MINERALOGY AND ORIGIN OF COPPER-GOLD

BEARING SKARN WITHIN THE BATU HIJAU

PORPHYRY DEPOSIT, SUMBAWA ISLAND,
INDONESIA
May Thwe AYE1,2, Subagyo PRAMUMIJOYO2, Arifudin IDRUS2, Lucas Donny
SETIJADJI2, Akira IMAI3, Johan ARIF4, Symsul KEPLI4
1
Department of Geology, University of Yangon, Myanmar
2
Department of Geological Engineering, Gadjah Mada University, Indonesia
3
Department of Earth Science and Technology, Akita University, Japan
4
Mine Geology Department, PT Newmont Nusa Tenggara, Indonesia

Abstract
The aim of this study i s t o emphasize on the origin of copper-gold bearing skarn mineralization
at the Batu Hijau deposit which is located at the southwestern corner of Sumbawa Island,
Indonesia. Although most skarn are derived from limestones, nolimestone is known in the Batu
Hijau deposit. Ca-rich andesitic volcaniclastic host rocks favor skarn alteration within the Batu
Hijau deposit. T h e t ype of skarn can be classified as calcic-exoskarn, and locally controlled by
faults and fractures. Two major stages consisting of four sub-stages of skarn forming processes
can be divided by the mineral assemblages of skarn as prograde and retrograde stages. The
prograde skarn consists of clinopyroxene and garnet ± magnetite formed at the trapping
temperature of 440°-480 °C with 34-38 wt% NaCl eq. while retrograde skarn alteration is
dominated by Fe-rich minerals such as amphibole and epidote formed at the trapping temperature
down to 340°-360°C with 4-8 wt% NaCl eq. Opaque minerals include chalcopyrite, pyrite,
sphalertite, and minor galena and bismuth- telluride. Gold was precipitated in the retrograde stage
associated with bismuth-telluride minerals. The sulfur isotope data of skarn ranges from +0.1 to
+1.7‰ (sulfide), and porphyry systems range from 0.04 to1.4‰ and 10‰ to 15‰ (sulfide and
sulfate respectively). According to the fluid inclusion and sulfur isotope data, the origin of skarn
and porphyry system can be suggested to be that the magmatic origin. Furthermore, the sulfur
isotope data of the deposit evidently shows that a porphyry- related skarn mineralization exhibiting
transition from one style to the next can be relatively rapid. The result of this research has
indicated that the range of porphyry-related deposits, skarn and porphyry systems can form during
a single prolonged hydrothermal event.

Key words: Batu Hijau Deposit, Copper-Gold Bearing Skarn, Fluid Inclusion, Indonesia,
Magmatic Origin, Sulfur Isotope

Introduction
A copper-gold bearing skarn was newly found in the deep level of the Batu Hijau deposit
which is an island arc porphyry deposit, located in the southwestern corner of Sumbawa
Island, in the west Nusa Tenggara Province, Sunda-Banda Archipelago of Indonesia. In
this paper, the mineralogical and geochemical data on skarn, fluid inclusion thermometry,
and sulfur isotope composition of skarn ore were investigated in order to understand the
process of skarn formation. Sumbawa lies along the tectonically active east-west trending
Sunda-Banda magmatic arc that is a product of the convergence of three major tectonic
plates: the Indian-Australian, the Eurasian and the Pacific Plates.

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 Figure 1 Map of Indonesia and surrounding areas showing the location of Batu Hijau
deposit (modified from Carlile and Mitchell, (1994)).

