of porphyry Cu–Au
108
deposits (K. Phillips, pers. comm., 2003), important
aspects in the Panguna discovery include a geologicalmodel-driven exploration program, use of government
data, comparisons with known deposits, early
recognition that young volcanic ash cover obscured
mineralisation resulting in non-anomalous soil
profiles, and a willingness to drill.
Resources and potential
Panguna began production in 1972 with a published
resource of 944Mt at 0.48% Cu, 0.56g/t Au and 3g/t
Ag. By 1988, the mine had milled 686.9Mt of ore at
0.53% Cu and 0.63g/t Au to produce 3.0Mt of
copper and 9.6 million ounces of gold, and retained a
mill feed of 710Mt at 0.4% Cu and 0.47g/t Au,
including low-grade upgrade by screening (Clark,
1990). Production was terminated by civil unrest in
1989.
While the higher grade upper portion has been
removed leaving material of questionable economics
in a difficult start-up environment, additional
prospecting should evaluate the potential of adjacent
porphyry stocks as well as the marginal gold vein
potential not fully prospected by CRA.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
PORGERA
Location and status
Porgera Mine lies 5km west of Porgera Township (lat.
5o28’S, long. 143o05’E) at an altitude of 2,500m in
the remote Enga Province of Papua New Guinea. The
Porgera Mine lies within a 2,227 hectare Special
Mining Lease (SML 4) that is held by the Porgera
Joint Venture. The lease is surrounded by two
exploration tenements (EL 454 and 848).
Discovery history
The first gold was officially reported from Porgera in
1938 as a result of investigations in the region from
1933–39 by Government Patrol Officers Jim Taylor
and John Black (Cotton, 1975; O’Dea, 1980). Black
spent several days prospecting alluvials below where
the Porgera Mine currently lies and then rejoined
Taylor for the remainder of the expedition. The
Porgera Valley was next visited late in World War II,
prior to a rush subsequent to the granting of
permission to enter restricted territory in 1948. After
the lifting of access to the area, Joe Searson applied for
a dredging and sluicing claim in 1949 (Jackson and
Banks, 2002).
In the early 1960s, Searson approached several
companies to test the hard-rock potential of the
mineralisation at Porgera, and the administration
provided the assistance of Government Geologist
R.G. Horne to follow up on the 1948 work of H.J.
Ward. Subsequently, in 1964–65 Bulolo Gold
Date
Ore type
Dredging bored 13 shallow drillholes but abandoned
the project. MIM showed interest by pegging around
Searson’s tenement. Meanwhile, Searson reached
agreement with Anaconda, and later with Australian
investors Kimberly and Rumple, and in 1969 formed
Ada Explorations, which employed Rudi Jezernik to
drive two adits at Waruwari. However, by 1971 funds
ran out and as Searson’s health failed he was forced to
sell to MIM, which entered into a joint venture with
Ada and continued trenching and drilling programs.
In 1975, Placer entered into a joint venture with
MIM, and as operator carried out detailed geological
mapping prior to drilling. In 1979 Renison Goldfields
Consolidated (RGC) brought funds and metallurgical
expertise and farmed in to the joint venture. 10% was
reserved for the people of Papua New Guinea should
the National Government elect to take up that equity.
Much to the concern of the private sector in PNG, the
equity figure has subsequently changed on two other
occasions. In the early 1980s, definition drilling was
carried out at Waruwari and geological studies defined
the ore types, the Porgera Intrusion Complex (PIC),
and the Roamane Fault (Fleming et al., 1986).
However, an initial study by Fluor on the Waruwari
mineralisation determined that it had poor
metallurgical characteristics.
The joint venture elected to continue low-budget
geological mapping and prospecting, and in late 1983
Geoff Handley identified outcropping Zone VII
mineralisation within the Roamane Fault that assayed
15g/t Au over 45m. Renewed drilling focused on this
Cut-off (Au)
Tonnes
Au Grade (g/t)
Moz
1982
All
3.0
2.5
6.9
0.6
1983
open pit
3.0
3.7
6.4
0.8
1988
open pit
3.5
50.6
4.1
3.3
underground
7.0
3.6
28.7
3.3
open pit
1.5
54.2
4.3
7.5
underground
7.0
5.9
27.0
5.1
58.4
3.5
6.6
1990
End 2001
All (proven + probable)
US$275
Table 8.5 Various Porgera resource figures (after Jackson and Banks, 2002).
The Geology and Mineral Potential of
PAPUA NEW GUINEA
109
Mineral Projects and Mines (cont.)
high-grade ore with good metallurgy, but it was not
until 1984 that the first geologically consistent, high
grade gold intercepts were encountered. This changed
the economics of the project by also allowing for a
much lower grade but larger open-pit resource to be
considered. A proposal was put to the Government of
Papua New Guinea in 1988, leading to construction
and initiation of underground mining in 1990 and
open pit mining in 1993 (Table 8.5).
In October 2003, DRD Gold of South Africa finalised
the purchase of a 20% stake in the Porgera Joint
Venture. Placer Dome holds 75% while Mineral
Resources Enga holds the remaining 5%.
Geological setting
Porgera gold mineralisation is intimately associated
with the Porgera Intrusion Complex (PIC), which has
been emplaced into a previously thrusted sequence of
sedimentary host rocks described by Davies (1983)
and Gunson et al. (1997). The Jurassic Om
Formation occurs as thinly bedded, dark grey
carbonaceous siltstone with pyritic calcareous nodules
and locally pyritised macrofossils. Cretaceous Chim
Formation comprises bioturbated and laminated grey
calcareous siltstone, mudstone and local sandy
horizons, which can be divided into discrete units
(packages) facilitating detailed geological correlations
in the Porgera region (Gunson et al., 1997). Eocene
to Miocene limestone crops out further south and east
of the mine, and includes the structurally thickened
package at Mt Paiam and Mt Kaijende.
Porgera Intrusion Complex in about 1992 showing
initiation of open-pit mining at Waruwari, and the two adit
levels into Zone VII as well as the Rambari, Roamane and
Peruk elements of the PIC to the right of Waruwari. A line
of drill pads across the face of Rambari–Roamane defines the
position of the Roamane Fault.
Porgera occurs within a corridor of regional-scale
faults interpreted to be related to deep crustal
fractures, formed normal to the structural grain of the
Papuan Fold Belt portion of the New Guinea Orogen.
Fold axes change in orientation across this structure.
Dextral strike slip movement is noted along
lineaments at Porgera (Smith, 1990; Hill, 1990), and
while some workers speculate that Porgera is localised
at a possible step over (fault jog) in the lineaments,
others suggest this movement localises ore within the
Roamane Fault (Corbett et al., 1995). The
sedimentary sequence is intensely thrusted (Davies,
1983; Rogerson et al., 1987a,b) by structures that
some workers interpret to represent reactivated earlier
basin-bounding faults (Hill, 1991; Gunson et al.,
1997). This complex thrusting abounds in the region
and is clearly apparent in exposures of the Porgera
open-pit mine.
Geology and mineralisation
Side-looking radar image of the Porgera Transfer Structure
showing the circular feature which roughly corresponds to the
Porgera aeromagnetic anomaly shown in Figure 8.29.
110
The gold mineralisation at Porgera displays a staged
geological development associated with evolution of
the PIC as a differentiating alkaline intrusion
(Fleming et al., 1986; Richards and Kerrich, 1993;
Figs 8.29, 8.30). Intrusions crop out as a ‘Y’-shaped
set of topographic highs defined by the harder
intrusions within softer shale. Syn-mineral thrusting
has placed calcarenite in contact with black shale, the
main host rock in the mine area, while post-mineral
thrusting on the Boundary Fault has resulted in the
emplacement of the calcarenite - black shale package
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
over fissile brown shale.
Aeromagnetic data indicate that
the intrusions exposed at the
mine occur as apophyses to a
much larger buried magmatic
source, which is inferred to have
domed the host sedimentary
rocks.
The largely buried
intrusion, only partly exposed by
erosion, is an important feature
of the Porgera setting, as ore
fluids have been bled from the
intrusion source at depth to
deposit mineralisation in the
cooler higher level setting
(Corbett et al., 1995).
The main open pit mineable
low-grade gold mineralisation
occurs at Waruwari, while the
Zone VII higher gold grade ore
occurs in the Roamane Fault and
parallel structures. Adjacent to
the intrusions, the incompetent
shale is indurated to form
bleached shale, which may host
veins. Recent age determinations
on Porgera intrusions (Ronacher
et al., 2002) updated earlier
findings
(Richards
and
McDougall, 1990) suggest that
intrusion and mineralisation
took place over a very short
time, as little as 0.26 million
years (including their error
estimate) at about 5.9Ma.
Most workers (Fleming et al.,
1986; Richards and Kerrich,
1993; Corbett et al., 1995, and
Fig. 8.29 The Porgera Intrusion Complex showing distribution of intrusions, transfer
numerous unpublished studies)
structures and the Roamane Fault.
distinguish
two
main
from equigranular to porphyritic, and approaches
mineralising events (Stages I and II), described below.
gabbroic composition; ‘andesite’ dykes and sills are
Stage I mineralisation is associated with the
interpreted to represent fine grained equivalents;
emplacement of generally porphyritic intrusions
• later hornblende diorite is characterised by
categorised as:
radiating hornblende rosettes within a fine-grained
• initial augite hornblende diorite, which ranges
matrix, locally forming needle hornblende diorite.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
111
Mineral Projects and Mines (cont.)
Fig. 8.30 Geological map of the Porgera Mine area.
Stage I gold mineralisation corresponds to
quartz–sulphide and carbonate–base metal–gold style
of intrusion-related low sulphidation epithermal gold
mineralisation. Strictly speaking, the mineralisation
at Porgera formed at what could also be termed a
mesothermal crustal level. Early classifications
(Fleming et al., 1986) described initial pyrite-rich "B"
veins that grade into "A" veins comprising
pyrite–sphalerite–galena–carbonate with local
tetrahedrite, freidbergite and chalcopyrite. These
workers also categorised "G" veins as pyrite–carbonate
stringers. High temperatures of ore formation are
apparent in the mine area from the presence of
pyrrhotite and dark sphalerite (Fe-rich), with fluid
inclusion temperatures in sphalerite and quartz of
273oC and 318oC, respectively (Corbett et al., 1995).
More recent exploration some 1,000m deeper has
identified hydrothermal magnetite, secondary biotite,
chalcopyrite, and pyrrhotite with quartz fluid
inclusion temperatures in the order of 350oC
(Ronacher et al., 1999).
