Thematic Articlerge_092. Arifudin Idrus, 1,2 Jochen Kolb 2 and F. Michael Meyer Introduction. Abstract

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1 doi: /j x Resource Geology Vol. 59, No. 3: Thematic Articlerge_092 Mineralogy, Lithogeochemistry and Elemental Mass Balance of the Hydrothermal Alteration Associated with the Gold-rich Batu Hijau Porphyry Copper Deposit, Sumbawa Island, Indonesia Arifudin Idrus, 1,2 Jochen Kolb 2 and F. Michael Meyer 2 1 Department of Geological Engineering, Gadjah Mada University, Yogyakarta, Indonesia and 2 Institute of Mineralogy and Economic Geology, RWTH Aachen University, Aachen, Germany Abstract This paper discusses the mineralogy, whole-rock geochemistry and elemental mass balance of the hydrothermal alteration zones within the Batu Hijau porphyry copper-gold deposit, Sumbawa Island, Indonesia. The hydrothermal alteration and mineralisation developed in four stages, namely (i) the early stage consisting of a central copper-gold-bearing biotite (potassic), proximal actinolite (inner propylitic) and the distal chloriteepidote (outer propylitic) zones; (ii) the transitional stage represented by the chlorite-sericite (intermediate argillic) zone; (iii) the late stages distinguished into the sericite-paragonite (argillic) and pyrophylliteandalusite (advanced argillic) zones; and (iv) the very late stage typified by the illite-sericite zone. In general, major elements (particularly Ca, Mg, Na and K) and some minor and rare earth elements decrease from the least altered rocks towards the late alteration zones as a consequence of the breakdown of Ca-bearing hornblende, biotite and plagioclase. Chemical discrimination by means of millicationic R 1-R 2 diagram indicates that R1 [4Si - 11(Na + K) - 2(Fe + Ti)] increases while R 2 [6Ca + 2Mg + Al] decreases with increasing alteration intensity, from least-altered, through early, transitional, to late alteration zones. Rare earth elements-chondrite (C 1) normalised patterns also exhibit the depletion of the elements through the subsequent alteration zones. These results are consistent with the elemental mass balance calculation using the isocon method which shows that the degree of mass and volume depletion systematically increases during alteration. A decrease of the elements as well as mass and volume from early, through transitional to late alteration stages may imply a general decrease of the element activities in hydrothermal fluids during the formation of the alteration zones. Keywords: Batu Hijau porphyry Cu-Au, Indonesia, lithogeochemistry, mass balance, mineralogy. 1. Introduction The Batu Hijau mine is a world-class deposit of copper and gold as defined by its position among the top 10 percent of deposits in the world in contained metal abundance. The deposit lies in the south-western part of Sumbawa Island, Indonesia (Fig. 1). It contains mineable reserves of 914 million metric tonnes with an average grade of 0.525% Cu and g t -1 Au (Clode et al., 1999), and being mined by Newmont Nusa Tenggara Company. Earlier descriptions of Batu Hijau since its its discovery in 1990 included the geology, petrology Received 1 August Accepted for publication 22 December Corresponding author: A. IDRUS, Department of Geological Engineering, Gadjah Mada University, Jl. Grafika 2 Bulaksumur (55281), Yogyakarta, Indonesia. arifidrus@ugm.ac.id Journal compilation 2009 The Society of Resource Geology 215

2 A. Idrus et al. of wallrocks and intrusions, alteration and mineralisation (Meldrum et al., 1994; Irianto & Clark, 1995; Maula & Levet, 1996; Mitchell et al., 1998; Clode et al., 1999). Detailed geological setting of intrusion-related hydrothermal systems in the Batu Hijau vicinity was studied by Garwin (2002). Arif and Baker (2004) studied the distribution of gold associated with the central biotite (potassic) alteration zone of the deposit. Fluid inclusion study at the shallow and deep part of the deposit werereported by Imai and Ohno (2005) and Imai and Nagai (2009), respectively. Idrus et al. (2007) determined the temperature and physico-chemical conditions of the emplacement of the