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Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
Gold, Silver, and Copper Metallogeny of the Eastern Sunda
Magmatic Arc Indonesia
Metalogeni Emas, Perak, dan Tembaga Busur Sunda
Bagian Timur, Indonesia
Adi Maryono1, Lucas Dony Setijadji2, Johan Arif3, Rachel Harrison4,
and Elang Soeriaatmadja3
PT Buena Sumber Daya, Jln. Radin Inten II No 2, Buaran Duren Sawit, Jakarta Timur
2
Geology Department University of Gadjah Mada, Bulak Sumur, Yogyakarta
3
Newmont Asia Pacific, #388 Hay Street Subiaco, Perth, Western Australia
4
Independent Geologist, Sading Sempidi Badung, Bali
1
Corresponding Author: [email protected].
Received: December 3, 2012; revised: May 16, 2014; approved: August 15, 2014
ABSTRACT
With the recent discovery of another world class gold-silver-copper deposit at the Tujuh Bukit Project
(30.1 million ounces of gold and 19 billion pounds of copper), the Eastern Sunda Arc has continued
to prove itself as an emerging economically important magmatic belt. This paper provides a general
description of the metallogeny of the Eastern Sunda Arc, covering a wide spectrum of topics, from
its tectonic setting, general geology, magmatic evolution, metal endowment and prospectivity,
mineralization styles and implications towards exploration. The Eastern Sunda Magmatic Arc is
constructed on a thinner island arc crust, bounded by the margin of Sundaland to the west and by the
Australian continental crust to the east. As one of five different ages of magmatic belts defined along
the arc, the Neogene magmatic belt is considered to be important as an overwelming number of gold,
silver, and copper deposits and prospects are spatially associated with Late Miocene-Pliocene age
intrusions. The metallogeny of the Eastern Sunda Magmatic Arc is dominated by gold, silver, and
copper which are predominantly contained in porphyry and epithermal deposit types. With a world
class gold-silver-copper endowment of 92.44 million ounces of gold, 279.17 million ounces of silver
and 61.92 billion pounds of copper, the Eastern Sunda Magmatic Arc has emerged as one of the
most prospective gold-copper belts in the world. Porphyry gold-copper and epithermal gold-silver
mineralization styles in the Eastern Sunda Magmatic Arc share similarities to those in typical island
arc settings, e.g. the Philippines. They also display some unique characteristics that are spatially and
genetically associated with ore and its environment and provide selection criteria for prospective
regions and a further basis for construction of exploration models. District and deposit exploration
models are refined on the basis of shared key features of the deposits in the region as guides during
exploration. These key features provide vectors to ore, applicable in identifying the central, proximal,
and distal parts of mineralized systems during exploration activities. Keys to exploration success
include understanding the characteristic features of ore systems, observing key geological features in
the field and determining vectors to ore.
Keywords: Gold-Silver-Copper, metallogeny, Eastern Sunda, magmatic arc, porphyry, epithermal
ABSTRAK
Dengan penemuan terakhir deposit emas-perak-tembaga tingkat dunia pada Proyek Tujuh Bukit
(30,1 juta ons emas dan 19 miliar pon tembaga), Busur Sunda Bagian Timur telah berlanjut dengan
85
Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
membuktikan dirinya sebagai suatu sabuk magma yang tumbuh menjadi penting secara ekonomi.
Makalah ini menyajikan uraian umum metalogeni Busur Sunda Bagian Timur yang mencakup topik
berspektrum luas, dari tataan tektoniknya, geologi umum, evolusi magma, sokongan dan prospek
logam, corak mineralisasi, dan implikasinya terhadap eksplorasi. Busur Magma Sunda Bagian Timur
terbentuk pada kerak busur kepulauan yang tipis, dibatasi oleh tepian Sundaland ke arah barat dan
oleh kerak benua Australia ke arah timur. Sebagai satu dari lima umur sabuk magma yang berbeda di
sepanjang busur, sabuk magma Neogen ini dianggap penting karena memberikan deposit emas, perak,
dan tembaga, dan prospeknya berkaitan dengan intrusi berumur Miosen Akhir - Pliosen. Metalogeni
Busur Sunda bagian Timur didominasi oleh emas, perak, dan tembaga yang terutama terkandung
dalam tipe deposit porpiri dan epitermal. Dengan sokongan emas-perak-tembaga berkelas dunia
dengan 92,44 juta ons emas, 279,17 juta ons perak, dan 61,92 juta miliar pon tembaga, Busur Magma
Sunda Bagian Timur telah muncul sebagai satu dari sabuk emas-perak-tembaga yang paling prospektif
di dunia. Corak mineralisasi emas-perak porpiri dan emas-perak epitermal di Busur Magma Sunda
Bagian Timur mirip dengan tataan tipe busur kepulauan, seperti Filipina. Keduanya juga memiliki
karakteristik yang unik, yaitu secara spasial dan genetis berasosiasi dengan bijih dan lingkungannya, serta memberikan kriteria bagi wilayah prospektif dan juga sebagai basis untuk merekonstruksi
model eksplorasi. Model distrik dan eksplorasi deposit dipertegas sebagai basis corak kunci deposit
pada suatu wilayah sebagai panduan selama eksplorasi. Corak kunci ini merupakan vektor bagi bijih, untuk mengidentifikasi bagian sentral, proksimal, dan distal sistem mineralisasi selama aktivitas
eksplorasi. Kunci bagi eksplorasi yang sukses meliputi pengetahuan pada karakteristik corak sistem
bijih, mengamati corak geologi di lapangan, dan menentukan vektor bagi bijih.