112
Thick Stage I A–B veins exploit pre-mineral NNEtrending structures, and thin veins become more
numerous in the vicinity of these fractures. While the
NNE-trend is consistent with the A–B veins
exploiting tension fractures during subduction-related
compression from the NNE, the transfer structures
may also have been dilated by doming of the
sediments during emplacement of the Porgera
Intrusive Complex at depth. Importantly, the sulphide
mineralisation is derived from a buried magmatic
source and not the high-level stocks in which it is also
hosted, as well as the sediments. Host-rock control is
evident as veins are best developed within intrusions,
and to a lesser extent in the adjacent indurated
(bleached) sediments, and poorly developed in the
incompetent black sediments.
Gold grades associated with the Stage I A–B veins
range up to a few g/t and, as demonstrated early in the
evaluation of Porgera (Fleming et al., 1986), may
display higher gold grades where they (A–B veins)
have acted as brittle host rocks and are overprinted
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
calcite and dolomite at depth (Corbett and Leach,
1998).
The Stage I mineralisation predominates in the
Waruwari area where it is exploited by open-pit
mining. There, mineralised fracture–vein–breccia
ores occur within many high-level stocks and dykes
and the enclosing bleached shale, although locally
overprinted by Stage II veins, the mineralisation is
best developed along N-NE trending structures.
Composite vein comprising mostly A–B vein pyrite–sphalerite
with overprinting quartz–roscoelite at the left hand end.
Stage II gold mineralisation occurs as quartz–
roscoelite–pyrite D veins (Fleming et al., 1986) and
corresponds to epithermal quartz–Au–Ag style
mineralisation described herein. Stage II gold
mineralisation is noted for bonanza metal grades
within free-milling ore. Prior to vein development,
quartz–feldspar porphyry stocks and dykes were
emplaced as the most differentiated later stage of PIC
magmatism, locally migrating along the Roamane
Fault and into hanging wall structures at Waruwari
(Corbett et al., 1995). Feldspar porphyry intrusions
post-date the Stage I A–B vein mineralisation and are
in turn cut by the quartz–roscoelite veins.
Stage I vein comprising initial pyrite followed by
sphalerite–galena and later carbonate, including
rhodochrosite.
The Roamane Fault is an E–W-trending, southdipping normal fault, with a slight sigmoidal shape, is
inferred to have been dilated by dextral movement on
the NNE-trending structures, and hosts NE-trending
dilatant D veins and flexures (Corbett et al., 1995;
Fig. 8.31). The fault was initiated between stages I
and II as it cuts A–B veins and is a host for D veins.
Parallel structures are primary controls of D vein
mineralisation.
by D veins (ie. quartz–roscoelite–pyrite, discussed
below), and are here termed A–D veins. Porgera A–B
veins display a mineralogy and paragenetic
sequence typical of southwest Pacific carbonate–base
metal–gold deposits as pyrite>sphalerite>galena>
chalcopyrite–tennantite>carbonate, with sphalerite
contents greater than galena (Corbett et al., 1995).
Gold, which displays an average fineness of 670,
occurs as minute inclusions (20–40µm) in sulphides
or as free gold in carbonate (Corbett and Leach,
1998). In type C veins (Fleming et al., 1986),
sub-microscopic gold occurs within early
pyrite–arsenopyrite veins, typical of a quenched
quartz–sulphide style gold deposit. Thus, the Stage I
mineralisation A–B and C veins commonly possess a
difficult and highly variable metallurgy. Carbonate
zonation is typical of carbonate–base metal–gold
deposits elsewhere and ranges from rhodochrosite and
siderite at both shallow levels, and occasionally deeper
when associated with major structures, to ankerite
(early) and dolomite (late) at intermediate levels, and
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Stage II mineralisation was initiated as localised
crosscutting milled matrix fluidised breccia dykes,
containing fine silica–pyrite, formed by explosive
hydrothermal activity at an elevated crustal setting.
The dilatant character of the ore environment is
evidenced by local banded vein/breccias and sheeted
veins. There are several ore shoots that host extensive
gold mineralisation at Porgera. Zone VII occurs as a
sub-horizontal ore shoot located roughly at the
intersection of the hanging wall split with the
Roamane Fault, where some workers suggest bonanza
gold mineralisation was promoted by fluid mixing
(Corbett et al., 1995; Wall et al., 1995). By contrast,
a steeper pitch is apparent for the Eastern Ore Zone,
113
Mineral Projects and Mines (cont.)
take on a wire form, and silver
tellurides (mainly hessite),
while later stage carbonate-rich
variants
contain
silver
sulphosalts
and
mercury
tellurides (Corbett and Leach,
1998). The latter veins at
Porgera have been termed type
"E" carbonate-rich silver ores
(Fleming et al., 1986). Pyrite is
commonly deposited first, gold
may be intergrown with
roscoelite, and these veins are
generally cut by later banded
comb quartz and local last-stage
carbonate–sulphate
(anhydrite–gypsum–barite).
Recent work by the Porgera
Joint Venture geologists has
identified a macroscopic-scale
association between sulphate
and Stage II mineralisation in
the mine, as an indication of
ore deposition by fluid mixing.
Minor amounts of red
sphalerite (indicative of a lower
temperature of formation than
the A–B veins), galena and
chalcopyrite
are
locally
intergrown with the early
quartz. Gold displays a high
fineness of 800, with fineness
Fig. 8.31 Cross-section through Porgera in the vicinity of the Roamane Fault.
declining in the later carbonaterich veins. Fluid inclusion data
developed at a flexure formed during dextral strikesuggest low-temperature, relatively dilute fluids
slip movement on the fault. The Eastern Deeps Zone
(135±14oC, 4.2–7.8 wt% NaCl; Richards and
is localised at the intersection of the Roamane Fault
Kerrich, 1993; Ronacher et al., 1999).
and an A–B vein-hosting NNE-trending structure
Thus, there has been a pronounced change from Stage
within a brittle intrusion. The Northern Zone
I mineralisation formed at higher temperatures in
comprises sheeted quartz–roscoelite veins formed
deeper crustal conditions, to the shallow-level lowadjacent to (hanging wall) a south dipping normal
temperature epithermal conditions prevailing during
fault, parallel to and in the footwall of the Roamane
the formation of the Stage II mineralisation.
fault. The steep-dipping Central Zone, located
between the Roamane and Northern Zone Faults,
Discussion
displays a more open-space breccia character and is
therefore interpreted as a large tension gash feature.
At Porgera, low-temperature epithermal (D vein Stage
Stage II veins occur as quartz with roscoelite
(vanadium bearing illite), pyrite, free gold which may
114
II) mineralisation is telescoped upon higher
temperature (A–B vein Stage I) quartz–sulphide–
carbonate–base metal–gold mineralisation, as two
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Bonanza-grade wire gold with quartz and roscoelite (green).
relatively distinct events within different structural
settings and, importantly, at different crustal levels.
This scenario requires 600–700m of uplift and
erosion (Corbett and Leach, 1998) between the two
events, yet Ronacher et al. (1999) suggested that the
Porgera intrusion–mineralisation event was very short
lived (<0.26 million years, including error). Munroe
and Williams (1996) also suggested that there is little
age difference between the Stage I hornblende diorite
(6.06±0.21Ma) and the Stage II feldspar porphyry
(5.87±0.15Ma). Thrusting, which is well developed
throughout the region, may provide an answer
(Corbett pers comm. 2003). Indeed, the Western
Boundary Fault occurs as a post-mineral fault that
locally provides a lower limit to ore where it separates
overlying
Waruwari
mineralisation
from
unmineralised fissile shale. Here, the calcarenite is
also thrusted into place and contains only later stage
The Geology and Mineral Potential of
PAPUA NEW GUINEA
feldspar porphyry intrusions. In fact, many thrusts
are apparent in the open pit. The dramatic change in
the crustal level of ore formation may have been
facilitated by the thrusting off of the upper portion of
the domed sediments, and the resultant pressure
reduction could have promoted emplacement of the
feldspar porphyry and initiation of Stage II
mineralisation. Standing (1994) goes so far as to
speculate that Mt Kare gold mineralisation, described
as 100m thick and 15km away (Laudrum, 1997),
could be a thrust-off part of the mineralisation at
Porgera.
At the Eastern Deeps D vein mineralisation, the
deepest (and hence highest temperature) part of the
Stage II event, there is a complete evolution from
initial thin bands of quartz–pyrite and lowtemperature carbonate–base metal mineralisation
115
Mineral Projects and Mines (cont.)
(with red-yellow sphalerite), to the epithermal
quartz–roscoelite event. Thus, it seems that the
Porgera Stage II quartz–roscoelite epithermal
quartz–Au–Ag is not a continuation of the Stage I
event, but an entirely new event of mineralisation, and
so Porgera comprises two distinct intrusion-related ore
systems.
Some trends in hydrothermal fluid flow can be
identified at Porgera (Corbett and Leach, 1998).
Stage I, derived from the major magmatic source at
depth, migrated from the central portion of the PIC
to the south along NNE-trending structures to cool
within competent host rocks, commonly as NNEtrending veins. The Stage II fluids are hosted within
more dilatant fracture systems, and in the Roamane
Fault migrated from an upflow at Waruwari and
deposited bonanza gold grades by rapid cooling.
In summary, Porgera has had a long history of
exploration from initial alluvial gold mining to
hard-rock exploration. While early exploration
focused on an ore system enabling Placer Dome to use
its expertise as a bulk open-pit miner, the Roamane
Fault, which was known to contain high gold grades,
was not targeted. The breakthrough came with the
1983 discovery of the outcropping bonanza gold
grade Zone VII ore within the Roamane Fault.
This came about as a result
of
continued
input
from government technical
advice
and continued
geological mapping. The
latter being a basic and
inexpensive tool. Geological
work at Porgera has greatly
contributed towards the
understanding of intrusionrelated low sulphidation
gold deposits.
which represents an average of 15.7% of the total
merchandise export value for PNG.
The mining feasibility investigation in 1990 cited the
Porgera resource as 85.8Mt at 5.7g/t Au for 15.7
million ounces (1.5g/t Au cut off ). Total proven and
probable mineral reserves at the end of 2003 were
48.85 million tonnes grading 3.4g/t gold, which
equates to 5.391 million contained ounces of gold.
This gives a projected operational life of 9 years.
Underground mining at Porgera is scheduled to
conclude in 2006 and at the open pit in 2007, while
milling of low grade stockpiled ore will continue until
2012. Porgera has been subjected to extensive
exploration over many years, culminating in more
than 800km of drill core to June 2003. In addition,
exploration is continuing from newly developed
underground workings. The current exploration
focus is to identify additional structurally controlled
quartz–roscoelite veins, particularly north of the
Roamane Fault. While this low-temperature,
structurally controlled mineralisation could travel
considerable distances from the magmatic source
within dilatant structures, it is likely to be limited by
the extent of competent host rocks, such as intrusions
or adjacent indurated shale, which are capable of
hosting fracture-controlled ore.