tonalite porphyry intrusions. In addition, Idrus et al. (2009) described occurrences of skarn-type calc-silicate rocks in the wall rock at the Batu Hijau deposit. Although numerous studies have been carried out in the deposit and its vicinity, a detailed investigation of some genetic variables including alteration geochemistry and elemental mass balance is needed to improve understanding of the deposit. This paper discusses the mineralogy, whole-rock geochemistry and elemental mass balance of the hydrothermal alteration zones within the Batu Hijau porphyry copper-gold deposit. Mass balance calculation following the example of Grant (1986) were used to quantify the effects of hydrothermal alteration on the host rock. The present study allows better understanding of the behavior (enrichment or depletion) of the elements during hydrothermal alteration processes and these might be useful in the chemical discrimination and origin of the hydrothermal alteration zones within the deposit. 2. Regional geology The majority of the Indonesian gold and copper resources are derived exclusively from six major Neogene mineralised magmatic arcs, namely, the Sunda-Banda, Aceh, Central Kalimantan, Sulawesi- East Mindanao, Halmahera and Medial Irian Jaya (Central Range-Papuan fold and thrust belt) (Carlile & Mitchell, 1994; Fig. 1). The Batu Hijau porphyry copper-gold deposit lies along the tectonically active east-west trending Sunda-Banda magmatic arc. The mineralisation in the western part of the Sunda- Banda arc is characterised by low-sulphidation epithermal vein systems including Mangani, Lebong Tandai and Lebok Donok in Sumatra as well as Gunung Pongkor, Cikidang and Cirotan in West Java. The mineralisation style changes to the east (e.g. Central-East Java and Sumbawa Island), marked by the porphyrytype mineralisation at the Selogiri area, Central Java and the Batu Hijau deposit in south-western Sumbawa Island (Maula & Levet, 1996; Imai et al., 2007). The Fig. 1 Copper-gold-mineralised Tertiary magmatic arcs in Indonesia (modified from Carlile & Mitchell, 1994). The location of the Batu Hijau deposit is also shown. 216 Journal compilation 2009 The Society of Resource Geology

3 Alteration geochemistry of Batu Hijau deposit south-western part of Sumbawa Island is underlain by Late Oligocene to Middle Miocene low-k calc-alkaline to weakly alkaline andesitic volcanic and interbedded volcaniclastic rocks, associated with low-k intermediate intrusions and minor shallow marine sedimentary rocks and limestones (Maula & Levet, 1996). The low-k intrusions include porphyritic andesite, hornblende diorite, quartz diorites and tonalite porphyries. 3. Geology of the Batu Hijau deposit The major wall rock units within the Batu Hijau deposit consist of interbedded andesitic volcaniclasitic rocks, intrusive andesite and at least two texturally distinct intrusive quartz diorite bodies (equigranular quartz diorite and porphyritic quartz diorite). These rock units are intruded by at least two texturally distinct stages of tonalite porphyries, which are referred to as intermediate tonalite and young tonalite. Andesitic volcaniclastic rocks are the most common rock-type within the deposit. The intrusive andesite is the earliest intrusive unit identified in the Batu Hijau area (Clode et al., 1999). The copper and gold-bearing tonalite porphyries form a north-north-easterly-elongate stock in the central part of the deposit. These intrusions were emplaced along the contact between the andesitic volcanic rocks and the equigranular quartz diorite. The U-Pb SHRIMP zircon dating indicates that the emplacement of the tonalite porphyry intrusions was rapid within thousand years from Ma (Garwin, 2002). The geological map of the Batu Hijau deposit (after Imai & Ohno, 2005) is presented in Figure Analytical methods Petrographic analyses were undertaken on thin, polished thin and polished sections using transmitted and reflected light. A total of 85 thin and polished thin sections and 30 polished sections from pit and drill holes were analysed in detail. This analysis is necessary in order to identify both altered