Keywords: emas-perak-tembaga, metalogeni, Sunda Bagian Timur, busur magma, porpiri, epitermal
INTRODUCTION
With the recent discovery of another world
class gold-silver-copper deposit at the Tujuh
Bukit Project, Banyuwangi, East Java, in
addition to another two known world class
deposits, the Eastern Sunda Arc (Figure 1)
has continued to prove itself as an emerging economically important magmatic belt.
Despite legal and social concerns, recent
positive exploration drilling results in Java,
Lombok, and Sumbawa have reinforced the
prospectivity of the arc.
This paper provides a general description of
the metallogeny of the Eastern Sunda Arc
with an emphasis on gold, silver, and copper. The description of the arc covers a wide
spectrum of topics from its tectonic setting,
general geology, magmatic evolution, metal
endowment and prospectivity, mineralization styles and implications to exploration.
The general description of regional and local
scale geology of the arc, given here, aims
86
to define the shared key geological features
related to criteria important for exploration
area selection, and construction of a goldsilver-copper deposit model for the region.
This paper provides exploration criteria for
this region and other regions with similar
tectonic and geologic settings to the Eastern
Sunda Arc. In addition, much recent understanding of the gold and gold-copper deposit
systems in the region relies on publications
on individual deposits; this paper provides
the first comprehensive deposit compilation
for the region.
Regional Perspective and Prospectivity
The islands of Java, Bali, Lombok and
Sumbawa constitute an east-west trending
Eastern Sunda Magmatic Arc with a total
length of about 1,800 km, part of the 3,940
km-long Sunda-Banda Arc extending from
the northern tip of Sumatra Island through
Java to east of Damar Island (Hamilton,
1979; Carlile and Mitchell, 1994; Setijadji
et al., 2006).
Gold, Silver and Copper Metallogeny of the Eastern Sunda Magmatic Arc Indonesia (A. Maryono et al.)
SE ASIA Tectonics
Cenozoic magmatic arc
Chinkuashih
Kyaukpahto
Sin Quyen
Oceanic or young island arc crust
Xepon
Baguio Gold
Santo Tomas II
Dizon
Pre Mesozoic Continental Basement
Sundaland Craton
Luconia Block
Australian Platform and related fragments
Rifted Crust and continental fragments
Lepanto
Guinaoang
Paracle District
Masbate
Atlas Biga
Bulawan
Raub
Lebong Tandai
Deposits
Au
Cu-Au
Kelian
Mount Muro
Lebong Donok
Trench
Trench approximate
Spreading centre
Strike-Slip fault
Pacific Plate
Solomon Sea Plate
Kingking
Tampakan
Mamut
Bau
Bismark Plate
Placer
Siana
Gosowong
Tombulilato
Gunung Pani
Ontong Java Plateau
Messel
Awak Mas
Gunung Pongkor
Wabu
Grasberg
Wetar
Batu Hijau
Simberi
Lihir
Freida
Porgera
OK Tedi
Panguna
Kainantu
Mt Kare
Golpu
Hidden Valley
Tolukuma
Wapolu Gameta
Gold Ridge
Misima
Figure 1. Gold-copper deposits and mineralization systems in the Eastern Sunda arc as one of the most prospective magmatic belts in the Southwest Pacific Region.
The Eastern Sunda Magmatic Arc was constructed on thin island arc crust, transitional
with the margin of Sundaland in the west
and bounded by Australian continental
crust in the east (Hamilton, 1979; Carlile
and Mitchell, 1994; Hall, 2002). It consists
of a chain of islands that have undergone
a similar geodynamic history from Java to
Sumbawa. These islands share similarities
in tectonic setting, regional geology with
dominantly Neogene and Quaternary volcanic rocks, magmatic evolution since the
Late Oligocene, and consistent northward
magmatic migration.
The Eastern Sunda Magmatic Arc is predominantly composed of Oligocene to Quaternary magmatic rocks with widespread
Late Miocene and Pliocene intrusions
exposed along the southern part of the belt.
The belt is considered to be prospective
with several fertile districts identified along
the islands on the basis of lithogeochemical studies (Setijadji et al., 2006; Loucks,
2009).
The Eastern Sunda Arc ranks among the
most endowed magmatic belts in the Southwest Pacific region. The region contains
more than fifteen magmatic belts in fifteen
countries with a total strike length of more
than 21,050 km and total gold endowment
of 744.8 million ounces. About 45.1% or
321.2 million ounces are hosted in Neogene
magmatic arcs and 42.6% or 317.3 million
ounces have been discovered in Indonesia.
The Eastern Sunda Arc contains 92.44 million ounces of gold, second to the Papuan
Fold Belt on Papua Island (Maryono and
Power, 2009).
For explorers, the competitive advantages
of the Eastern Sunda Arc include a proven
record of exploration successes including
the recent discovery of another world class
gold-copper deposit at Tujuh Bukit, existing
infrastructure with the presence of an active
world-class operating mine at Batu Hijau
on Sumbawa, and a copper-gold smelter at
Gresik East Java.
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Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
Regional Tectonics and Geology
The Eastern Sunda Arc is located along
the tectonically active zone that marks the
convergence of three major tectonic plates:
Eurasian, Indo-Australian, and Pacific Plates
(Hamilton, 1979). The western segment of
the arc (West to East Java) developed on
thick continental crust on the southern margin
of Sundaland, whereas the eastern segment
(East Java to Sumbawa) was constructed on
a thinner island arc crust bounded by Australian continent crust further east (Sumba and
Timor) (Hamilton, 1979; Carlile and Mitchell, 1994; Hall, 2002; Setijadji et al., 2006).
