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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 11. 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