and ore minerals as well as their textural relationships. It also allows the recognition of diagnostic hydrothermal minerals, typical of each alteration zone. More than 100 fresh/ least-altered and altered rock samples were analysed using XRF spectroscopy for major oxides and infra-red optical emissions spectroscopy (IROES) for the volatile components, particularly carbon and sulphur at the Rheinische Westfaelische Technische Hochschule Fig. 2 Geological map of the Batu Hijau porphyry copper-gold deposit (as of early 2002) (after Imai & Ohno, 2005). (RWTH) Aachen University, Aachen, Germany. Representative samples were sent to Activation Laboratories Ltd. (Actlabs, Ancaster, Canada) for both trace and REE nalyses using instrumental neutron activation analysis (INAA) and ICP-MS. The geochemical data were used to evaluate quantitatively the mass balance of major and minor elements that accompanied hydrothermal alteration processes. The method of Gresens (1967), as modified by Grant (1986), was employed to calculate the mass balances. The isocon diagram is constructed using a doublelogarithmic plot following the suggestions of Baumgartner and Olsen (1995). The identified immobile elements were used to graphically define an isocon line, using the standard deviation of the immobile element analyses (1s error bars; Selverstone et al., 1991; Leitch & Lentz, 1994). The gradient of the isocons were defined by the mass of the original samples against the Journal compilation 2009 The Society of Resource Geology 217

4 A. Idrus et al. mass of the altered samples (M /M a ). Once this reference frame of immobile elements has been established, geochemical variations can be discussed in terms of element mobility (i.e. gains or loss of elements). Elements that plot above the reference isocon were enriched during alteration, whereas elements that plot below were depleted (Grant, 1986). 5. Results 5.1 Mineralogy of least-altered rocks Mineralogy of the intermediate and young tonalite porphyries are relatively identical, characterised by phenocrysts of hornblende, plagioclase, quartz, biotite, magnetite and ilmenite. The phenocrysts are set in aplitic medium-coarse-grained groundmass. In the intermediate tonalite, plagioclases are the predominant phenocryst, generally euhedral-subhedral and uniform in size from 3 4 mm. Quartz phenocrysts (2 6 mm across) are usually rounded bipyramids ( quartzeyes ). In the young tonalite, rounded bipyramid quartz phenocrysts range from 8 to 10 mm across and mostly contain small inclusions of other minerals. Euhedral-subhedral plagioclase has a variable size from 2 to 8 mm. This rock is typified by easily identifiable large hornblende phenocrysts, which are locally rimmed by prismatic biotite. Magmatic biotite is frequently identified as euhedral-subhedral single grain, with a typical length of 1 4 mm. Andesitic volcaniclastic rocks contain grains of 10 20% of broken plagioclase and minor mafic silicate minerals (amphibole > clinopyroxene) as well as minor volcanic lithic fragments, which are set in an ash-size matrix (<2 mm). The grain shapes are commonly anhedral and up to 1 mm in length. The abundance of grains commonly range from 50 to 60% and locally exceed 80%. Chlorite and calcite partially to completely replace the mafic silicate minerals. Plagioclase is locally replaced by epidote. Equant grains of pyrite and of magnetite are also present. The lithic clasts are subrounded to subangular, 2 64 mm (granule to pebble) in size, locally exceed 100 mm, and are dominantly of volcanic origin. Equigranular quartz diorite is characterised by light grey, subporphyritic with a medium- to coarse-grained and hypidiomorphic equigranular texture. The phenocrysts consist of 70 80% of plagioclase, ranging 1 3 mm (commonly 0.8 mm in length), 5 15% of quartz (up to 1 mm), and 1 3 mm in size of minor mafic minerals, i.e. amphibole > pyroxene. The phenocrysts account for 60% of the rock volume, which are set in an interstitial groundmass of fine- to medium-grained quartz and plagioclase ( mm in size). The plagioclase shows complex compositional zonings from normal to oscillatory, with relatively narrow rims. The composition of plagioclase varies from labradorite at the cores, and andesine at the rim. Minor biotite and pyroxene are also present locally. 