The geology of the islands of the Eastern
Sunda Arc is characterized by island arc-type
volcano- sedimentary successions of Oligocene to Quaternary age (Hamilton, 1979;
Carlile and Mitchell, 1994; Metcalfe, 1996;
Garwin, 2002; Setijadji et al., 2006) (Figure
2). Igneous rocks of Paleocene-Eocene age
are thought to be present locally along the
southernmost parts of Java at Bayah dome
Jakarta
(Cikotok Formation), and at the Cikotok
Formation and Jatibarang Volcanic Formation (JVF) in west Java (Hutchison, 1982;
Sujatmiko and Santoso, 1992; Setijadji et al.,
2006). The earliest magmatic activities can
be traced, scattered toward east of Java as far
as Pacitan, western part of East Java (JICAJOGMEC, 2004; Setijadji et al., 2006).
While the Paleocene-Eocene volcanic centers are poorly defined and restricted in the
area, Late Oligocene to Middle Miocene
magmatic rocks are widespread and continuously distributed along the whole belt. Volcaniclastic rocks of Late Miocene to Pliocene
age are more abundant than the older volcanic rocks, following the southern margin of
the belt with a relative northward shift over
time. Low-K calc-alkaline to weakly alkaline
andesitic volcanic and interbedded volcaniclastic rocks, associated low-K intermediate
intrusions and minor shallow water marine
sedimentary rocks extend from Java to Bali,
Lombok and Sumbawa (Maula and Levet,
1996; Garwin, 2002; Setijadji et al., 2006).
JAVA
Bandung
Semarang
Surabaya
Yogyakarta
Legend
Quaternary sedimentary rock
BALI
Mataram
SUMBAWA
Denpasar
Quaternary volcanic rock
Quaternary intrusive rock
Tertiary sedimentary rock
Tertiary volcaniclastic rock
Tertiary volcanic rock
N
Tertiary intrusive rock
Pre-Tertiary metamorphic rock
200 km
Figure 2. Regional geology of the Eastern Sunda Arc (summarised from Hamilton, 1979; Carlile and Mitchell,
1994; Hall, 2002; Setijadji et al., 2006).
88
Gold, Silver and Copper Metallogeny of the Eastern Sunda Magmatic Arc Indonesia (A. Maryono et al.)
The islands display progressively younger
volcanic complexes of Pleistocene to Quaternary age towards the north, with recently
active volcanoes, Mt. Krakatau in westernmost Java, Mt. Agung in Bali, Mt. Rinjani
in Lombok, and Mt. Tambora and Mt. Sangeangapi in Sumbawa Islands. In total there
are more than fifty-six Quaternary volcanoes
along the belt from Java to Sumbawa.
Numerous Eocene to Pliocene (50.9 Ma
to 2.7 Ma) intrusions occur scattered along
the belts from Java to Sumbawa. Gabbro
in Ciletuh, West Java is dated at 50.1-50.9
Ma (Pertamina-ITB, 2002), a dioritic dyke
in Karangsambung, central Java at 37.6 Ma
(Soeria-Atmadja et al., 1994), an andesitic
intrusion in the Pacitan area at 38.7 Ma (JICA-JOGMEC, 2004). Miocene felsic intrusions were recognized include quartz diorite
at Ciletuh - Ciemas (13.7 Ma) and rhyolite
at Cirotan (9.6Ma). At Cineam in West Java,
Widi and Matsueda (1998) reported ages
from 13.5 to 8 Ma for hydrothermal activity related to magmatism and epithermal
mineralization in this area. Further east,
late stage quartz diorite to tonalite dykes at
5.0 to 2.7 Ma have been reported from East
Java, Lombok, and Sumbawa where they
are associated with gold and gold-copper
mineralization (Clode et al., 1999; Garwin,
2002, Maryono et al., 2005).
The Eastern Sunda Arc is segmented by a series of north-northeast trending arc-normal
structures that are evident in topographic
and satellite image data. Tectonic factors
appear to have localized volcanic centres
along the arc-normal structures
RESULT AND DISCUSSION
Magmatic Evolution
In total, the Eastern Sunda Arc consists
of five different ages of magmatic belts:
Pre-Tertiary, Paleocene-Eocene, Oligocene-
Middle Miocene, Late Miocene-Pliocene
(Neogene) and Quaternary (Hamilton, 1979;
Carlile and Mitchell, 1994; Hall, 2002; Setijadji, 2006) (Figure 3). The Arc is defined
by a similar tectonic setting, constructed
on thinner island arc crust, bounded by the
margin of Sundaland in the west and by
Australian continent crust in the east.
Earliest volcanism in the Eastern Sunda
Arc is poorly understood and restricted to
Java. It is thought to have developed during the initiation of the Java Trench in the
Paleocene-Eocene as older volcanic rock
units are restricted to a belt from West to
East Java as far as Pacitan. The earliest
magmatism resulted from the development
of the Western Sunda Arc (Sumatra) that
migrated to the east, in which the resulting
magmatic arc was located along the edge
of Sundaland, from Sumatra through Java,
Sumba, and western Sulawesi (Hamilton,
1979; Carlile and Mitchell, 1994; Hall,
2002).
The spatial evolution of the volcanic arcs
since the Oligocene is better understood
and can be reconstructed. Collision between
West Sulawesi and micro continents in the
Miocene led to change the location of the
subduction system to a more southerly position. The following volcanic activity then
gave rise to the islands of Bali, Lombok,
and Sumbawa. During the Oligocene to
Pliocene, the volcanic centres have shifted
northward (towards the back arc-side). The
shifting distances increase relatively eastwards, and such spatial movement may have
resulted from counter-clockwise rotation of
the volcanic arcs, with westernmost Java
(around Bayah dome) as the rotational pole.