Resources and
potential
From the commencement
of mining in 1990 to the
end of 2003, Porgera had
produced 12.22 million
ounces of gold and 2.23
million ounces of silver,
116
Porgera mill and open pit in 2004.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Helicopter supported exploration at the Frieda Copper prospect.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
117
Mineral Projects and Mines (cont.)
TOLUKUMA
Location and status
The Tolukuma gold mine (lat. 8o34’S, long.
146o08’E) lies within Mining Lease 104, located some
100km north of Port Moresby between Fane (5km
west) and Woitape (12km east) in mountainous
terrane between 1,400m (including underground)
and 1,750m altitude. The lease is held by Tolukuma
Gold Mines Ltd, a subsidiary of DRD Gold of South
Africa.
As the nearest road is 50km from Tolukuma at
Kubuna, the mine was constructed using materials
transported by Russian Mil 8 and 26 helicopters, with
payloads of 3.5 and 5.5 tonnes respectively (at that
altitude). The mine is still serviced by a Mil 8
helicopter. Mining and detailed exploration for more
ore is being undertaken on ML 104, while the
surrounding Exploration Licences are the subject of
regional exploration.
Discovery history
Alluvial gold is reported to have been worked about
1,900m downstream from Tolukuma in the Auga
River 1.5km north of Mondo, and in Ongimolo
Creek, a tributary of Mase Creek near Fane (Davies
and Williamson, 1998).
In 1983, the Tolukuma region was targeted by
Newmont Pty Ltd in a search for epithermal gold on
the basis of information supplied by geological staff of
the Geological Survey of Papua New Guinea, and also
on data available on open file that described the area
as containing intermediate volcanics, high-level
intrusions, and alluvial and hard-rock gold
occurrences (Langmead and McLeod, 1991). By mid1985, a helicopter-supported stream sediment
program utilising the bulk leach cyanide extractable
gold (BLEG) analytical technique had indicated
anomalous samples assaying up to 20ppb Au in the
headwaters of the Auga River between Fane and
Woitape. Despite the rugged terrane, which made
even helicopter assess difficult, limited quartz float
typical of banded adularia–sericite veins was
identified, sampled and assayed yielding results of
10–136ppm Au. Outcropping veins were identified
in July 1986 at Tolukuma Hill, which contained a
distinct negative vegetation anomaly, and by
September that year a trenching program had defined
a 2–10m wide vein system over a 1km strike length.
Tolukuma - located 100km north of Port Moresby, in the Goilala district of Central Province.
118
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Although other vein systems in the region were
prospected, Tolukuma remained the focus of activity
through drilling and eventually reached feasibility
study stage. By 1990, Newmont had defined a drillindicated resource estimated at 1.47Mt at 13.7g/t Au
using a 4g/t cut off, for 654,000oz of gold (Langmead
and McLeod, 1991). As the Tolukuma vein system
was of limited size (for a major mining company such
as Newmont), of difficult access, and represented
Newmont’s only activity in Papua New Guinea, the
company sold Tolukuma to Dome Resources in early
1993.
Dome Resources continued the drilling program and
by late 1993 published an upgraded measured
resource for Zone C in the vicinity of Tolukuma Hill
of 440,000t at 17g/t Au and 46g/t Ag (250,000oz
gold equivalent), and an inferred resource of 120,000t
at 16g/t Au (62,000oz of gold). This resource allowed
for two and a half years of mainly open-pit mining
and a minor underground operation, although mine
life was expected to be extended by further
underground development combined with additional
resources from known vein extensions. Construction
began in May 1995 and processing of ore commenced
in December 1995.
As anticipated by most geologists familiar with
Tolukuma, exploration since construction has
continued to identify additional mineralisation within
existing veins (Gulbadi and Tolimi), in extensions to
veins, in splays from existing veins, and within newly
discovered outcropping veins (Kunda).
grading to later shoshonitic affinity, with lahars and
lesser tuff beds and sedimentary intercalations. Duck
(2001) suggested that a dismembered volcano north
of the mine, termed the ‘Boundary Volcano’, is
genetically related to ore formation, while Pieters
(1978) described a suspected caldera 25km to the
south at Mt Cameron as a source of volcanic material.
Subvolcanic intrusions, including diorite, quartz
diorite, latite porphyry and dacite, locally display high
sulphur contents and magnetic susceptibilities typical
of porphyry Cu–Au alteration and mineralisation.
Hydrothermal breccias are indicative of some
explosive activity associated with intrusion
emplacement. A Late Miocene–Pliocene (5.9–4.7Ma)
age is provided for the Mount Davidson Volcanics
(Davies and Williamson, 1998; Dekba, 1983), and a
hornblende porphyritic andesite (4.8±0.88Ma) in the
mine area (Langmead and McLeod, 1991).
Basement to the Tolukuma area is Cretaceous Kagi
Metamorphics of the Owen Stanley Metamorphic
Complex, which crops out as slate and phyllite
hosting metamorphic sweat-out veins (Davies and
Williamson, 1998; Langmead and McLeod, 1991).
The Tolukuma mineralisation is hosted within a
corridor of N–S structures that lie close to, and locally
define, the western margin of the Mount Davidson
Volcanics. Normal and sinistral movement on a
faulted contact between basement and volcanics may
have contributed towards the formation of volcanichosting basins and the graben interpretation cited
previously, as well as dilatant splay vein formation.
In mid-2000, ownership of Dome Resources passed to
DRD Gold.
Geological setting
Tolukuma is on the western margin of a N–S-trending
portion of the Mount Davidson Volcanics between
Fane and Woitape (Langmead and McLeod, 1990,
1991). Although cited as a graben by early workers,
more detailed studies demonstrate that the volcanic
rocks mostly lie unconformably on basement,
although faulted contacts are apparent in the vicinity
of the mine.
In the mine area, the Mount Davidson Volcanics
comprise mostly andesitic and dacitic agglomerates
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Tolukuma vein underground.
119
Mineral Projects and Mines (cont.)
Geology and mineralisation
Gold mineralisation at Tolukuma is typical of the low
sulphidation adularia–sericite–Au–Ag epithermal style
mineralisation. At Tolukuma the mineralisation lies
within thin banded veins comprising mostly
chalcedonic quartz, and associated with quartz
pseudomorphing platy calcite, adularia, black
sulphidic ginguro bands and clay (Corbett, 2002;
Langmead and McLeod, 1990; Semple et al., 1998).
Regional propylitic alteration is overprinted
by illite–smectite wall-rock alteration adjacent
to
mineralised
veins,
while
phyllic
(silica–sericite–pyrite–clay) alteration in the mine area
is attributed to intrusion emplacement that took place
prior to the previously mentioned epithermal
mineralisation.
Fig. 8.33 Cross-section through the Tolukuma vein at
22400N, on the southern side of Tolukuma Hill.
The corridor of N–S structures that host the
Tolukuma mineralisation has been traced for over
10km, continuing north of the Auga River, and
remains open to the south of the mine (Fig. 8.32).
Where the Tolukuma vein was originally exposed at
Tolukuma Hill, the basement–volcanic contact dips
easterly with an interpreted local normal fault
movement, and so the Tolukuma vein locally occurs as
a 10m wide hanging wall vein (Fig. 8.33). Elsewhere,
most banded quartz veins are in the 0.5–2m range
and the well-developed banding is indicative of
repeated extensional fault activation. The northern
continuation of the Tolukuma vein to form the Gifinis
vein is unimpressive. Current mining is focused on
the Gulbadi vein south of Tolukuma.
Fig. 8.32 Map of Tolukuma vein distribution.
120
The best gold grades at Tolukuma are recognised in
NW-trending vein segments and splay fracture/veins
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
60g/t Au in 1998, while free gold was recognised
within splays at the margins of the Tolukuma vein.
Many NW-trending cross-veins such as the Gulbadi
cross-veins also produce high-grade ore (Fig. 8.34).
Tolukuma vein showing banded quartz and ginguro band,
which hosts most Au-Ag mineralisation.
High grade vein from Tolimi showing dark ginguro bands,
which host electrum.
Some mineralogical variations within the veins are
apparent. The Tolukuma and Tolimi banded quartz
veins contain the best gold mineralisation of the
deposit, within black sulphidic ginguro bands that are
typical of adularia–sericite epithermal Au–Ag
deposits. The veins also host pyrite, chalcopyrite,
galena, argentite and silver sulphosalts, with electrum,
native gold, and silver. High-grade gold identified in
quartz pseudomorphing platy calcite textures was
deposited as later stage open-space fill. The Gulbadi
vein occurs as a quartz–adularia–clay vein with later
overprinting pyrite–marcasite–stibnite, ranging up to
several percent sulphide in some instances. Here,
higher gold grades are associated with pyrargyrite.
Prior to mining, gold fineness for Tolukuma was
estimated to be in the range of 597–771 with an
average of 686 (Semple et al., 1995).
Exploration
Fig. 8.34 Cross-section through the Gulbadi vein at 270N.
formed by interpreted sinistral movement on N–S
faults. The non-outcropping Tolimi vein possessed
bonanza grades and produced a head grade of
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Immediately south of the mine, exploration at Degot
Creek has identified narrow fault zones with minor
brecciated quartz that contain locally consistent
anomalous gold grades (e.g. 100mm at 58g/t Au in 10
samples), followed up by impressive drilling results
(DDH IV004, 2m true width of 20g/t Au and
185g/t Ag). Further south, ore from the Kunda vein
yielded bonanza gold grades at outcrops where NWtrending splay faults intersect the main NNW vein
trend. As the topography rises steeply to the south of
Gulbadi, these veins are expected to be accessed by the
121
Mineral Projects and Mines (cont.)
Gulbadi vein showing early banded quartz cut by later sulphide.
2km Miliahamba Deeps adit being driven from the
existing mine workings. Continued exploration
further south of the mine is accessing targets, such as
high gold grades in association with a W-NW vein
flexure in the overall NNW-trending Seri Seri vein,
4km south of Tolukuma hill.
The Saki Prospect, located 3km east of Tolukuma, has
been the site of most recent exploration. There, veins
varying between 0.8 and 4.2m wide, have yielded drill
results between 1.5 and 13.6g/t Au. A resource
containing about 35,000oz of gold has been
estimated. Regional epidote–chlorite–magnetite
alteration is overprinted by quartz–sericite–illite
alteration
associated
with
NW-trending
quartz–pyrite–stibnite–realgar–clay shears. The best
gold in the Saki Prospect area occurs in localised
dilatant sites such as intersections with cross-veins in
quartz–adularia–pyrite veins, which are commonly
overprinted by poorly mineralised carbonate–base
metal veins.
122
North of the Auga River, the silicification and veins at
Mt Sen are inferred to represent a continuation of the
Tolukuma and adjacent Kimino mineralised trends.