5.2 Alteration mineralogy The hydrothermal alteration and mineralisation within the Batu Hijau deposit developed in four temporally and spatially overlapping stages: early, transitional, late and very late (Fig. 3; Table 1). The andesitic volcaniclastic rocks and equigranular quartz diorite have been altered to form the early central biotite (potassic), early proximal actinolite (inner propylitic), early distal chlorite-epidote (outer propylitic), transitional chloritesericite (intermediate argillic), late pyrophylliteandalusite (advanced argillic), late sericite-paragonite (argillic) and very late illite-sericite zones, whereas the tonalite porphyries are only altered to form the early central biotite (potassic) zone. Highest grade of copper Fig. 3 Simplified map of hydrothermal alteration zone of the Batu Hijau porphyry copper-gold deposit (refer to Table 1 for the abbreviations). 218 Journal compilation 2009 The Society of Resource Geology

5 Alteration geochemistry of Batu Hijau deposit Table 1 Hydrothermal alteration and mineralisation sequences of the Batu Hijau porphyry copper-gold deposit Magmatic Early Transitional Late (feldspar destructive) Very late (Illite-sericite) Chlorite-epidote (outer propylitic) Actinolite (inner propylitic) Biotite (potassic) Chlorite-sericite (interm. argillic) Pyrophylliteandalusite (advanced argillic) Sericiteparagonite (argillic) Hbl Chl, Cal, Ep Act, Chl, Rt Bt, Chl, Rt Chl, Bt, Rt Prl, Ser + Pg, Bt Chl, Cal, Ep Act, Bt, Chl Bt, Chl Dkt, Rt Pl (An90-30) Pl (An70-40), Ep, Cal Pl (An70-30), Ep, Cal Pl (An30-2), Anh, Cal Pl (An30-5), Ser, Cal And, Ser + Pg, Kln, Ill-Smc, Dsp, Cal Chl, Ser + Pg, Rt Ser, Pg, Ill-Smc, Cal, Tlc Ill-Ser-Smc, Rt, Chl Mag, Ilm Mag, Hem, Ilm Mag, Hem, Ilm Mag, Ilm, Hem Mag, Hem, Ttn Hem, Py, Ccp Hem, Py, Ccp Py, Hem Ccp, Bn, Py, Mo Sulphides Py, Ccp Py, Bn, Ccp Bn, Bn Dg Cc, Ccp Veins and Veinlets Primary rock texture Qtz + Py + Chl vein Qtz + Ep Py Chl veins/veinlets Qtz + Act veinlets Act Qtz + Mag Chl Ep veinlets Mag + Bt Qtz stringers, EDM -like, Am, Ab, A, AB and B veinlets C veinlets (Ccp Bn Py) D veins/veinlets, (Py Ccp Qtz) Py + Ser Pg Qtz veinlets Qtz + Py + Sp + Gn Tnt Ccp veins Preserved Destroyed Preserved Preserved to destroyed along fault zones Increasing in distance Central deposit Increasing in distance Notes: Quartz is ubiquitous; bold font indicates high abundance; regular font indicates moderate abundance; italics font indicates minor or rare abundance. Vein/veinlet nomenclature and description follow those of Gustafson and Hunt (1975); EDM -early dark micaeous. Mineral abbreviations: Act-actinolite, And-andalusite, Anh-Anhydrite, Bt-biotite, Bn-bornite, Cal-calcite, Cc-chalcocite, Ccp-chalcopyrite, Chl-chlorite, Dg-digenite, Dsp-diaspore, Dkt-dickite, Ep-epidote, Gn-galena, Hbl-hornblende, Hem-hematite, Ill-illite, Ilm-ilmenite, Kln-kaolinite, Mag-magnetite, Mo-molybdenite, Pg-paragonite, Pl-plagioclase, Prlpyrophyllite, Py-pyrite, Rt-rutile, Ser-sericite, Smc-smectite, Sp-sphalerite and Tlc-talc. Journal compilation 2009 The Society of Resource Geology 219

6 A. Idrus et al. and gold is associated with the early central biotite (potassic) zone. The term biotite (potassic) alteration is used in this paper for description for alteration in the deposit, in contrast to the common terms of potassic or K-silicate, which was initially defined by Meyer and Hemley (1967). The early hydrothermal alteration at the Batu Hijau deposit is nearly circular and centred on the tonalite porphyries, which typically lacks the two other diagnostic minerals, i.e. K-feldspar and sericite. The potassically altered volcaniclastic rocks are mainly characterised by fine-grained plagioclase, quartz and secondary biotite replacing the amphibole. The altered equigranular quartz diorite is typified by plagioclase, secondary biotite and scattered iron oxides. The medium-coarse plagioclase grains are locally altered to