The Early Tertiary volcanic arcs occupied the
southern coast of the island, and perhaps
also offshore to the south, as indicated by a
high gravity anomaly. In the Late Miocene
significant northward migration of volcanic
centres was noteworthy in the eastern part
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Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
Creataceous
Paleocene-Eocene
Oligocene-Miocene
Late Miocene - Pliocene
Jakarta
Bandung
Quaternary
JAVA
Semarang
Surabaya
Legend
BALI
Yogy
a
karta
SUMBAWA
Denpasar
Quaternary sedimentary rock
Fertile Neogene Belt
Mataram
Quaternary volcanic rock
Quaternary intrusive rock
Tertiary sedimentary rock
Tertiary volcaniclastic rock
Tertiary volcanic rock
N
Tertiary intrusive rock
Pre-Tertiary metamorphic rock
200 km
Figure 3. Five magmetic belts of the Eastern Sunda Arc that show consistent northward migration from Early
Tertiary to Quaternary (Hamilton, 1979; Carlile and Mitchell, 1994; Hall, 2002; Setijadji et al., 2006).
of West and Central Java, but not in East
Java. Again during the Pliocene, volcanic
centres shifted northward. The back arc-ward
volcanic shift ended after the Pliocene, and
the trench-ward volcanic shift started in the
Quaternary. The trench-ward shift is demonstrated by the ‘invasion’ of Late Tertiary
volcanic centres by Quaternary volcanoes
in West and Central Java, and the ‘missing’
Pliocene volcanoes in most of East Java. This
may be due to their being completely covered
by Quaternary volcanoes. Radiometric data
from the Quaternary cross-arc volcanic chain
of Merapi-Merbabu- Telomoyo-Ungaran
also suggest that the Quaternary volcanism
gradually moved trench- ward (Kohno et al.,
2005). An exception occurs at westernmost
Java, where Quaternary volcanism (Krakatau
and Danau) migrated back arc-ward due to
the Sunda Strait opening (Nishimura et al.,
1986).
Back arc magmatism only took place since
the latest Miocene to Quaternary. With recent physiography as reference, there are
two main locations of back arc magmatism,
90
i.e., on Java where the current depth of the
subducted slab is around 320 - 350 km
(Quaternary Muria and Lasem volcanoes),
and offshore Java where the current depth
of the subducted slab is around 600 km
(Quaternary Bawean island). Analogous to
the volcanic arc trench-ward shift after the
Pliocene, the locations of back arc magmatism seem also to shift trench-ward from the
Karimunjawa Islands to the Muria-Lasem
volcanoes.
Gold-Copper Endowment and Prospectivity
Total metal endowment of the Eastern Sunda
Arc is dominated by gold, silver, and copper
with very insignificant other metals (iron,
lead, and zinc). The arc contains 92.44 million ounces of gold, 279.17 million ounces
of silver, and 61.92 billion pound of copper
from 14 deposits and prospects. This large
metal endowment is mainly contributed
from three world class gold-copper deposits
at Batu Hijau, Elang, and Tumpangpitu. The
gold endowment of the Eastern Sunda Arc
Gold, Silver and Copper Metallogeny of the Eastern Sunda Magmatic Arc Indonesia (A. Maryono et al.)
accounts for 26.1 % of the total gold endowment of Indonesia (317.3 million ounces;
Maryono and Power, 2009).
prospective magmatic belts, with potential
similar to that of the Papuan Fold Belt and
the Solomon-Lihir Magmatic Arc in the region. With remarkable known metal endowment and potential for new discoveries, the
Eastern Sunda Arc has high prospectivity.
On that basis the arc is considered to be one
of the world’s emerging gold-copper belts.
In the regional context, the Eastern Sunda
Arc stands among the top in the Southwest
Pacific region (Maryono and Power, 2009).
The region covers fifteen countries and hosts
fifteen magmatic arc belts with total of more
than 21,050 km strike length of magmatic
arc belts and total gold endowment of 744.8
million ounces (Figure 4). The Eastern
Sunda arc contains 11.1 % of total region
gold endowment, second to the Papuan Fold
Belt (281.0 million ounces). A similar rank
is seen in the Indonesian context that the
arc contains 26.1% of the total Indonesian
gold endowment (317.3 million ounces),
second to the Indonesian part of the Papuan
Fold Belt.
Almost 100% of the metal endowment in
the Eastern Sunda Arc is related to the Neogene magmatic stage, one of five stages of
magmatic activities identified along the belt.
Dating of mineralization age and/or related
intrusion age shows similar features to the
magmatic host rocks where mineralizing
intrusions have been dated as Neogene in
age (3.6 - 3.8 Ma) at Batu Hijau, 2.7 Ma at
Elang, 7.5 Ma at Selodong, 2.5 Ma at Pongkor, and 3 Ma at Arinem. This is consistent
with the Western Pacific region where the
largest gold endowment (about 45.1% or
321.2 million ounces) is hosted in Neogene
magmatic arcs (Maryono and Power, 2009).