Deposit Geology
More than 98% of Indonesian gold and copper resources are derived exclusively from
six major Neogene mineralized magmatic arcs which include Sunda-Banda, Aceh, Central
Kalimantan, Sulawesi-East Mindanao, Halmahera and Medial Irian Jaya (Central Range-
Paouon fold and thrust belt) (Carlile and Mitchell, 1994). The Batu Hijau porphyry Cu-
Au deposit is located in the southwestern corner of Sumbawa Island, in the west Nusa
Tenggara Province, Indonesia. Sumbawa lies along the tectonically active east-west
trending Sunda-Banda magmatic arc that is a product of the convergence of three major
tectonic plates: the Indian-Australian, the Eurasian and the Pacific Plates (Hamilton,
1979).
The Batu Hijau deposit is dominantly underlain by andesitic volcaniclastic rocks
(Early to Middle Miocene). A premineralization porphyritic quartz diorite was intruded
by equigranular quartz diorite and tonalite porphyries (Clode et al., 1999). Detailed
mapping within the Batu Hijau deposit identified five major structural trends: N-S, E-W,
NE-SW, NW-SE and radial pattern (Priowarsono and Maryono, 2002). The most
common two major structures; the northeasterly trending Bambu- Santong fault zone
and northwesterly trending Katala-Tongoloka Puna fault zone transect the Batu Hijau
district at the Santong diatreme, 2 km NW of the Batu Hijau deposit. Figure 2 shows
the geology of the deposit illustrating the borehole sample location, and Figure 3 shows
the lithology distribution along cross-section (A) and the 3D schematic diagram showing
the skarn distribution at deeper levels around the intermediate tonalite porphyry intrusion
(B).

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Figure 2 Geological map Batu Hijau deposit. Black circles show the location of the
boreholes and location of skarn mineralization (modified from Newmont, (2008)

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Figure 3 (A) Lithologic distribution of the cross-section (Section 070) along A-A’ in Fig.
2 and the vertical profile of the borehole SBD-196 and SBD-284, (B) 3D schematic
diagram showing the skarn distribution at deeper levels around the intermediate tonalite
porphyry intrusion (modified from Newmont, (2008)).

Occurrences of Skarn Zonation and Ore Mineralization

The skarn and ore mineralization is developed as a result of metasomatic reaction
between the calcium-rich andesitic volcaniclastic rocks and intermediate tonalite
porphyry intrusions at deeper levels (Fig. 3). Skarn and related ore mineralization are
extensively faulted caused by the geology in the Batu Hijau deposit and which is
complicated in structure.
Whereas most skarn deposits are derived from limestone or carbonate rocks (Einaudi
et al., 1982), copper-gold bearing skarn at the Batu Hijau deposit clearly defines skarn
occurrences that resulted from the complete replacement of Ca-rich volcaniclastic units
and with no evidence of limestone in the Batu Hijau deposit (Idrus et al., 2009). The
skarn in Batu Hijau is typified by banded structures and characterized by anhydrous and
hydrous calc-silicate minerals (garnet, diopside, amphibole, epidote), chlorite, quartz,
clay minerals, sulfides (pyrite, chalcopyrite, sphalerite, galena, bornite), oxides
(magnetite, hematite) and carbonates (calcite) developed along the contact between
intermediate tonalite porphyry stock and the volcanic host rocks.

Mineralogy
Skarn Minerals and Associated Ores
Andraditic garnet and diopsidic clinopyroxene are the dominant skarn minerals in the
Batu Hijau deposit. Most garnets are coarsed-grained, massive and brecciated in
nature associated with magnetite. Under the microscope, garnet is euhedral and exhibits
zoning. Aggregates of coarse- grained garnets are commonly observed while fine-grained
garnet and occasional crystals of feldspar are often filled within the calcite matix (Fig. 4a).
Most garnet shows distinct concentric zoning (Fig. 4b). Veins and veinlets of magnetite
cut across the clinopyroxene (Fig. 4c). Under the microscope, euhedral clinopyroxene
coexists with a lesser amount of quartz, sphene, and hematite. Epidote- bearing skarn
replaced garnet and is widely distributed. Microscopic observation reveals that epidote is
anhedral when it is associated with garnet, implying a late formation after garnet (Fig.
4d). Moreover, it is also commonly associated with magnetite and minor sericite,
sphene and quartz. Magnetite occurs both as massive and brecciated varieties. Most ores
are composed of magnetite associated with chlorite, calcite and quartz, filling fractures or
spaces. High grade ores are found in the broken magnetite zone. Ore minerals include

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chalcopyrite, pyrite, sphalerite; galena associated with gold and bismuth-telluride fills the
cracks and cavities within the magnetite-rich zone.

Figure 4 Photomicrograph of representative samples from the Batu Hijau drill hole
showing skarn and ore mineral assemblages. (a) epidote (Epi) showing granoblastic
texture and replace garnet associated clinopyroxene (Cpx); (b) zoned plagioclase
associated with replacement of clinopyroxene (Cpx) by chlorite (Chl); (c) garnet (Grt)
shows concentric zoning; (d) magnetite (Mag) veinlets cut cross clinopyroxene (Cpx);
Polished thin section under reflected light, (e) blebs of native gold in chalcopyrite (Ccp)
associated with sphalerite (Sp) and magnetite (Mag) and (f) replacement of pyrite (Py) by
chalcopyrite (Ccp) associated with sphalerite (Sph).