sericite and dusted by biotite magnetite sulphides. A typical characteristic of the early potassically altered tonalite porphyries is their brownish grey colour in hand specimen, which reflects the presence of abundant secondary biotite (10 20%) and the development of discontinuous (lengths less than a few centimeters) magnetite-biotite quartz stringers and EDM -like (early dark micaeous) veinlets (cf. Meyer, 1965) as well as the abundance of early A -family veins/veinlets (cf. Gustafson & Hunt, 1975). The early proximal actinolite (inner propylitic) zone is typically characterised by the presence of finegrained (<0.4 mm, commonly < 0.2 mm), acicular and fibrous mats of actinolites after hornblende, plagioclase, as well as minor biotite, chlorite, and epidote. Secondary biotite appears to be crudely oriented and in fine-grained aggregates. The early distal chloriteepidote (outer propylitic) zone is made up of chlorite and epidote. This alteration zone typically consists of quartz + pyrite + chlorite and quartz + epidote pyrite chlorite veins/veinlets. These veins/veinlets exhibit drusy texture, continuous pattern with regular wallrock contacts and internal banding. The transitional chlorite-sericite alteration occupies a temporal position between the early biotite and late argillic alteration zones. It is characterised by weak to moderate sericitation of the silicate phases, as well as the presence of copper-bearing sulphides and magnetite. Sericitisation is commonly initiated from the plagioclase cores, whereas the rims are converted to albitic composition. Bornite plus magnetite are converted to chalcopyrite, either as disseminated grains or in the C veinlet. The chalcopyritic C veinlet commonly contains quartz, pyrite and minor bornite with chlorite+sericite selvages. Two major alteration zones of the late feldspardestruction can be distinguished within the deposit, i.e. pyrophyllite-andalusite (advanced argillic) and sericite-paragonite (argillic). The late alteration zones crosscut and replace the mineral assemblages in all four older alteration zones. The advanced argilicallyaltered rocks are characterised by the presence of the index minerals, andalusite, pyrophyllite and clay minerals (e.g. kaolinite and illite). Diaspore and dickite may also be present. The late sericite-paragonite (argillic) zone shows complete destruction and replacement of pre-existing mafic minerals (e.g. hornblende, biotite) and feldspar by fine- to medium-grained white micas, i.e. sericite and paragonite. Preservation of the original rocks is also occasionally observed. The late pyritic D veinlets containing sericite paragonite + quartz assemblage are also recognised within the alteration zone. The very late illite-serite zone is relatively narrow with irregular alteration envelopes, which developed along the structure-controlled quartz + pyrite + base metal veins. The very late vein does not exceed 10 cm in width, with a massive to brecciated-structure. The base metal minerals are dominated by sphalerite and galena with minor tennantite and chalcopyrite. 5.3 Alteration geochemistry and elemental mass balance The average geochemical compositions of the leastaltered rocks and various alteration zones associated with the andesitic volcaniclastic rocks, equigranular quartz diorite and tonalite porphyries are tabulated in Table 2. Mass balance calculations, however, are only emphasised on the four major alteration zones, i.e. early central biotite (potassic), early distal chloriteepidote (propylitic), transitional chlorite-sericite (intermediate argillic) and late sericite-paragonite (argillic). Mass balance calculations for the early central biotite (potassic) and early distal chlorite-epidote (propylitic) zones were based on samples from the tonalite porphyries (intermediate tonalite) and andesitic volcaniclastic rocks, respectively, whereas mass balances for the transitional chlorite-sericite (intermediate argillic) and late sericite-paragonite (argillic) zones were calculated from samples of the equigranular quartz diorite. These were chosen because they are strongly altered, mineralised and have mineral assemblages representative of the alteration zones. As outlined in the previous section, the method of Gresens (1967), as modified by Grant (1986) is employed to calculate the mass balances. Mass and 220 Journal compilation 2009 The Society of Resource Geology