For such a short extent of magmatic arc
length (1,800 km), with world-class gold,
silver, and copper endowment the Eastern
Sunda arc ranks it as one of the world’s most
4 Moz Au
Cibaliung
Jakarta
Pongkor
0.5 Moz Au
JAVA
Bandung
Ciemas
Semarang
1.0 Moz Au; 200Mlb Cu
Wonogiri
Yogyakarta
BALI
Trenggalek
Tujuh Bukit
Legend
Quaternary sedimentary rock
Surabaya
30.1 Moz Au; 19.0 Blb Cu
Denpasar
Quaternary volcanic rock
Quaternary intrusive rock
Tertiary sedimentary rock
Tertiary volcaniclastic rock
Tertiary volcanic rock
Tertiary intrusive rock
Pre-Tertiary metamorphic rock
5 Moz Au
Mataram
Gapit
Batu Hijau
Elang Pangulir
20 Moz Au; 19.6 Blb Cu
Mineral Occurrences
Pophyry
High Sulphidation Epithermal
Low Sulphidation Epithermal
Skarn
Sedimen-Hosted
SUMBAWA
1 Moz Au
25.4 Moz Au; 16.3 Blb Cu
N
200 km
Figure 4. The Eastern Sunda Magmatic Arc with three world class-porphyry Cu-Au deposits discovered along
the belt, making it one of the world’s most fertile and prospective magmatic belts.
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Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
Three of 26 gold deposits containing >5
million ounces of gold in the western Pacific region occur along the belt in the form
of porphyry Cu-Au deposits. These three
world-class Cu-Au deposits are Batu Hijau
(19.9 million ounces of gold and 19.6 billion ounces of copper; Clode et al., 1999;
Newmont Annual report, 2009), Elang in
Sumbawa (25.4 million ounces of gold and
16.3 billion ounces of copper; Newmont
Mining Corporation, 2012) and at Tumpangpitu in East Java (27.4 million ounce
of gold and 15.4 billion pounds of copper;
Intrepid Mines Ltd, 2012). Active mine
operations along the belt that are major
contributors to Indonesia’s gold and copper
production include gold mines of Pongkor,
Cibaliung, and Cikotok in West Java, and
Batu Hijau Cu-Au mine in Sumbawa, NTB
Province.
Recent intense exploration programmes
have delineated another world-class porphyry copper- gold deposit at Tumpangpitu,
the Tujuh Bukit Project, recently discovered
through intense drilling programmes by
Intrepid Mines Ltd since September 2007.
Other recent intense exploration drilling
programmes have been carried out at Hu’u
and Pangulir in Sumbawa, Brambang,
Pelangan, and Mencanggah in Lombok and
Selogiri and Trenggalek in Java; they have
all intersected significant copper and gold
mineralization of porphyry and high sulfidation epithermal styles.
Gold-Silver-Copper Mineralization Styles
Porphyry Cu-Au mineralization style is a
prime metal source for gold and sole source
for copper in the Eastern Sunda Arc, contributing about 90.3% of total gold or 74.5
million ounces and 100% of copper endowment or 53.1 billion pounds. Epithermal
mineralization styles are second with 8
million ounces of gold endowment or about
9.7%. This endowment is similar to that for
92
the Western Pacific region in that about 88%
of total gold endowment or 655.1 of 744.5
million ounces of total gold endowment are
contributed by porphyry mineralization.
Other deposit types, e.g. skarn and sedimenthosted, are insignificant.
In the eastern segment of the arc, significant
porphyry deposits or districts are spaced
approximately every 100 km along the
east-west trending arc from Empang/Hu’u
in the east, Elang, Batu Hijau in Sumbawa,
Selodong/Brambang in Lombok, and Tumpangpitu in East Java, to the west. A long gap
is seen further west to Selogiri in Central
Java. Paucity of significant porphyry occurrences in the west segment of the arc is in
marked a contrast to the east segment.
Three world class porphyry deposits at
Tumpangpitu, Batu Hijau, and Elang are
thought to be restricted in the eastern segment on thin island arc crust. In contrast the
western segment is dominated by low to intermediate sulfidation epithermal gold-silver
deposits at Pongkor, Cikotok, Cikondang,
Cibaliung, and Arinem, with no significant
porphyry copper-gold deposits. As for porphyry deposits, high sulfidation epithermal
deposits/prospects are also confined to the
eastern segment at Empang, Sane/Rinti,
Pangulir, Ladam/Elang, Sabalong/Lantung
in Sumbawa, Pelangan and Mencanggah in
Lombok, and Zone A, B. C (Tumpangpitu)
in East Java.
Porphyry and epithermal mineralization
styles in the Eastern Sunda arc have their
own distinctive characteristics that have
developed across the arc, resulting from
their specific tectonic setting and host lithologies. Porphyry and epithermal deposits
in the Eastern Sunda Arc share many characteristics with those in other island arcs in
the Western Pacific region e.g. Philippines,
Solomon Islands, PNG, and Fiji. They display significant differences to the Lamaride
Gold, Silver and Copper Metallogeny of the Eastern Sunda Magmatic Arc Indonesia (A. Maryono et al.)
porphyry systems in continental margin
and cratonic settings in the eastern Pacific
region.
Characteristics of gold-silver-copper mineralization systems along the Eastern Sunda
Arc can be seen from regional, district to
deposit scale. At a regional scale porphyry
Cu-Au and epithermal Au-Ag deposits are
located along active convergence plate
boundaries.
Strong conjugate northwest (NW) and
northeast (NE) fault systems are the dominant structural features of the islands, both
at regional and district scales. North-west
and north-eastern trending lineaments are
evident from air photo analysis and satellite-airborne image interpretation and are
thought to be cross structures, related to the
emplacement of intrusions, and consequently to formation of major porphyry Cu-Au
deposits. Major NE trending structures can
be seen in the mineralized districts at Batu
Hijau, Elang, Empang, and Hu’u (Garwin,
2002; Maryono et al., 2005), whereas NW
trending major structures are observed at
Tumpangpitu, Selodong, and Brambang.