Mineralised high sulphidation alteration with
anomalous gold and mercury has been identified at
the Yemi Prospect, 10km east of Tolukuma, and traced
for several kilometres south along strike along the host
Dykoku structure.
Resources and potential
The Tolukuma vein was initially mined (1995) by
open pit and then underground in the early mine life,
followed by the rich Tolimi vein (1998), while recent
(2003) production has been from the Gulbadi and
newly opened 120 vein. The mine has been
consistently producing about 7000oz of gold per
month since mid-2003. Total production to mid
2004 has been 986,438t yielding 514,969oz of gold
and 1,821,397oz of silver. Ore reserves at June 2004
stood at 349,000 tonnes grading 18.15g/t.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
UMUNA (MISIMA ISLAND)
Location and status
The Umuna Au–Ag deposit (lat. 10o40’S, long.
152o48’E) lies in eastern Papua New Guinea on
Misima Island in Milne Bay Province. Placer-Dome
managed the open-pit mining operations until closure
in May 2004.
History
Alluvial gold was identified on Misima Island in 1888
by miners departing from nearby Sudest Island.
Although the Umuna Lode was discovered in 1904,
most of the 200,000 ounces of gold produced to 1911
was from alluvial workings (Lewis and Wilson, 1990).
Production from some of the pre-World War II hardrock mines on Misima (Williamson and Rogerson,
1983) is listed in Table 8.6.
Mining at Misima ceased as a consequence of the
Japanese invasion in 1942, although small-scale
mining supplemented post-World War II exploration
activities. Bill Bloomfield and George Buchanan
extracted 52.8t of ore grading 20.2% Pb, 36.1% Zn,
28.3g/t Au and 91.8g/t Ag from number 7 level in the
Umuna Mine in 1951 (Williamson and Rogerson,
1983). In 1959–69, an exploration joint venture
between Pacific Islands Mines and Cultus
Explorations Ltd unsuccessfully attempted to assess
high-grade ore below the old Umuna workings using
two adits and diamond drilling, the latter of which
yielded only poor recovery (Lewis and Wilson, 1990).
However, assay results from regional stream sediment
samples indicated numerous base-metal anomalies.
Noranda Australia Ltd joined the joint venture from
Mine
Period
1969 to 1972 and drill tested (15 diamond-drillholes)
a porphyry copper target that provided an estimated
resource of 70Mt at 0.1–0.16% Cu. The tenement
then fell vacant until 1976 when it was taken up by
Peter Macnab, who in 1977 formed a joint venture
with Placer (PNG) Pty Ltd. CRA Exploration farmed
into the joint venture in 1978. Placer carried out
intensive drilling of the Umuna Lode and by 1982
bought Macnab’s interest, and in 1985 CRA
withdrew. By December 1987, Placer’s subsidiary,
Misima Mines Pty Ltd, had reached an agreement
with the government to proceed with development.
Under provisions of the day the government took a
20% equity provision in the venture (Lewis and
Wilson, 1990). Open-pit mining commenced in
mid-1989 and continued until 2002 after which
recovery of gold was from low-grade stockpiles until
closure in May 2004.
Geological setting
The oldest rocks on Misima are Cretaceous to
Paleogene metamorphic rocks, which can be subdivided into the western Awaibi association and the
younger overthrust eastern Sisa association that is host
to the gold and copper mineralisation (Williamson
and Rogerson, 1983; Fig. 8.35). The Awaibi
association comprises stratified metabasic rocks
similar to the core complexes on the D’Entrecasteaux
Islands. The Sisa association has been divided
(Williamson and Rogerson, 1983; Lewis and Wilson,
1990) into several units, which form a sequence from
top to bottom as:
•
Umuna Schist - micaceous and carbonaceous
schist.
Production
Au (oz)
Au
Ag (oz)
Ag
grade
grade
(g/t)
(g/t)
Misima Gold Mines
1914–22
37,000
not recorded
11.2
New Misima Gold Mines
1928–35
19,000
not recorded
Cuthberts Misima Gold Mine
1935–42
47,900
140,000
7.3
21.4
Gold Mines of Papua
1933–39
7,700
22,000
5.9
16.7
7.75
Table 8.6 Selected pre-World War II production from Umuna Lodes.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
123
Mineral Projects and Mines (cont.)
•
•
•
Halibu Schist - marble and graphitic schist.
Ara greenschist - massive to foliated metabasalt
comprising amphibole, albite, quartz, chlorite,
magnetite, and biotite.
Bulpat Schist - metapelite and metapsammite
with lesser volcanic units metamorphosed to
greenschist facies.
The metamorphic rocks are intruded by
sheet-like bodies and stocks of undeformed Boiou
Microgranodiorite intrusions, which have provided an
11.3±0.6Ma 40Ar/39Ar age (Adshead, 1997), and
8.1±0.4Ma zircon age (Adshead and Appleby, 1996).
Younger thrusts and high-angle normal faults also
transect these intrusions.
A post-mineral Quaternary to Recent sequence of
basal conglomerate, sandstone and coralline limestone
contains some detrital gold.
Geology and mineralisation
The Umuna Fault Zone occurs as NNW-trending,
3km long 50–500m wide fault and breccia zone with
a normal displacement of 200m.
It has a
configuration similar to a dilational fault jog
characterised by down drop within the dilatant
segment, formed by a component of dextral strike-slip
movement on the bounding regional WNW-trending
fault system (Corbett and Leach, 1998; Adshead,
1997). Brittle deformation on Misima is partly
associated with the post-5Ma extension of the
Woodlark Basin rift.
Two phases of magmatic arc mineralisation are
apparent on Misima Island. Early porphyry copper
skarn occurrences are related to the commonly sheeted
Miocene Boiou Microgranodiorite porphyry
intrusions. Endoskarns occur at the contact between
intrusions and carbonate (marble) units within the
Sisa association host rocks. Typical skarn mineralogies,
paragenetic sequences and zonations (Adshead, 1997;
Adshead and Appleby, 1996) are characterised by
wollastonite–grossularite proximal to the intrusion
grading to more distal andradite–diopside-dominated
mineral assemblages, which are overprinted by
magnetite–pyrite–chlorite, while later retrograde
chalcopyrite–calcite–chlorite–haematite–epidote–
sphalerte accounts for sub-economic (<0.3%) copper.
This mineralisation is of Miocene age.
Epithermal Au–Ag mineralisation is classed as two
styles within the Umuna Lode and adjacent greenschist
(Jones, 1991). The Umuna Lode style comprises
multiphase extensional breccia banded vein infill of
quartz and carbonate with associated pyrite, galena,
sphalerite, barite and minor tetrahedrite. This
mineralisation can be classified as low sulphidation
carbonate–base metal–Au–Ag style, with associated
additional quartz, possibly as a transition to
an adularia–sericite-style vein–breccia system.
Mineralisation displays many characteristics of
carbonate–base metal deposits, including pronounced
vertical zonation. Banded quartz and quartz replacing
carbonate textures predominate in the upper portions,
passing to breccias at depth.
Surface
mineralisation
is
associated with manganese wad
derived from the weathering of
rhodochrosite, a common
constituent of carbonate–base
metal deposits, along with base
metal sulphides. Also typical of
this type of deposits (Corbett
and Leach, 1998), the sphalerite
at Misima varies from Fe>Zn
pale sphalerite at high levels to
dark Zn>Fe at depth (Adshead
and Appleby, 1996).
Fig. 8.35 Geological relationships at Misima (after Lewis and Wilson, 1990; Corbett
and Leach, 1998).
124
Much of the silver is reported
to have been derived from
oxidation of argentiferous
minerals associated with galena,
while alluvial gold yielded a
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Mining on the Umuna Lode in 1989.
Umuna Lode mineralisation characterised by banded quartz
with open vughy manganese wad typical of carbonate–base
metal–gold deposits.
fineness of 795–827 (Williamson and Rogerson,
1983), but this may have been depleted in silver.
Much of the vertical silver zonation — from Ag:Au
ratios of 15:1 to as much as 40:1 in the upper
portions, grading to 3:1 in some deeper intersections
(Jones, 1991) - may be attributed to silver enrichment
during oxidation and supergene enrichment (Lewis
and Wilson, 1990), and is most pronounced in the
upper portions. Vein and mineralogy style, as well as
the 240–275oC fluid-inclusion temperatures from
quartz and sphalerite (Williamson, 1984), further
support the epithermal crustal regime for this
mineralisation.
The Ara greenschist hosts quartz and carbonate veins
with higher base metal and copper values that display
Au:Ag ratios of 3-4:1, are also of carbonate–base metal
style.
Discussion
Sericite alteration within the wall rocks hosting the
epithermal Au–Ag mineralisation has been dated at
4.0–3.2Ma (Adshead, 1997). Thus, the exposure of
deeper level porphyry copper skarn mineralisation at
the same crustal level as epithermal mineralisation is
accounted for by the differing Miocene (11–8Ma) and
Pliocene (4–3.2Ma) ages of the two mineralising
events. The extensional regime enabling the
localisation of epithermal mineralisation may be
attributable to the opening of the Woodlark Basin in
Pliocene times.
Resources and potential
Production began in 1989 with a published (Lewis and
Wilson, 1990) resource estimate in 1988 of 55.9Mt at
1.38g/t Au and 21.0g/t Ag using a 0.7g/t Au cut off of
(2.5 million ounces of gold, 37.9 million ounces of
silver). To the end of May 2004, the mine had
produced a little over 3.7 million ounces of gold and
18.4 million ounces of silver.
Misima mine re-vegetation, October 2003.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
125
Mineral Projects and Mines (cont.)
WAFI
History
The Wafi Cu–Au prospect is a complex hydrothermal
system comprising porphyry (Golpu prospect) as well
as high and low sulphidation Cu–Au mineralisation,
often with overprinting alteration relationships (Erceg
et al., 1991; Funnell, 1990; Ryan and Vigar, 1999;
Leach, 1999; Leach and Erceg, 1990; Tau-Loi and
Andrew, 1998; Corbett and Leach, 1998).
• the Golpu copper–gold porphyry deposit has an
estimated resource based on historical drill data of
114.25Mt @ 1.43% Cu and 0.72g/t Au;
There is a report of gold having been worked in the
Wafi River by Spec Warton in the 1930s. The CRAE
Star porphyry copper search, which did not assay the
samples for gold, identified several low-order base
metal anomalies in the Watut Valley during a 1967
program that led to discovery of the Wamum
porphyry copper deposit. Follow up of these
anomalies in 1977 led to the identification of a pyritic
float boulder which assayed 22g/t Au, 0.57% Pb and
89g/t Ag. Detailed ground follow up by Peter
Macnab and Steve Shedden in 1979 led to
identification of altered pyritic volcanics of the Wafi
high sulphidation gold system, from which initial rock
chips assayed 3.25g/t Au on the eastern side of Wafi,
and 6.2g/t Au on the western side. Further surface
exploration in 1982–83 defined Zone A as a soil
anomaly and drill testing was initiated. By 1984, a
soil auger program had defined a >0.2ppm Au
anomaly of 2.3sq km, which included Zones A, B, C
and D (Fig. 8.36). Ground and airborne geophysics
were also carried out.