7 Alteration geochemistry of Batu Hijau deposit Table 2 Whole-rock geochemical data (mean) of various fresh/least and hydrothermally altered rocks associated with three major wall-rocks, including tonalite porphyries, equigranular quartz diorite and andesitic volcaniclastic rocks Major oxides/ elements Fresh/least altered rocks Hydrothermally altered rocks Biotite (potassic zone) Chlorite-epidote (propylitic zone) Chlorite-sericite (interm. argillic) Sericite-paragonite (argillic) Pyrophyllite-andalusite (advanced argillic) It (N = 2) Yt (N = 3) Qde (N = 4) Vxl (N = 6) It (N = 10) Yt (N = 10) Qde (N = 8) Vxl (N = 9) Qde (N = 2) Vxl (N = 21) Qde (N = 8) Vxl (N = 10) Qde (N = 12) Vxl (N = 9) Qde (N = 3) Vxl (N = 2) SiO TiO Al2O Fe2O MnO nd 0.11 nd nd nd MgO CaO Na2O K2O P2O LOI Total Density S C Cr Co Sc V Cu Zn Sn Mo nd nd nd nd nd As nd nd Se nd nd Au nd nd Rb Ba Sr Ga Hf Zr Nb Y Th U nd La Ce Pr Nd Sm Eu Gd Tb nd Dy Ho nd 0.14 Er Tm Yb Lu Notes: Major oxides, LOI (Loss of Ignition), S and C are in wt%, all trace elements are in ppm with an exception of Sc and Au in ppb, and density is in g/cm 3 ; abbreviation of rock types: It-intermediate tonalite, Yt-young tonalite, Qde-equigranular quartz diorite and Vxl-andesitic volcaniclastic rocks. N, numbers of analyzed samples; nd, not detected. Journal compilation 2009 The Society of Resource Geology 221

8 A. Idrus et al. volume changes are calculated using immobile element concentrations in an altered sample against those in its least-altered precursor. Therefore, the selection of immobile elements is the most important aspect of the mass balance analysis. Titanium, Al and Zr have been shown to be relatively immobile during hydrothermal alteration, and have been used often as basis of mass balance calculations in many copper-gold porphyry deposits (e.g. Hezarkhani, 2002; Ulrich & Heinrich, 2002). The correlation coefficient, r, provides an estimate of the relative immobility of one element with respect to another element with relation to any change in mass or volume in a sample suite (MacLean & Kranidiotis, 1987). Selected elements Al, Zr, Ga, Hf and the heavy REE (HREE) plotted against Ti indicate a correlation coefficient r 3 0.6; these elements are considered as immobile (Idrus, 2006). The absolute composition change (DC) is defined as the concentration change of each oxide/element referenced to its original concentration, and can be visualised as the vertical distance between the isocon and the individual elements, calculated using the following equation: [ ] a ΔC= ( 1 S) ( C C ) 1 where S is the gradient of the immobile isocon and C a /C is the ratio of the element concentration in the altered and least-altered rock(s). For the quantification of the volume and mass changes (DV; DM), the following equations were used (Grant, 1986): a ΔV = ( 1 S) [( r r ) 1] 100 ΔM = [( 1 S) 1] 100 where DV and DM are volume and mass losses or gains in percent and r a /r is the ratio of the specific density (g cm -3 ) in the altered against least altered rocks. The isocon diagrams of elemental mass balance between the altered rocks associated with the four major alteration zones and their precursors are shown in Figure 4a d, whereas the gains and losses of major and trace element concentration for the selected sample pairs are diagramatically illustrated in Figure 5a d Early central biotite (potassic) zone When normalised to the least-altered rocks, the biotitealtered intermediate tonalite shows a slight decrease in the mass and volume ( and %, respectively; Fig. 4a). The rocks indicate strong gains of K 2O, S and CO 2, and minor increases in SiO 2, MgO and Na 2O as well as losses in Fe 2O 3 and CaO (Fig. 5a). Rubidium strongly increases with increasing K 2O, which is related to the high abundance of secondary biotite in the rocks. The gain of Na 2O may