Some deposits, e.g. Batu Hijau and Elang,
are localized at fault intersections (arc- parallel and NE trending major structures).
The deposits are spatially associated with
Neogene intrusive bodies with low-K calcalkaline to weakly alkaline, dioritic to tonalitic composition (Garwin, 2002; Maryono et
al., 2005; Setijadji, 2006; Roe, per comm.,
2012). Intrusion ages range from 2.7 Ma at
Elang, 3.7 Ma at Batu Hijau to 7.5 Ma at
Selodong. The causative intrusions generally form a series of nested, small dioritic to
tonalitic intrusive complexes. Mineralizing
intrusive bodies consist of multiple phases,
mostly early, intermediate and late tonalite intrusions. These multiphase intrusive
complexes are generally part of remnant
volcanic centres or stratovolcanoes with
dioritic to andesitic batholiths/stocks as
premineralization intrusions.
Intrusive bodies are elongate, with pencillike apophyses 200 m to 500 m in diameter
with >2 km vertical extent. The apophyses
rise within or from the margins of coarsegrained, equigranular stocks/batholiths.
The depth of porphyry intrusions ranges
from 1 to 2 km below the paleo surface and
extend a further 5 km depth. The intrusions
are characterized by porphyritic textures,
with 30 to 60% phenocrysts consisting of
abundant plagioclase, minor alkali feldspar,
hornblende, and quartz.
Host stratigraphy is generally characterized
by Miocene volcanic rocks and associated
volcaniclastic rocks as a volcanic edifice.
The volcaniclastic rock sequence contains
thin calcareous sedimentary rocks and
limestone, which form thin skarn mineralization, e.g. at Elang, Batu Hijau, and
Tumpangpitu.
Structural fabrics at district scale are
dominated by strong conjugate systems
of NW and NE faults that are apparent in
some mineralized districts. NW-trending
structural corridors exist at Pelangan,
Mencanggah, and Brambang in Lombok,
and Tumpangpitu in East Java. A series of
ore-bearing quartz ledges of intermediate to
high sulfidation epithermal character with
NW to NNW orientation are developed at
Pelangan, Mencanggah, and Tumpangpitu.
A similar NW alignment of mineralized
porphyry centres is seen at Brambang and
Tumpangpitu. NE alignment of porphyry
prospects or intrusion centres occurs further east in the eastern segment of the arc
at Batu Hijau, Elang, Rinti, and Gapit in
Sumbawa, where porphyry Cu-Au prospects and other Cu-Au mineralized centres
in the district are aligned along a NE trending structural corridor with different levels
of exposure.
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Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
District Mineralized System and Exploration Model
District mineralized systems as illustrated
by conceptual deposit models in Figures 5
and 6, are zoned with a spatial association of
a central porphyry and overprinting high sulfidation epithermal, marginal low sulfidation
epithermal veins, skarn and sediment-hosted
gold-silver mineralization. Intermediate to
high sulfidation epithermal mineralization
forms a telescoped system above or adjacent
to underlying porphyry systems, e.g. Elang,
Gapit, and Tumpangpitu. Low sulfidation
systems are developed further away from
the porphyry centres.
Most significant porphyry and related epithermal mineralization occurrences are associated with diatreme breccia bodies. The breccia is developed at the margin or adjacent to
the porphyry systems, resulting in disruption
to the mineralized bodies, e.g. Rinti, Elang,
Batu Hijau, Selodong, Brambang, and Tumpangpitu. Major disruption can be observed
at Selodong, SW Lombok where the Motong
Botek porphyry mineralization system has
been fragmented by late diatreme breccia.
Rootless porphyry mega fragments occur
within large breccia bodies.
An example showing the full spectrum is the
Elang District, Sumbawa, in a telescoped
system where a high-sulfidation system at
Ladam occurs on top of porphyry Cu-Au
mineralization (25.4 million ounces of gold
and 16.3 billion pounds of copper; Newmont
Mining Corporation, 2012). Gold-bearing
quartz vein sets of low sulfidation epithermal character is developed 1 km to the
south at Sebu and 1.5 km north-northeast
at Kokar Ika within the diatreme body
(Maryono et al., 2005). Insignificant skarn
mineralization with calc-silicate alteration
is developed in thin calcareous intercalations in the host volcaniclastic sedimentary
sequence of Miocene age.
Another good example occurs in the Tumpangpitu District in East Java. An overlying
1000
500
Quartz vein stockwork
Silica-pyrophylitis ledges
sericite selvages
Advanced argilic vuggy silica centers
leached cap
Distreme breccia
Advanced argilic
Dacite
pyroclastic
Limestone interbed
Quartz vein Sericte alteration
0
Biotite alteration
Andesitic
dioritic
dyke
Late Tonalie Intrusion
Chi-sericite-mg
alteration
Tonalite Intrusion
-500
-1000
Volcanic sedimentary seccecion
Supergene Cu
enrichment
Silification
Subvolcanic Basement
Bernite Dominant
Chalcopyrite Dominant
Pyrite Dominant
-1500
Equigranular Diorite stocks
-2000
Zn-Pb-Au-AgAs+Cu
Low grade core
Multiphase Tonalte
dyke Complex
Cu+Au-Mo+Ag-As
Anomalies
0
1000
m
Zn-Pb-Au-AgAs+Cu
Figure 5. Conceptual district scale exploration deposit model in section view showing central porphyry goldcopper deposit with peripheral epithermal, skarn, and sediment-hosted deposits.
94
Gold, Silver and Copper Metallogeny of the Eastern Sunda Magmatic Arc Indonesia (A. Maryono et al.)