• the Wafi gold epithermal prospect has resources of
72.2Mt @ 2.72g/t Au.
By 1986, CRA had completed 31 drillholes, but the
low gold grade refractory sulphide ore typical of the
Location and status
The Wafi Prospect is 60km SW of Lae (lat. 6o53’S,
long. 146o27’E) centred on a N–S ridge on the
southern side of the Wafi River. At the time of writing
(2004), the prospect was being actively drill tested by
the owner, Wafi Mining Ltd, a wholly owned
subsidiary of Harmony Gold from South Africa. The
current resource estimates at Wafi are:
Wafi Gold and Golpu deposit - looking north
126
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
depth (e.g. DH WR 147 included 32m at 2.08g/t Au
from 470 m; Ryan and Vigar, 1999). Two follow-up
drillholes established the ‘Link Zone’ with assays
including 20m at 8.5g/t Au and 70m at 11.0g/t Au
(DDH WR 154), and 86m at 5.7g/t Au and 46m at
4.3g/t Au (DDH WR 154). Definition drilling by
CRA proceeded until August 2002 when ownership
passed to Aurora Gold NL, and then to Abelle Ltd in
February 2003, and finally ended with Harmony after
their takeover of Abelle.
Although the Link Zone mineralisation is very deep,
this low sulphidation style has significantly better
metallurgical characteristics and higher gold grade
than the high sulphidation system, and so significantly
changed the exploration potential of the project.
Potential for additional mineralisation of this
(carbonate–base metal–gold) style is evident from the
extensive untested zinc anomaly apparent in the
CRAE database (Tau-Loi and Andrew, 1998).
Geological setting
Fig. 8.36 Wafi geological relationships and prospect locations.
Wafi high sulphidation mineralisation led CRA to
joint venture the project to Elders Mining (PNG) in
1988.
Although Elders immediately commenced to drill an
additional 54 diamond-drillholes into Zones A and B,
the joint venture agreement required Elders to
complete four conceptual drillholes. A model
provided by a consultant derived from a synthesis of
interpreted structure with the available alteration data,
suggested testing the upflow source of the high
sulphidation fluid (Corbett, 1990). In late 1990,
Elders as operator of the joint venture completed
drillholes WR 92 and 95 and identified the Golpu
(formerly Rafferty’s) porphyry Cu–Au deposit.
However, Elders was taken over by Carter Holt
Harvey which elected to sell Elders 45% stake in Wafi,
and CRA exercised its pre-emptive right and resumed
total ownership of the property. For the next several
years, CRA evaluated the Golpu porphyry.
Wafi lies within a corridor of N–S structures that in
part divide the western and eastern portions of the
New Guinea Orogen (New Guinea Thrust Belt and
Owen Stanley Thrust Belt), and is immediately north
of the Aure Deformation Zone. One of the most
intense of these structures, the Wafi Structure,
localises the Wafi hydrothermal system and provides a
dextral offset on the Ramu–Markham Fault. Thus, a
major structural corridor may have facilitated
overprinting of intrusion-related hydrothermal events.
Host rocks at Wafi comprise sandstone, siltstone,
mudstone and locally conglomerate of the Cretaceous
In late 1996, definition drilling of the margins of
‘Zone A’ identified low-grade gold mineralisation at
Silica–alunite-altered milled matrix (diatreme) breccia.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
127
Mineral Projects and Mines (cont.)
Owen Stanley Metamorphic Complex. These are cut
by a 800m x 400m maar–diatreme breccia pipe with
steep-dipping margins and comprising a polymictic
milled matrix breccia. Several phases of xenolithbearing Wafi dacite porphyry intrusions cut the milled
matrix breccia about the pipe margins, as possible
endogenous domes. The late-mineral Heking
Andesite cuts the dacite porphyry. Potassic alteration
within the non-outcropping Golpu diorite porphyry
is dated at 14Ma and is cut by the diatreme breccia,
which is inferred by many workers to be associated
with the same later intrusion event as the epithermal
gold mineralisation, which yielded a date of 13Ma
from alunite within the high sulphidation alteration
(Tau-Loi and Andrew, 1998).
Fig. 8.37 High sulphidation alteration zonation at Wafi.
Golpu stockwork quartz veins within diorite porphyry
showing intense silica–sericite–pyrite overprint.
Geology and mineralisation
The three distinct styles of alteration and
mineralisation apparent at Wafi (Fig. 8.37) are
discussed below.
The Golpu porphyry Cu–Au system is associated with
a cylindrical porphyry intrusion, exceeding 900m
vertical extent and measuring 300m at the widest
point. The intrusion displays typical porphyry-style
alteration which is zoned in time and space, from
inner and early potassic (biotite–quartz–magnetite±Kfeldspar), grading to marginal inner propylitic
(actinolite–epidote),
and
outer
propylitic
(epidote–chlorite), which are overprinted by phyllic
(silica–sericite–pyrite) alteration. A and D style
quartz veins (in the classification of Gustafson and
128
Hunt, 1975), occurring at the contact between the
intrusion and host metasediments grades up to 1% Cu
and 2g/t Au, but decreasing in tenor away from the
intrusion contact. Changes in Cu–Au grade occur as
a result of the overprinting of the original porphyry
Cu–Au mineralisation by high sulphidation
epithermal alteration, after which, this composite
system has then been subject to supergene processes
(Ryan and Vigar, 1999). Zonation is apparent from
the surficial younger down to older deeper levels, as:
• no surface geochemical manifestations of the
porphyry within the barren quartz–alunite cap
have been recognised;
• supergene copper enrichment grading 2.5–3.5%
Cu extends from below the base of oxidation at
100m below surface to 250m depth, as a severalmetre-thick chalcocite–diginite high-grade zone,
underlain by transition zone silica–alunite-altered
porphyry containing covellite–enargite–pyrite
(chalcocite–tennantite). Here, porphyry Cu–Au
mineralisation is overprinted by high sulphidation
alteration and remobilised mineralisation;
• the upper portion of hypogene phyllic–argillicaltered porphyry extending to a depth of 350m
yields grades of 1–2% Cu and 0.3–0.8g/t Au in
association with pyrite–covellite;
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
Wafi during 2003, showing drill roads which cut Zone B (left), Zone A (right) and overlie the Link Zone buried in the lower
portion of the photo. The camp is to the left, and alteration associated with the Link Zone is in the background.
• at the deepest levels where the potassic alteration
is preserved, pyrite–chalcopyrite±gold±bornite±
molybdenite ores yield 1–3% Cu and 1–2.5g/t Au,
although much of this pyrite is interpreted to have
been introduced during the later high sulphidation
event. Native gold occurs as minute inclusions
within chalcopyrite and bornite.
The Zone A and B style high sulphidation epithermal
gold mineralisation overlies and overprints the Golpu
porphyry Cu–Au, the contact being discernible in the
change from >1% as within the advanced argillic
alteration, to <0.1% as within the underlying
hypogene porphyry. The zoned high sulphidation
alteration is manifest as initial advanced argillic
grading to argillic alteration (Erceg et al., 1991; Leach
and Erceg, 1990; Leach, 1999; Corbett and Leach,
1998). This zoned alteration has been produced by
the progressive cooling and neutralisation through
rock reaction of hot acid fluids, which may have
entered the systems from the vicinity of the Golpu
porphyry, and grades outwards from the silica cap
The Geology and Mineral Potential of
PAPUA NEW GUINEA
through assemblages dominated by alunite,
pyrophyllite–dickite, dickite–kaolinite, with an
illite–chlorite assemblage being the most distal.
Mineralisation of the Zone A–B style overprints
alteration as fine-grained arsenic-rich pyrite, in which
fine (1–3µm) refractory gold is encapsulated in the
lattice, and is associated with minor enargite–luzonite.
Several mineralisation zones defined from early soil
sampling cluster about the diatreme breccia, which
provided original permeability and a locus of
enhanced fluid cooling, aided by mixing with
groundwater.
The high sulphidation alteration overprints the Golpu
porphyry style alteration and mineralisation, and
the diatreme that hosts fragments of the earlier
porphyry alteration. It is interpreted that the high
sulphidation event has remobilised pre-existing
porphyry-related copper from the phyllic–argillic
altered porphyry and deposited this as
zoned enargite–tennantite–covellite–chalcopyrite
mineralisation, while most gold was introduced in
129
Mineral Projects and Mines (cont.)
style mineralisation to fluids associated with lower
sulphidation style mineralisation.
Discussion
Quartz–pyrite vein with later rhodochrosite overprint.
association with pyrite of the high sulphidation event.
The Link Zone low sulphidation gold mineralisation,
localised at the diatreme margin between
and below Zones A and B, is characterised
by veins comprised mainly of pyrite with
lesser quartz (quartz–sulphide–gold style), which
are overprinted by more than one generation
of
pyrite–sphalerite–galena–carbonate
veins
(carbonate–base metal–gold style). Selective sampling
and petrology have demonstrated that while the gold
occurs within pyrite (of the quartz–sulphide event) as
the main sulphide, marcasite and arsenean pyrite are
also present, providing a correlation between arsenic
and gold. The gold content of the lower arsenic-poor
carbonate–base metal event is ‘volumetrically
insignificant’ (Ryan and Vigar, 1999; Leach, 1999).
As is evident from the initial discovery drillholes, gold
grades are significantly higher than in either of the
earlier events. Wall-rock alteration to the veins is
typically an illite–smectite–carbonate–chlorite
assemblage.
In the model proposed (Leach, 1999), the high
sulphidation fluid (pH 2 and >250oC) has been
progressively cooled and neutralised (to pH 6–7 and
<100–150oC) at the diatreme margin and so
alteration
passes
from
advanced
argillic
(alunite±pyrophyllite), to intermediate argillic
(dickite–kaolinite±sericite–illite), and thence argillic
(carbonate–smectite–chlorite), and is accompanied by
a zonation in sulphide mineralisation from enargite
through luzonite, tennantite, to pyrite and base metal
sulphides. Gold mineralisation resulted from fluids
that changed from those typifying high sulphidation
130
Wafi is interpreted to represent a porphyry Cu–Au
system that has been overprinted by epithermal
mineralisation, which at a diatreme margin passes
from
high
to
low
sulphidation
gold
(quartz–sulphide–gold with minor carbonate–base
metal–gold) and so demonstrates a progression to an
ore style that has the ability to substantially change the
economics of the project. At present, this higher
gold grade, metallurgically simpler low sulphidation
mineralisation
has
been
identified
in
only one location (Link Zone) marginal to the high
sulphidation system. The presence of auriferous
sphalerite–galena–quartz–carbonate veins at the
Nabonga Creek Zone and the extensive zinc soil
anomaly shown on the published CRA data (Tau-Loi
and Andrew, 1998) demonstrate that this low
sulphidation gold mineralisation (Link Zone style)
could be more widespread than is thought at present.