reflect sodic replacement on the rims of plagioclase phenocrysts. The local plagioclase breakdown may lead to the depletion of Ba and Sr. The replacement of primary Ca-rich hornblende (CaO = 10 wt percentage) by secondary biotite may contribute to the depletion of CaO and probably Cr. It is assumed that some CaO lost from hornblende during biotitization may be fixed as anhydrite (cf. Mitchell et al., 1998), and may partly be introduced during the formation of actinolite, calcite and epidote in the distal part of deposit. As expected, Cu, Au and Se are strongly added with enrichment factors of 2.79, 2.10 and 2.11, respectively. These are consistent with the high abundance of the early copper-bearing sulphides, including chalcocite, digenite and bornite Early distal chlorite-epidote (outer propylitic) zone The outer propylitic-altered volcaniclastic rock at the periphery of the deposit is unchanged or slightly depleted in their mass and volume ( and %, respectively; Fig. 4b) with respect to the least-altered rocks. The altered rocks typically show a strong increase of CO 2, but are anomalously slightly depleted of CaO (Fig. 5b). The depletion of CaO in the propylitic alteration has also been reported by Garwin (2002), and may suggest that calcium concentration was controlled by replacement of calcic plagioclase rather than the presence of Ca-bearing hydrothermal minerals, e.g. calcite and epidote in the rocks. Silica and Fe 2O 3 are relatively unchanged or slightly enriched, whereas MgO, Na 2O, K 2O and S are slightly depleted. Barium and Rb are moderately added, probably absorbed by plagioclase and sericite. The rocks show a moderate increase in Cu and minor increase in Au, reflecting the presence of minor chalcopyrite and rare early sulphides (chalcocite, digenite and bornite), respectively Transitional chlorite-sericite (intermediate argillic) zone The transitional chlorite-sericite (intermediate argillic) altered rock is normalised to the early biotite (potassic) altered rock, its precursor rock, as indicated by their overprinting relationships. The altered rocks are characterised by a slight depletion in their mass and volume ( and %, respectively; Fig. 4c). This 222 Journal compilation 2009 The Society of Resource Geology

9 Alteration geochemistry of Batu Hijau deposit (a) (b) (c) (d) Fig. 4 Elemental mass balance plots using the isocon diagram method (after Grant, 1986): (a) early central biotite (potassic) zone (N = 10) with respect to the least-altered intermediate tonalites (N = 2) (b) early distal chlorite-epidote (outer propylitic) zone (N = 10) normalised to the least-altered andesitic volcaniclastic rocks (N = 4) (c) transitional chloritesericite (intermediate argillic) zone (N = 8) compared with the early biotite (potassic) zone (N = 8) of equigranular quartz diorite, and (d) late sericite-paragonite (argillic) zone (N = 12) in comparison with the transitional chlorite-sericite (intermediate argillic) zone (N = 8) of equigranular quartz diorite. Notes: The major oxides are in wt% and elements in ppm, with the exception of S and C in wt% and Au in ppb. N = number of rocks analysed. is consistent with overall losses of major oxides, including SiO 2,Fe 2O 3, CaO, Na 2O and K 2O as well as S. The chloritisation of secondary biotite and the plagioclase breakdown may be represented by the depletion of the total iron and alkalis. Magnesium oxide tends to be conserved or may be slightly enriched during the replacement of those silicate minerals to chlorite and sericite. Barium and Mo are commonly enriched, with an enrichment factor of (Fig. 5c), which represents the local occurrence of molybdenite within the alteration zone. Copper and Au decrease with decreasing abundance of the early copper-bearing sulphides Late sericite-paragonite (argillic) zone In comparison to the transitional chlorite-sericite (intermediate argillic) alteration, the late sericiteparagonite (argillic) alteration displays a moderate decrease in mass and volume ( and %, respectively; Fig. 4d). The mass and volume decrease would be even greater, if it is normalised to the least-altered