Au-Cu-As
PB-Zn-Au-Ag-As-Cu Anomalous Zone
Anomalous Zone
Underlying Supergene
Cu enrichment
Sericite alteration
Hematitic
leached cap
Underlying Supergene
enrichment
Cu-Au-Mo-Ag-As
Anomalous Zone
al
Co
rid
or
Diatreme
breccia
EM
ine
ral
ize
ds
tru
ctu
r
Window of
advanced argillic
Cu-Mo
alteration
Anomalous Zone
Porphyry Cu-Au system
(biotite alteration)
Pyrite-enargite-tenantite veins
associated with vuggy silica centers
Multiphase
Tonalite
Dyke Complex
NN
Quartz veins
with narrow sericite
kaolinite selvedges
N
advanced argillic
alteration
Quartz veins
stockwork with
sericitic selvedges
0
2 km
Figure 6. Conceptual district scale exploration deposit model in plan view showing central porphyry goldcopper deposit with peripheral epithermal deposits and diatreme breccia body.
high-sulfidation epithermal deposit (2.4
million ounces Au and 80 million ounces
Ag) in Zones A, B and C penetrates as deep
as 1 km below the current surface over
a porphyry Cu-Au deposit (28.0 million
ounces Au and 19.0 billion pounds Cu;
Intrepid Mines Ltd, 2012). Low sulfidation
epithermal mineralization is developed at
Gunung Manis approximately 3 km east of
the main porphyry Cu-Au deposit. Minor
skarn mineralization occurs in thin beds
of calcareous sedimentary rocks and limestones in the Miocene volcaniclastic host
rocks.
Alteration systems at district to deposit scale
with underlying porphyry Au-Cu systems
and peripheral intermediate to high sulfidation epithermal Au-Ag systems are generally manifested at surface by large lithocap
alteration bodies (Figure 7). Low sulfidation
epithermal systems have limited alteration
envelops at the surface, confined to quartz
vein selvages.
Large surface lithocap features more than 20
km2 in area with barren to very weak stream
geochemical signatures can be seen at Hu’u
Sumbawa and Brambang Lombok, where
the porphyry Au-Cu systems are totally
concealed. Mineralized overlying lithocap
bodies with obvious surface geochemical
signatures occur at Tumpangpitu (Zones A,
B and C) and Elang (Ladam) where high
sulfidation epithermal Au-Ag mineralization
styles overprint porphyry Cu-Au systems
at depth. The lithocap bodies are composed
of central mineralized vuggy and pervasive
quartz ledges. They are zoned outward to
peripheral advanced argillic, argillic and
outermost propylitic alteration zones. The
advanced argillic alteration is composed
of acid clay minerals, dominant kaolinitedickite at shallow levels and dominantly
phyrophyllite- diaspore-topaz at depth.
Argillic alteration zones are made up of
neutral clay minerals illite, and montmorillonite with little kaolinite. Alunite in the
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Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
N
0
1
N
3
5
0
km
1
N
3
5
Brambang
1
3
5
km
Legend:
Lithocaps
Porphyry
Coastline
N
1
0
km
Tujuh Bukit
0
Selodong
Elang
Batu Hijau
N
3
5
km
0
1
3
5
km
Figure 7. Large surface lithocap alteration footprints with underlying porphyry gold-copper systems at some
porphyry gold-copper deposits/prospects along the Eastern Sunda Arc.
form of crystals or pervasively dispersed
is observed in the central quartz ledge and
residual vuggy quartz zones.
A gradual alteration change to the underlying porphyry system can be seen with the
presence of abundant illite, quartz veins, and
relict magnetite/hematite. Shreddy chlorite
(after hydrothermal biotite) and hydrothermal magnetite increase downwards, giving
way at depth to preserved early porphyry
alteration (biotite-magnetite-actinoliteplagioclase and potassium feldspar assemblages). Porphyry alteration types and
zones recognized on a district scale include
early, transitional, late, and very late assemblages. Early biotite-actinolite- oligoclase-magnetite alteration is overprinted by
retrograde chlorite-magnetite to form a chlorite- actinolite-biotite-magnetite±oligoclase
alteration assemblage. Zones of early alteration contains porphyry vein types “A”,
“EB”/’EDM”, and “A- family” of Gustafson
and Hunt (1975) and Brimhall (1977) with
96
dominant chalcopyrite and bornite mineralization. Early porphyry alteration is spatially
associated with porphyry Cu-Au ore which
measures more than 1.5 km in diameter at
the three world-class porphyry systems,
Batu Hijau, Elang, and Tumpangpitu. Dominance of biotite alteration, a lack of potassium feldspar alteration, and the presence of
significant actinolite are marked differences
compared to porphyry Cu-Au systems in
other parts of the world (Lowell and Gilbert,
1970; Sillitoe and Gappe, 1984).
Transitional porphyry alteration produced
widespread zones of chlorite-sericitemagnetite+clay assemblages overprinting
early porphyry alteration. This transitional
alteration tends to form an alteration shell
and is closely associated with early chloriteactinolite-biotite- magnetite alteration. Typical characteristics of this transitional alteration are marked by the presence of shreddy
chlorite, green sericite and hematite. Weak
sericite-clay- chlorite+magnetite assem-
Gold, Silver and Copper Metallogeny of the Eastern Sunda Magmatic Arc Indonesia (A. Maryono et al.)
blage is associated with the late Echo Tonalite intrusion (Maryono et al., 2005). The
sericite-chlorite- magnetite/hematite+clay
assemblage is thought to be comparable to
the sericite- chlorite-clay zone (SCC) of Sillitoe and Gappe (1984) and pale green mica
assemblage of Clode et al. (1999). Porphyry
type veins, especially “B” and “C” veins,
are associated with transitional alteration.