Paragenesis
Paragenetic sequence of skarn at the Batu Hijau deposit appears similar to other skarns
(Einaudi et al., 1982; Newberry, 1987; Meinert, 1993; Kwak, and White, 1982).
Mineralogical and textural evidence suggest that the process of skarn formation can be
categorized into two discrete stages of prograde and retrograde events which consist of
four sub-stages. The early prograde stage is hornfelsic skarn characterized by fine-grained
garnet and clinopyroxene. The early retrograde stage is typified by precipitation of a
large amount of magnetite whereas epidote and a small amount of quartz precipitated
simultaneously with magnetite. Sulfide minerals such as chalcopyrite, pyrite, sphalerite,
galena associated with gold and bismuth-telluride and hydrous minerals precipitated
during this stage. The generalized paragenetic sequence of formation of the skarn and
ore minerals from the Batu Hijau deposit is shown in Figure 5.

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 Figure 5 Paragenetic diagram for skarn and ore minerals at the Batu Hijau deposit

Fluid Inclusion Study

Analytical Procedures
Fluid inclusions were examined in six samples from quartz veins/veinlets associated
with prograde and retrograde skarn minerals in order to estimate the spatial and temporal
variations of temperatures and the composition of the hydrothermal fluid. The
homogenization and the ice melting temperatures of fluid inclusions were obtained on
double polished thin sections using a Linkam THMS-600 stage and also a USGS-adopted
Fluid Inclusion heating/freezing stage. The melting temperatures of several metals and
the ice melting temperatures of NaCl solutions of known concentration were measured in
order to confirm the accuracy of the stage.

Results

Fluid inclusions from the quartz sample associated with prograde and retrograde stages
vary in size from 10-50 µm. The type of fluid inclusion follows the classification by
Nash (1976), as follows: Liquid-rich and vapor-rich two-phase inclusions mainly occur
in the quartz associated with the retrograde stage and polyphase inclusions containing a

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daughter mineral are found mainly in quartz associated prograde minerals (Fig. 6). Fluid
inclusions in the prograde stage homogenized over a broad range, from 340ºC to 515ºC
with a salinity between 24 and 48 wt% NaCl eq. whereas fluid inclusions in the
retrograde stage are homogenized between 200ºC and 396ºC with the salinity between
1 and 10wt% NaCl eq. (Figs. 7 & 8). However, the trapping temperature ranges from
440°- 460°C for the prograde stage and 340°-360°C for the retrograde stage. The high
temperature and high-salinity fluid in skarn is usually interpreted to represent a
magmatic fluid (Burnham, 1979). Table 2 shows the summary of the fluid inclusion data.

Figure 6 Photomicrographs of morphology and various types of fluid inclusion from

skarn minerals at the Batu Hijau deposit.

Figure 7 Frequency distribution diagrams of homogenization temperature (Th) of fluid

inclusions in quartz associated prograde and retrograde stages.

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 Figure 8 Frequency distribution diagrams of salinity (wt% NaCl eqn.) of fluid
inclusions in quartz from prograde and retrograde stages.

Table 2 Summary of fluid inclusion microthermometry of skarn mineralization from

the Batu Hijau deposit
Salinity
Sample Depth N Size Trap
Host Origin Stage
wt%
No. (m) Phase NaCl eq.
h ping
(µm)
226 694 21 Qtz P 5-30 L+V+h 156- 29-36 257- 251- Prograde
273 510 470
252 1031 14 Qtz P 5-30 L+V+h 233- 30-48 340- 330- Prograde
416 562 515
286 710 12 Qtz P 10-30 L+V+h 243- 30-39 380- 362- Prograde
263 504 467
814 19 Qtz P 5-33 L+V+h 159- 30-37 320- 308- Prograde
288 511 473
273 930 22 Qtz P 10-30 L+V (-1.9)- 3-10 262- 255- Retrogra
(-3.6) 404 382 de
284 20 Qtz P 10-50 L+V (-1.6)- 3-6 246- 200- Retrogra
997 (-6.3) 420 396 de
Abbreviation: Tm: temperature of melting (ice), Th: vapor-liquid homogenization
temperature, Td: dissolution temperature of halite, Qtz: quartz, N: number of
measurements