Curiously, despite 25 years of exploration, the source
of the original 1977 lead-anomalous float sample has
not been identified. This float may be derived from
the Nabonga Creek Zone, or be associated with
alluvial gold that is derived from further upstream.
Since the original prospect identification, discoveries
of additional mineralisation at Wafi have resulted
from the application of conceptual modelling
(Golpu), or occurred during the course of definition
drilling (Link Zone).
Potential
In the immediate prospect area, drill testing of the
Link Zone is continuing. As the understanding of this
mineralisation style, recognised as far afield as
Nabonga Creek, extends the prospective terrane about
the margin of the diatreme breccia, Wafi clearly has
the potential for more ore to be identified by
innovative explorationists. Renewed regional
prospecting may be rewarded because upstream of the
Wafi River local people are working alluvial deposits
in the vicinity of three known gold anomalies — the
Klondike Creek, Bonanza Creek and Tovu Zones.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
WOODLARK ISLAND
Location
Woodlark Island lies well north of Misima in Milne
Bay Province (lat. 9o05’S, long. 152o45’E), and forms
part of the Papuan Islands on the northern rise of the
Woodlark Basin.
History
In 1895, word reached miners on Misima Island that
Lobb and Ede, traders and prospectors, had won a
‘half billy can’ of gold in a creek on Woodlark Island,
and a gold rush began. The Murua Goldfield was
declared that year and by 1897 over 400 miners were
working the area, but that number soon dwindled to
160 by 1898 as the easy, shallow gold was worked, and
disease (malaria) took its toll. Alluvial gold
production is estimated to have reached 42,000oz by
1898, when lode mining had begun (McGee, 1978a).
The first lode-mine workings were at Kulumadau,
followed by Boniavat and Busai, but most had closed
by the end of World War I. Some attempts were made
to restart old mines, and limited tailings treatment
continued to the 1960s (McGee, 1978b; Russell,
1990). Total production prior to World War I is
estimated at 100,000oz of lode gold and 83,000oz of
alluvial gold.
that older auriferous rocks crop out only as inliers
(Fig. 8.38). Basal conglomerate to the coral contains
exotic alluvial gold. The oldest basement rocks are the
Eocene Loluai Volcanics, comprising low-K oceanridge basalts and volcaniclastics (Ashley and Flood,
1981), overlain unconformably by the Early Miocene
Nasai Limestone. This in turn is overlain by the
volcanolithic Early to Mid-Miocene Wonai Hill Beds,
comprising sediments, agglomerate and andesite, and
Mid- to Early Miocene Okiduse Volcanics, comprising
high-K to calc-alkaline epiclastics, porphyritic andesite
to dacite flows, breccias and tuffs with co-magmatic
porphyritic microdiorite and andesite intrusions
(Ashley and Flood, 1981; Joseph and Finlayson, 1991),
dated at between 16.5Ma and 13±0.4Ma (Smith and
Milson, 1984). Most workers associate gold
mineralisation with the porphyritic microdiorite
intrusion suite, which in hand specimen resembles
other mineralised porphyry throughout Papua New
Guinea (Corbett, Leach, Shatwell et al., 1994).
From the 1960s, government attempts to promote
prospecting on the island included geological
mapping, sampling and limited diamond drilling
(Trail, 1967). Exploration programs were carried out
by Esso (porphyry copper), BHP (iron ore) and CRA
(bauxite). CRA initiated gold exploration from 1967
to 1971, followed by BHP from 1978, in JV with
Nord Resources from 1981. In 1990, Highlands Gold
Ltd purchased PA 455 from the BHP–Nord Joint
Venture and carried out an intensive exploration
program there during the early 1990s (Corbett, Leach,
Shatwell et al., 1994).
Geological setting
The regional geology and gold mineralisation on
Woodlark Island are well documented (Stanley, 1912;
Trail, 1967; Russell and Findlayson, 1987; Joseph and
Finlayson, 1991; Corbett, Leach, Shatwell et al.,
1994), from which this text is taken.
Most of Woodlark Island is covered by soft coralgal
limestone of the Pleistocene Kiriwina Formation, such
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Fig. 8.38 Geology of the central Woodlark Island horst block
(after Joseph and Findlayson, 1991; Corbett, Leach,
Shatwell et al., 1994).
131
Mineral Projects and Mines (cont.)
Aeromagnetic data support earlier geological mapping
and aerial photo interpretation to indicate that the
central portion of Woodlark Island is a 12km wide
horst block separated by NE-trending faults from the
marginal down-dropped blocks. Mineralisation at
Kulumadau is localised on one major horst-bounding
structure, while Busai lies on the intersection of NWtrending linears that transect the horst and the margin
of a large circular feature evident on the aeromagnetic
data. Other workings on the rim of this circular
feature display high-temperature alteration,
supporting the speculation that it could represent an
early collapsed volcanic edifice (Corbett, Leach,
Shatwell et al., 1994).
Busai
From 1902 to 1916, the open-pit Murua United
Mine produced approximately 3,500oz of gold from
ore grading 4.3g/t Au. The gold fineness was 771–846
(McGee, 1978a).
Feldspar porphyry intrusions are commonly emplaced
as sills into Okiduse Formation andesitic lavas. A
variety of pre-mineral breccias occur as red-brown
haematite-matrix breccias formed during deuteric
alteration of the lavas. This was followed by fluidised
breccia dykes with rounded polymictic fragments in a
milled matrix containing pyrite and chalcedony,
which are interpreted to represent phreatomagmatic
precursors to mineralisation.
hanging wall to the flatter dipping Main and Blue
Lode Shears. These locally host narrow bonanza gold
mineralisation.
The flat-dipping fracture-controlled mineralisation,
constrained between the lodes, is interpreted to have
formed in dilatant settings created by reverse fault
movement on the steep structures. A sequence of
alteration and mineralisation (Corbett, Leach,
Shatwell et al., 1994; Corbett and Leach, 1998)
includes initial phreatomagmatic fluidised breccias,
overprinted by banded chalcedonic quartz–pyrite with
local jasper as an indication of shallow oxidising
conditions (quartz–sulphide alteration). The
mineralisation is interpreted to have been deposited
from a rapidly cooling dilute (<2wt% NaCl) fluid at
about 280oC. Later carbonate–base metal–gold
mineralisation displays carbonate zonation from
shallow Fe–Mn, to intermediate Mn–Mg–Fe–Ca, and
deeper Ca–Mg carbonates, indicative of the mixing of
collapsing cool, dilute CO2-rich waters, with rising
hot (>250°C) saline (>6wt% NaCl) ore fluids. The
gold at Busai is of 830 fineness and is associated with
carbonate and minor sulphides.
Fluidised milled matrix breccia dyke with later
carbonate–base metal mineralisation.
Two styles of structurally controlled mineralisation are
apparent from the initial diamond drilling (Corbett,
Leach, Shatwell et al., 1994; Fig. 8.39). Sheared
steeply dipping NW-trending lodes occur in the
132
Fig. 8.39 Cross-section through Busai at 735N.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
derived from a porphyry
intrusion
at
depth,
speculated as a potential
source for the ore fluids
(Corbett, Leach, Shatwell et
al., 1994). Furthermore,
phyllic
(sericite–dickite)
alteration occurs at Bomagai
immediately to the north,
where
BHP
reported
covellite and pyrophyllite.
Drill core (top to the left) showing initial fluidised breccia, early chalcedony and later
carbonate–base metal mineralisation.
Busai mineralisation is dated
from illite wall rock
alteration
adjacent
to
mineralised
veins
as
12.2±0.1 and 12.4±1.6Ma
(Russell and Finlayson,
1987).
Kulumadau
Bonanza gold in a section of the Busai Lode.
The steep structures are interpreted to have acted as
feeders for the mineralising fluids, and locally host
bonanza-grade lode gold interpreted to have deposited
from a quenched fluid, while the bulk of the
carbonate–base metal mineralisation is believed to
have been deposited by fluid mixing in the dilatant
flat fractures.
The zonation from Mn–Mg carbonate at Busai, to
manganese carbonate at Federation, 500m north, is
indicative of a north to south fluid flow, possibly
derived from Muniai in the centre of the circular
feature where inner propylitic alteration
(actinolite–epidote–chlorite–carbonate) may be
The Geology and Mineral Potential of
PAPUA NEW GUINEA
From 1901 to 1950 (but mainly to 1918),
Kulumadau produced 77,000oz of gold, primarily
from the Ivanhoe Lode, at an estimated average grade
of 15.9g/t and 776–859 fineness (McGee, 1978a,b).
The Ivanhoe Lode and other adjacent gold showings
all occur in a major horst-bounding structure
characterised by intense shearing, and consequently
the mine closed in 1918 due to difficulties in dealing
with water in the permeable host structure, although
later attempts were made to reopen the mine. The
Ivanhoe Lode contains inclusions of gold of 790–800
fineness within base metal sulphides consisting of
pyrite, galena and sphalerite, with minor chalcopyrite
and tetrahedrite (McGee, 1978a,b; Russell and
Finlayson, 1987).
Several features suggest that Kulumadau
mineralisation occurs marginal to a porphyry
intrusion. Intense aeromagnetic highs correlate with
areas of fracture-controlled magnetite that are
interpreted here to represent porphyry style
potassic–propylitic alteration, which is overprinted by
sericite–clay alteration. Fluid inclusions within quartz
veins in this area have high salinities (7–8 wt% NaCl)
and are associated with anhydrite. Although early
breccias display phyllic alteration, most brecciation at
Kulumadau occurs as intense post-mineral shearing
associated with movement on the graben-bounding
133
Mineral Projects and Mines (cont.)
fault, and has hampered tracing the continuity of the
lodes. Anomalous Cu, Pb and Zn display a concentric
zonation about the inferred porphyry centre.
Boniavat
Anomalous gold values are associated with
quartz–carbonate–base metal veins localised on NWtrending fractures on the margins of a feldspar
porphyry dyke. There are several old workings in the
field which includes the Woodlark King Mine that
produced 9,145oz of gold from 1904 to 1928
(McGee, 1978a,b; Corbett, Leach, Shatwell et al.,
1994). The presence of small amounts of
manganese (Russell, 1990) at the nearby
Little McKenzie Mine, is consistent with a
quartz–sulphide–carbonate–base metal–gold style of
mineralisation for this area.