Late alteration overprints involve broad
zones of advanced argillic and argillic alteration. A broad argillic zone (sericite-illitekaolinite) extends for 5 km along the NNE
trending alteration corridor from Ladam
to Sepekat.
Three main zones of advanced argillic
alteration (pyrophyllite-dickite-kaolinitealunite) occupy high topography at Elang,
Gerbang, and south Sepekat. High sulfidation (quartz-enargite- tennantite) and intermediate sulfidation (quartz-base metal)
epithermal veins are associated with late
advanced argillic and argillic alteration
respectively. Very weak argillic alteration
(weak sericite-clay±chlorite) associated
with the diatreme breccia and post-mineralization dacite porphyry is considered to
be a product of a very late alteration stage.
Similarly, narrow sericite-kaolinite selvages
on quartz-base metal veins are related to a
very late hydrothermal stage. Copper and
gold ore- bearing alteration is intense biotite-magnetite alteration (lacking potassium
feldspar) measuring from 1 to more than 1.5
km in diameter (Maryono et al., 2005).
Porphyry mineralization in the Eastern
Sunda Arc is typified by gold-rich porphyry
systems, similar to those in island arc settings in the Philippines. Copper-gold mineralization is formed during emplacement of
cogenetic porphyritic intrusion. Hypogene
mineralization at three world-class deposits
at Batu Hijau, Elang and Tumpangpitu is
typical of porphyry Cu-Au deposits. It is
associated with a series of small multiphase
porphyry intrusions (early, intermediate and
late tonalite phases) emplaced close together
in space and time in an area 1.5 km by 1 km.
The mineralized zone, as marked by 0.3 %
Cu zones in surface projections of drill hole
data, measures on average more than 1 km
in diameter, centred at small porphyritic
dioritic to tonalitic mineralizing intrusions.
Porphyry mineralization forms an annular or
inverted shell that lies within and around the
margins of deep tonalite intrusive bodies.
Significant supergene copper enrichment is
developed beneath advanced argillic alteration only at Batu Hijau and Elang. A weak
chalcocite blanket averaging 40 m thick and
0.5 to 0.7 % Cu has been intercepted in drill
holes. The copper enriched zone measures
in excess of 500 m by 750 m in plan with
variable thickness and is characterized by
an overlying goethite-hematite leached cap
at the surface. Very thin supergene copper
mineralization (0.3 to 0.5% Cu, 10 to 20 m
thick) intersected at Brambang does not
form a significant chalcocite blanket.
CONCLUSIONS
The metallogeny of the Eastern Sunda Magmatic Arc is dominated by gold, silver, and
copper which are predominantly contained
in porphyry and epithermal deposit types.
With world class gold-silver-copper endowment of 92.44 million ounces of gold, 279.17
million ounces of silver and 61.92 billion
pounds of copper, the Eastern Sunda Magmatic Arc has emerged as one of the most
prospective gold- copper belts in the world.
Porphyry gold-copper and epithermal goldsilver mineralization styles in the Eastern
Sunda Magmatic Arc share similarities to
those in typical island arc settings, e.g. the
Philippines, and display some unique characteristics. At district scale the mineralized
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Majalah Geologi Indonesia, Vol. 29 No. 2 Agustus 2014: 85-99
system is zoned with a spatial association
of central porphyry and overprinting high
sulfidation epithermal mineralization, and
marginal low sulfidation epithermal, skarn,
and sediment-hosted gold- silver mineralization. Post-mineralization diatreme breccia
bodies are common, developed at the margin
or adjacent to the porphyry systems, and
disrupt the mineralized bodies.
At regional to district scales, key deposit
features can be summarised to include an
association of deeply eroded volcanic centres, NNE and NW trending major structures, Oligocene to Miocene volcaniclastic
host rocks, Neogene multiple small nested
intrusive complexes, a spatial association
of central porphyry and distal epithermal
systems and large surface lithocap footprints. At deposit scale key deposit features
include gold-copper ore bearing intense
biotite-actinolite-magnetite alteration, goldsilver bearing vuggy and pervasive quartz
alteration, gold-silver bearing quartz veins,
porphyry veining types and patterns, sulfide
species and pattern, hypogene Fe-oxides,
supergene Fe-oxides, mineralization forms
and textures, and detailed alteration types
and patterns. These key deposit features are
spatially and genetically associated with ore
and its environment that provide selection
criteria for prospective regions. Exploration
area selection is based on the presence or
absence of these specific geological features,
and geochemical, geophysical, and geomorphological features which reflect underlying
geological features.
District and deposit exploration models can
be further refined on the basis of shared
key features of the deposits in the region
as guides during exploration. These key
features provide vectors to ore, applicable
in identifying the central, proximal, and
distal parts of mineralized systems during
exploration activities. Keys to exploration
success include understanding the characteristic features of ore systems, observing
98
key geological features in the field, and
determining vectors to ore.
ACKNOWLEDGEMENTS
This paper benefitted from contributions of many
colleques during work on several projects along
this magmatic arc. The authors would like to thank
site exploration/geology teams at Batu Hijau, Elang,
Selodong, Tujuh Bukit and Brambang for their work,
site assistance and contribution. Noel White is specially thanked for constructive review and editing.
The authors would also like to thank the Management
of PT Buena Sumber Daya, in particular Bambang
Irianto and Rayes Sembiring, for the support to
publish this paper.
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