Sulfur Isotope Study

Analytical Procedures
The sulfide mineral grains from skarn were hand-picked from a specimen under a
binocular microscope. The sulfur isotope analysis for the sulfides from the skarn was
performed at the Activation Laboratories Ltd. Ontario, Canada. Additionally, sulfide and
sulfate minerals (pyrite, chalcopyrite, bornite, gypsum and anhydrite) from twelve
samples of vein type deposit of porphyry at the Batu Hijau deposit (Data analyzed by
Imai and Nagai, 2009) (unpublished data) were previously determined at the Department
of Earth and Planetary Sciences, University of Tokyo by Dr. Dana Anton using a
Thermo Finnigan delta plus mass spectrometer for SO2 gas prepared by the procedure
described by Yanagisawa and Sakai (1983).

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Results
The δ34S value (sulfide) from the Batu Hijau skarn ranges from +0.1 to +1.7‰, and
sulfide and sulfate values from porphyry systems range -0.04 to1.4‰ and 10‰ to 15‰
respectively. The δ34S values for sulfides fall in the narrow range -3 to +1 per mil close
to the accepted mantle range (Ohmoto and Rye, 1979). Histogram of δ34S values from the
sulfides and sulfate are shown in Fig. 9 and Table 3 showing the summary of sulfur
isotope data.

Figure 9 Histogram of δ34S values from sulfur. A. Sulfide values from skarn and
porphyry mineralization and B. Sulfate value from porphyry with reference to the mineral
sample type.

Table 3 Summary of sulfur isotopic composition of sulfide and sulfate from skarn
and porphyry within the Batu Hijau deposit
Depth
Sample no.
(-m)
Elevation Mineral Locality, Type δ34S(%)

SBD-240 734 450.3 py skarn 1.7

SBD-286 776 322.7 py+cp skarn 0.1
907 py+cp skarn 0.2
969 py skarn 1
SBD-021 285.4 220 bn+cp porphyry -0.1
441.7 bn+cp porphyry 0.4
SBD-183 461.2 460.5 bn+cp porphyry 0.1
722 py porphyry 0.6
792 bn+cp porphyry -0.9
957.9 cp+py porphyry 0.3
SBD-194 155.4 405 cp porphyry 0.2
891 cp+py porphyry 0.6
SBD-221 186.5 250 bn+cp porphyry 0.4
SBD-257 1263.2 -320 py porphyry 1.4
1263.2 anh porphyry 15.8
1292 gyp porphyry 12.6
1294.2 py porphyry 0.7
1294.2 anh porphyry 12.9
1322.5 anh porphyry 12.9
1428.2 py porphyry 0.7

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 1472.9 py porphyry 0.8
1472.9 anh porphyry 13.5
1484.3 gyp porphyry 11.9
1541.7 py porphyry 0.5
1541.7 gyp porphyry 12.3
1579.5 py porphyry 1.3
1579.5 anh porphyry 10.9
SBD-265 1327.8 340 py porphyry 0.8
1327.8 anh porphyry 12.1

Abbreviation: anh: anhydrite (sulfate), bn: bornite (sulfide), Cp: chalcopyrite (sulfide),
py: pyrite (sulfide), gyp: gypsum (sulfate)