Discussion
Intrusion-related low sulphidation gold mineralisation
on Woodlark Island occurs as quartz–sulphide–gold
and carbonate–base metal–gold styles within
structurally controlled lodes and also as fracture fill.
These styles grade to localised bonanza-grade
epithermal–quartz–Au–Ag-style
mineralisation.
Many of the early eluvial workings featured ground
sluicing of what were probably areas of supergene gold
concentrated by deep tropical weathering.
Kulumadau post-mineral breccia.
The extensive post-mineralisation cover hampers
regional
prospecting,
although
geological
interpretations have been made using aeromagnetic
data. Most of the 1990s exploration was concentrated
in the vicinity of old workings, which is usually where
there is the most outcrop.
Resources and potential
In May 2002, Auridiam Ltd provided an estimated
resource of approximately 810,000t of mineralisation
grading 5.09g/t Au at Kulamadau, and 530,000t
grading 4.91g/t Au at Busai, using a cut-off grade of
2g/t Au for 365,000oz of gold. In July 2003, this
figure was upgraded to 373,000oz of gold.
Highlands Gold field camp in 1992.
134
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
YANDERA
Location and status
The Yandera porphyry Cu–Au prospect spans 1000m
between elevations of 1600–2600m, and lies 100km
SW of Madang and 18km west of Bundi (lat. 5o45’S,
long. 145o10’E) within the New Guinea Thrust Belt.
History
Yandera was identified during regional geological
mapping by BMR geologists in the late 1950s
(McMillan and Malone, 1960; Watmuff, 1978). It
was explored by Kennecott from 1965 to 1970 and
later (from 1973) in joint venture with Amdex
Mining. BHP farmed in to the joint venture and
explored the area from 1974–5. Major exploration
ceased by the late 1970s after an economic resource
could not be defined. Initial prospecting consisted of
extensive regional stream traversing as well as handdug contour trails in the project area and culminated
in over 25,000m of diamond drilling. Triako
Resources inherited the project from Amdex but being
unable to maintain interest in the low-grade project
surrendered it in 1992. An EL over Yandera was then
taken up in 1992 by Highlands Pacific who compiled
the existing data and carried out an aeromagnetic
survey and some regional exploration for gold, but
surrendered the EL in June 2003.
Geological setting
Yandera lies within the New Guinea Thrust Belt,
which is part of the New Guinea Mobile Belt of Dow
(1977). The region is characterised by extensive
deformation on regional WNW-trending major
structures which parallel the Bundi and
Ramu–Markham Faults. High-grade gold lode
mineralisation in association with barely unroofed
porphyry Cu–Au intrusions at Kainantu lies some
70km along strike to the ESE.
An obducted fragment of oceanic crust to the north of
Yandera hosts the Ramu Ni–Co laterite deposit.
Yandera occurs within the batholithic Mid-Miocene
(13.5Ma; Page and McDougall, 1972a) Bismarck
Intrusive Complex (Bain and Mackenzie, 1975), and
comprises tonalite, granodiorite and quartz
monzonite typical of island arc calc-alkaline intrusions
(Watmuff, 1978). The 51km x 19km batholith has
been emplaced into Goroka Formation rocks (Bain
The Geology and Mineral Potential of
PAPUA NEW GUINEA
and Mackenzie, 1975), which were metamorphosed
in the 35–20Ma period (Page, 1976), prior to 2.5km
of uplift since the Pliocene (Watmuff, 1978) and
subsequent erosion that has only barely unroofed the
Bismarck Intrusive Complex (Titley et al., 1978). The
pronounced uplift is evidenced by the presence of Mt
Wilhelm 45km SW of Yandera, which at 4,509m is
the highest point in Papua New Guinea. There may
be an association between rapid uplift and mineralised
intrusion emplacement.
The porphyries at Yandera are emplaced into the older
Bismarck Intrusive Complex along a NW-trending
system traceable for about 10km, oblique to the strike
of regional structures (Titley et al., 1978). While
some mineralisation may have been introduced by an
early Maramuni Event (12.5Ma; Page and
McDougall, 1972a) associated with the Bismarck
Intrusion Complex, it is most likely that the porphyry
Cu–Au prospect is related to younger intrusions
dated at 8–7Ma (Page and McDougall, 1972a) or
6.6Ma (Grant and Nielson, 1975).
Geology and mineralisation
From an analysis of detailed accounts of Yandera
geology (Grant and Neilson, 1975; Titley et al., 1978;
Watmuff, 1978), it is possible to suggest a sequence of
events in which tectonism, intrusion, alteration and
mineralisation are all interrelated.
The older porphyries intruded the Bismarck
Granodiorite (12.5Ma; Page and McDougall, 1972a)
under conditions of right lateral deformation (Titley
et al., 1978) and compression from the north (Dow et
al., 1974; Dow and Dekker, 1964; Fig. 8.40). These
intrusions occur as quartz diorite porphyry and related
microquartz diorite porphyry emplaced into NWtrending shear fractures, while early barren quartz
veins display orthogonal orientations. Quartz veins
cut secondary biotite–magnetite (potassic) alteration
which is developed as a replacement of primary
hornblende (in the granodiorite) and biotite veining
which has a halo of epidote–chlorite–calcite
(propylitic) alteration.
A slightly younger group of porphyry intrusives occur
as phenocryst-rich, locally ‘crowded’, leucocratic
tonalite and dacite dykes.
No significant
mineralisation was associated with the two intrusive
episodes.
135
Mineral Projects and Mines (cont.)
Potassic alteration is well developed in the areas of
aplitic quartz monzonite plugs as fracture halos,
veinlet and pervasive quartz, orthoclase, biotite,
epidote and magnetite as well as albite (Grant and
Nielson, 1975). Biotite is more widespread than
orthoclase and displays a spatial relationship to
fracture-controlled pyrite–chalcopyrite mineralisation.
Propylitic alteration is developed throughout the
prospect as chloritic alteration of primary hornblende
and biotite with scattered epidote, carbonate and clay
alteration. Some workers have noted a 5km wide
thermal aureole overprinting the granodiorite and
dacite porphyry (Grant and Nielson, 1975).
Phyllic alteration is characterised by quartz–
sericite–kaolin alteration that occurs as selvages to
veins that both overprints, and forms a wide halo to,
the earlier prograde potassic alteration.
Fig. 8.40 The geology of the Yandera Prospect
(after Titley et al., 1978).
Analysis of quartz vein orientations suggests that the
major mineralisation event at Yandera was initiated by
a change in stress orientation (Titley et al., 1978), as
noted more recently in many other Pacific magmatic
ore systems (Corbett and Leach, 1998). Compression
from the NE and rapid uplift were active during the
forceful emplacement of the NE–NNE-trending
Intermediate porphyries. The mineralisation is
associated with aplitic quartz monzonite porphyry
emplaced into the centre of the pre-existing NW
structural grain, and dated at 6.6Ma (Grant and
Nielson, 1975). This central emplacement of later
intrusions is common in many Pacific Rim porphyry
systems (Grasberg, West Papua; Ridgeway, eastern
Australia).
Extensive chlorite-altered milled matrix breccias form
dyke-like bodies up to 1500m long, developed as
intrusion breccias associated with emplacement of
quartz diorite, and represent passive hosts for later
mineralisation (Grant and Nielson, 1975).
Alteration
Alteration and mineralisation at Yandera display a
strong fracture control, extending from the younger
intrusions into the granodiorite host.
136
Mineralisation
Porphyry Cu–Mo–Au mineralisation is strongly
fracture controlled. There is an early phase of fracture
hosted and disseminated mineralisation characterised
by pyrite–chalcopyrite in association with potassic
alteration (biotite>orthoclase). This phase commonly
grades to only 0.3% Cu (Grant and Nielson, 1975).
Younger, structurally controlled mineralisation occurs
as 1–2mm veinlets of chalcopyrite with lesser bornite,
pyrite and magnetite with selvages of
biotite–chlorite–epidote, or as 20–100mm veins of
pyrite–chalcopyrite with quartz, chlorite, epidote and
carbonate gangue (Watmuff, 1978; Grant and
Nielson, 1975). This younger vein mineralisation
ranges in grade from 0.4–1% Cu, with the best
mineralisation occuring where the two styles are
coincident in areas of greatest fracture intensity.
Molybdenite is less abundant than the copper
sulphides and has an erratic distribution. The zones
with the greatest concentration of pyrite are associated
with retrograde quartz–sericite–clay–chlorite–pyrite
alteration developed peripheral to and overprinting
thin fracture/veins of copper sulphides. There is a
broad pyritic halo to the entire deposit. Sphalerite
occurs as an accessory in the fracture/veins with
chalcopyrite but galena is rare.
The zonation at Yandera is not as well developed as it
is in many other porphyry copper deposits. An inner
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Mineral Projects and Mines (cont.)
zone of potassic (biotite–orthoclase) alteration gives
way to a broad propylitic (chlorite–carbonate)
alteration that is overprinted by a broad phyllic
(quartz–sericite–clay–pyrite) alteration.
Metal
zonation is basically copper core grading to more
marginal Zn>Pb soil geochemistry (Fig. 8.41).
Bornite is not a major constituent and is restricted to
the centre of the deposit (Grant and Nielson, 1975).
There is no obvious correlation between
mineralisation and the so called ‘quartz core’ which
may predate emplacement of the main mineralising
intrusion.
Discussion
Many features suggest that although the
Yandera porphyry Cu–Au deposit occurs within a
batholith, only the upper part of the
hydrothermal–mineralisation system related to later
intrusions is exposed. There is a strong fracture/vein
control suggesting upwards transport of ore fluids
from the main buried intrusion body, evident at the
level of exploration as the aplitic quartz monzonite
stocks and dykes. The higher temperature potassic
(biotite–orthoclase–magnetite) alteration displays
fracture control within the host granodiorite, spatially
associated with the quartz monzonite intrusions,
while propylitic alteration is more widespread. The
pyrite–chalcopyrite–chlorite–carbonate–epidote veins
and strong accessory sphalerite are also consistent with
a higher level setting marginal to the source intrusion,
with a bias towards a propylitic rather than potassic
alteration assemblage. Collapsing late-stage phyllic
(quartz–sericite–pyrite–chlorite–clay) alteration is
typically associated with low pH fluids, which may
promote sulphide deposition by mixing with ore
fluids.
Fig. 8.41 Alteration and geochemistry of the central Yandera
Prospects; see Fig. 8.40 for location (after Grant and Nielson,
1975).
targets during a program of regional stream sediment
sampling by Highlands Pacific and an aeromagnetic
survey by a joint venture partner. Yandera was
essentially bypassed by the 1980–90 gold exploration
boom.