Discussion
The compositions of Batu Hijau skarn minerals indicate an oxidizing environment of
deposition. The clinopyroxene (diopside) co-exists with or is replaced by andradite which
suggests that clinopyroxene and andradite are formed in an oxidized environment (Kwak
and White, 1982; Meinert, 2000). In addition, the common occurrence of magnetite
associated with chalcopyrite and pyrite supports the conclusion of an oxidizing
environment during skarn formation.
The paragenesis of skarn evolution from the Batu Hijau deposit shows two main stages
consisting of four sub-stages. Sub-stages I and II mineral assemblages are dominated by
clinopyroxene and garnet. These two stages are considered to represent prograde
anhydrous skarn development, whereas stage III, which is dominated by hydrous
minerals (amphibole, epidote, chlorite), and stage IV, which comprises of hematite and
calcite, are considered to represent retrograde hydrous skarn development. Small blebs of
gold occur as inclusions in chalcopyrite associated with sphalerite.
According to the fluid inclusion data, the high temperature of prograde stage up to
515°C (trapping temperature of 440°-480 °C) and the salinity of 48 wt% NaCl eq.
correspond to a fluid pressure of ~400 bars and lithostatic depth of ~1.5 km (hydrostatic
depth of 4 km). For the retrograde stage, temperature up to 396°C (trapping of 340°-
360°C) corresponds to a fluid pressure of ~180 bars which is equivalent to a
lithostatic depth of 0.8 km (hydrostatic depth of 1.8 km). The high- temperature and
high-salinity fluid in skarn is usually interpreted to represent an orthomagmatic fluid
(Burnham, 1979) as it is interpreted at the Mid-Patapedia prospect (Williams-Jones and
Ferreira, 1989) and Mines Gaspe (Shelton, 1983).
In addition, the δ34S values for sulfides fall in the narrow range -3 to +1 per mil close
to the accepted mantle range and porphyry copper deposits are the most likely candidate
for magmatic, igneous source of sulfur (Ohmoto and Rey, 1979). It can either be
explained by magmatic–hydrothermal processes, or by incorporation of an external,
isotopically light, sulfur source such as biogenic sulfide, which is characteristically
depleted in δ34S. In this paper, sulfur isotope data range from -3 to +1 per mil. It can be
suggested that the source of Batu Hijau deposit is of magmatic origin. Figure 11
illustrates the δ34S values for sulfur-bearing minerals in hydrothermal deposits showing
the Batu Hijau deposit (Ohmoto and Rye, 1979).

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 Figure 11 he δ34S values for sulfur-bearing minerals in hydrothermal deposits
(modified from Ohmoto and Rye, 1979).

The genetic development of skarn in the Batu Hijau deposit documents that a
hornfelsic skarn was first formed in response to the intrusions into Ca-rich layer of host
rocks, which converted the volcanic rocks as i n t o the prograde isochemical skarn. The
skarn development is controlled predominantly by temperature, pressure, composition
and texture of the host rock. Subsequently, the skarn system was later influenced by the
presence of calcium-rich host rock to produce massive amount of calc- silicates (garnet
and pyroxene skarn) as prograde skarn (metasomatic stage). The mineralogy formed
during the prograde stage is characteristically coarser-grained. Sulfide and oxide
deposition commences during the latter stage of metasomatic skarn development.
Magnetite dominates over sulfides forming either by replacement of garnet or pyroxene
at the tonalite intrusive contact. This stage is characterized by the replacement of earlier
prograde anhydrous minerals by late stage hydrous minerals. The retrograde skarn is
composed of complex mineral assemblages of many phases which are the main stage of
sulfide and oxide formation in skarn. Sulfide mineralization and retrograde alteration in
skarn system is typically structurally-controlled and cuts across the prograde skarn due to
its brecciated nature.

Conclusions
The copper-gold bearing skarn within the Batu Hijau deposit is a unique style of skarn
mineralization as hosted by Ca-rich andesitic volcanic rocks. It is a calcic exo-skarn.
The Ca-rich volcanic rocks favor metasomatic alteration to form the skarn within the

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Batu Hijau deposit. The skarn system is characterized by a Fe-rich and oxidized
paragenetic sequence as an early hornfels stage, a massive prograde skarn replacement,
and a somewhat restricted retrograde alteration. Gold mineralization occurs closely
associated with sulfide minerals of the late epidote and/or amphibole by the retrograde
alteration of pyroxene-bearing zones, or magnetite-rich shear zone. Fluid inclusion
analysis and sulfur isotope study showed that skarn and porphyry were from a similar
source, and the origin of the mineralization is predominantly a magmatic sulfur source,
hydrothermal magmatic origin. The Batu Hijau deposit holds evidence that the porphyry-
related skarn deposits are mineralogically zoned and that transition from one style to the
next can be relatively rapid.

Acknowledgments
The first author was provided a scholarship by AUN/SEED-Net, JICA (Japan
International Cooperation Agency) at Gadjah Mada University, Indonesia. The authors
are very grateful to Prof. Koichiro Watanabe, Earth Resources Engineering
Department, Kuyshu University, for his kind support and guidance. Our sincere
gratitude also to the management of PT Newmont Nusa Tenggara, Sumbawa, Indonesia,
for kind support and help during field work.

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