Resources and potential
In recent times, local villagers have been working
alluvial and eluvial gold in and around the Yandera
prospect, further enhancing the possibility that
peripheral structures could host Irumafimpa-style
quartz–sulphide vein mineralisation, which might
enhance the economics of the prospect.
Yandera was discovered early in the porphyry Cu–Au
era (later 1950s) and most exploration was completed
prior to the 1980s. The 1970s exploration program
enabled a resource estimate for Yandera of 338Mt
grading 0.42% Cu, 0.1g/t Au and 0.018% Mo, of
which 124Mt is indicated and 214Mt inferred
(Watmuff, 1978; Titley et al., 1978). This estimate
predates the current JORC classification use of these
terms. Exploration since then appears to have been
restricted to the identification of some peripheral gold
If the magmatic source for structurally controlled
mineralisation observed at Yandera lies buried, and if
only the upper part of the hydrothermal system is
well exposed, then potential may exist for the
discovery of additional, possibly structurally
controlled mineralisation at depth. As Yandera was
evaluated in the 1970s, the application of new
concepts for porphyry Cu–Au deposits that have
evolved over the past 20 years may prove worthwhile.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
137
9. Environment
- Level 2B Permit, plus EIA if of national
importance.
- Level 3 Permit and EIA.
ENVIRONMENT ACT 2000
During the 1st quarter of 2004, the Environment Act
came into force from the 1st January 2004. The new
Environment Act has amalgamated the previous
environment acts, namely the Water Resources Act,
the Environmental Contaminants Act and the
Environmental Planning Act. The Environment
Council is yet to be established to fully implement the
new Act.
Some of the basic ‘mechanics’ surrounding the new
Environment Act 2000 are summarised below from a
speech made in 2004 by Mr Vincent Bull of the law
firm Allens Arthur Robinson.
General Environmental Duty
"A person shall not carry out any activity that causes
or is likely to cause environmental harm unless he
takes all reasonable and practicable measures to
prevent or minimise the environmental harm."
Permitting Process - EIA
1.
2.
3.
4.
5.
6.
7.
8.
9.
Register intention to carry out preparatory work
Receive notice to undertake EIA
Submit Inception Report
Submit Environmental Impact Statement •
Public review
•
Environment Consultative Group
•
Provincial Environment Committee
Director’s acceptance of EIS
Environmental Council’s acceptance of EIS
Approval in Principle by Minister
Application for Environment Permit
Grant of Permit by Director
Permitting Process - NON EIA
Level 2B - if no EIA required:
Administrative Structures
•
•
•
•
•
Director of Environment
Environment Council
Environment Consultative Group
Working Committees
Provincial Environment Committees
Tools - Admin & Enforcement
•
•
•
•
•
•
•
•
•
•
Environment Policies
Environmental Codes of Practice
Provincial Environment Policies
Environmental Impact Assessments
Environmental Audit and Investigations
Environment Improvement Plans
Environment Protection Order
Clean-up Order
Emergency Direction
Warning Notice
1.
2.
3.
4.
5.
6.
7.
8.
Register intention to carry our preparatory work
Lodge application
Acceptance of application by Director
Referral to NG, PG, LLG by Director
Notification of application (radio, newspaper)
If objection - Conference of interested parties
Independent expert
If cannot agree on independent expert Environment Council tie breaker
9. Assessment
10.Grant of permit
From acceptance to grant - 90 days
Level 2A - Fast Track
1.
2.
3.
4.
Lodge application
Application accepted by Director
Assessment of application
Grant of application
Environmental Permits
From acceptance to grant - 30 days
• Carrying out an activity
• Existing activity or new?
• What level of activity? -
Example:
- Level 1 No permit required.
- Level 2A Permit, but no EIA.
138
• Drilling contractor signs contract to drill 2000m
on EL.
• Driller is "carrying out activity" so caught by the
Act.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Environment (cont.)
• Activity is Level 2A.
• Does EL holder have Permit? He should, but
if not;
• Driller must apply for a Permit or risk prosecution.
• Can use fast track for Level 2A.
• Should take 30 days or less from acceptance of
application.
• If proceeds without Permit: -
Emergency Direction to stop
Environment Protection Order
Clean up Order
Criminal penalty of up to K250,000 for
company and K50,000 for company officers /
imprisonment
Further information on any of these matters can be
obtained from:
The Director
Department of Environment & Conservation
P.O Box 6601
Boroko, N.C.D
Papua New Guinea
Tel: (675) 3250180
Fax: (675) 3250182
The Department of Environment & Conservation
previously administered the Environmental Planning
Act, which deals with environmental impact
assessment; Environmental Contaminants Act which
deals with prevention, abatement and control of
environmental contaminants: Water Resources Act
which regulates water abstraction and discharges into
water and various other legislation that deals with
nature protection and conservation including:
• National Parks Act.
• Conservation Areas Act.
• Fauna (Protection & Control) Act.
• International Trade (Fauna & Flora) Act; and
• Crocodile Trade (Protection) Act.
Various Approvals or Licences and Permits were issued
under this legislation. Examples of these are Approval
of Environmental Plans, Water Use Permits, Water
Investigation Permits, Permits to export fauna
(protected and non-protected), Licence to discharge
contaminants and others.
Exploration and development of mineral resources has
an impact on the physical and social environment and
therefore these activities require careful impact
assessments.
The Geology and Mineral Potential of
PAPUA NEW GUINEA
139
10. Other Relevant Agencies of Government
INVESTMENT PROMOTION
AUTHORITY (IPA)
The IPA was established by the Investment Promotion
Act. This Act is one of the most important business
laws for any investor in the country. Compliance with
requirements of the IPA is a prerequisite for any
investor intending to engage in mining business in the
country. The IPA’s functions are to:
• provide information to investors in the country
and overseas
• encourage and facilitate investment in the country
by assisting investors to obtain all necessary
licenses, compliances and approvals
• provide a system of certification of a foreign
enterprise and to require that a foreign enterprise
may only carry on business if so certified
• monitor the activities of foreign enterprises and
other functions as specified in the IPA Act.
All foreign enterprises (either an individual or
company) must be certified by the IPA prior to
carrying on business in Papua New Guinea. This
requirement for certification is contained in the IPA
Act. An overseas company that commences business
in Papua New Guinea must also apply for registration
of their business with the Registrar of Companies,
which is an office within the IPA establishment. Both
certification and registration may be done
simultaneously. Foreign investors must also be aware
that there are certain business activities that are
restricted to citizens and national enterprises, and
there are severe penalties for noncompliance with the
Act.
The IPA also administers the Companies Act 1997,
Securities Act 1997, Associations Incorporation Act,
Business Names Act, Business Groups Act and
Trademarks Act. Further information in respect of
doing business in Papua New Guinea can be obtained
direct from the IPA.
DEPARTMENT OF LABOUR &
EMPLOYMENT
This Department administers the Employment Act
which deals with general employment matters and the
Employment of Non-Citizens Act which deals with
work permits for Non-citizens and prohibited
occupations for Non-citizens.
A Training and Localization Program is required
where approval is being sought to recruit non-citizens
to fill positions. A Minimum Wages Board is also set
140
up within the Department and the Board determines
minimum wages in Papua New Guinea. Further
information on these matters can be obtained from:
The Secretary
Department of Labour & Employment
PO Box 5644
Boroko, NCD
Papua New Guinea
Tel: (675) 3217408 3214160
Fax: (675) 3201062
BANK OF PAPUA NEW GUINEA
& FOREIGN EXCHANGE
CONTROLS
The Central (Reserve) Bank of Papua New Guinea
carries out many functions of importance to the
economy, including regulation of financial
institutions in the country and implementation of
Government’s policy on foreign exchange controls.
Further information on foreign exchange controls can
be obtained from:
Controller of Foreign Exchange
Bank of Papua New Guinea
PO Box 121
Port Moresby, NCD
Papua New Guinea
Tel: (675) 3227200
Fax: (675) 3211617
INTERNAL REVENUE
COMMISSION & TAXATION
The Internal Revenue Commission (IRC), headed by
the Commissioner General of Internal Revenue,
administers the Income Tax Act 1959 (as amended),
which contains the tax laws of Papua New Guinea, the
Customs Act, Stamp Duties Act and the Value Added
Tax Act. Different taxation regimes (including tax
incentives) apply to mining and petroleum operations
in the country. The IRC is also responsible for all
customs import and export duties and excise duties.
Further information on the various taxes, including
tax incentives, and customs and excise duties, can be
obtained from:
The Commissioner General
Internal Revenue Commission
PO Box 777
Port Moresby, NCD
Papua New Guinea
Tel : (675) 3226600
Fax: (675) 3214249 / 3213484
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Other Relevant Agencies of Government
(cont.)
APPLICATION OF OTHER LAWS
USEFUL CONTACTS
Various other laws may apply depending on the type
of other activities chosen by the investor and
consequently, the investor would be required to
consult the respective Government Agencies
(including Provincial Governments), or local
Municipal Authorities that administer the legislation,
to obtain further information on their specific
requirements. Besides the Mining Act, there is various
other sectoral legislation which governs various
natural resource development activities. Examples of
these are the Oil and Gas Act administered by the
Department of Petroleum & Energy, which governs
petroleum activities; the Foresty Act administered by
the National Forest Authority, which governs forestry
activities and the Fisheries Management Act,
administered by the National Fisheries Authority,
which governs fisheries activities.
Papua New Guinea Chamber of Mines and
Petroleum
Executive Director
Papua New Guinea Chamber of Mines and
Petroleum
PO Box 1032
Port Moresby Post Office
Papua New Guinea
Fax (+675) 321 2988
Investment Promotion Authority
The Managing Director
E-mail [email protected]
Phone (+675) 321 7311
Fax
(+675) 321 2819
Department of Mining
The Secretary
Phone (+675) 322 7675
Fax
(+675) 321 7958
The Deputy Secretary
Phone (+675) 321 2945
Fax
(+675) 321 7958
Director, Mining Division
Phone (+675) 322 7624
Fax
(+675) 321 3701
Mining Registrar, Tenements Administration
Phone (+675) 322 7615
Fax
(+675) 321 3701
Assistant Director, Project Coordination
Phone (+675) 322 7622
Fax
(+675) 321 3701
Chief Inspector of Mines
Phone (+675) 322 7602
Fax
(+675) 321 7707
Assistant Director, Project Assessment
Phone (+675) 322 7628
Fax
(+675) 321 3701
Assistant Director, Small Scale Mining
Phone (+675) 322 7626
Fax
(+675) 321 3701
The Geology and Mineral Potential of
PAPUA NEW GUINEA
141
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The Geology and Mineral Potential of
PAPUA NEW GUINEA
The Geology and Mineral Potential of PAPUA NEW GUINEA
The Geology and Mineral Potential of
PAPUA NEW GUINEA
Edited by Anthony Williamson and Graeme Hancock