rocks due to the complete destruction of early mafic minerals and plagioclase. The altered rocks are depleted in all major oxides, with an exception of Fe 2O 3, which is slightly enriched (Fig. 5d). The addition of Fe 2O 3 may correspond to the high abundance of hematite (after magnetite). Calcium oxide exhibits the strongest depletion among alkalis. This may reflect the destruction of calcic plagioclase at a rate greater than the depletion of K 2O and Na 2O, which are incorporated into sericite and paragonite, respectively. Sulfur in the rocks shows an enrichment factor of This is consistent with the high abundance of pyrite and minor chalcopyrite. The change in concentration of Journal compilation 2009 The Society of Resource Geology 223

10 A. Idrus et al. (a) (b) (c) (d) Fig. 5 Concentration change (DC) of selected oxides/elements during hydrothermal alteration processes: (a) early central biotite (potassic) zone (N = 10) with respect to the least-altered intermediate tonalites (N = 2) (b) early distal chloriteepidote (outer propylitic) zone (N = 10) normalised to the least-altered andesitic volcaniclastic rocks (N = 4) (c) transitional chlorite-sericite (intermediate argillic) zone (N = 8) compared with the early biotite (potassic) zone (N = 8) of equigranular quartz diorite, and (d) late sericite-paragonite (argillic) zone (N = 12) in comparison with the transitional chlorite-sericite (intermediate argillic) zone (N = 8) of equigranular quartz diorite. Notes: The major oxides are in wt% and elements in ppm, with the exception of S and C in wt% and Au in ppb. N = number of rocks analysed. the trace elements involves a moderate enrichment of Rb, which may be fixed into sericite and paragonite, and loss in Ba and Sr reflecting feldspar destruction. Copper, Mo and As are enriched, whereas Au tend to be depleted. The Cu gain and Au loss correspond to the presence of minor chalcopyrite and the general absense of early sulphides, respectively. 6. Discussion 6.1 Chemical discriminations between hydrothermal alteration zones The chemical discrimination between the hydrothermal alteration zones enable a comprehensive understanding of the processes and fluids involved, which may also be applied to potential exploration targets. There are essentially two possible methods to chemically distinguish between the alteration zones: (i) variations in the bulk rock chemistry, which is illustrated using, e.g. millicationic R 1-R 2 diagram (De La Roche et al., 1980) and REE-chondrite (C 1) normalised patterns; and (ii) chemical compositional variations of diagnostic hydrothermal minerals. This study focuses on the first method Major element geochemical discrimination In this section, the chemical discriminations are only emphasised on the bulk rock chemistry of the andesitic volcaniclastic rocks and equigranular quartz diorite. The rock types are selected due to their presence in all hydrothermal alteration zones within the deposit. For comparative purposes, the geochemical data of both least-altered rock types are also incorporated. The R 1-R 2 diagrams take into consideration all the major cations, the degree of silica saturation and the combined change in Fe/(Fe + Mg) and (Ab + Or)/An, in 224 Journal compilation 2009 The Society of Resource Geology

11 Alteration geochemistry of Batu Hijau deposit the formulas R 1 = 4Si - 11(Na + K) - 2(Fe + Ti) and R 2 = 6Ca + 2Mg + Al (plotted along the x and y axis, respectively, Fig. 5a, b; De La Roche et al., 1980). Although initially applied to discriminate between volcanic and plutonic rocks, it has since been used to characterise chemical variations (e.g. Robb & Meyer, 1987). To increase the efficiency of the plots, the compositions of the major mineral constituents including some diagnostic hydrothermal minerals have been added. In relation to the mineral compositions, the trends in the alteration zones are more conspicuous. The R 1-R 2 diagram appears to be more suitable in evaluating the dispersion of the hydrothermal mineral assemblages. Both the andesitic volcaniclastic rocks and the equigranular quartz diorite show relatively similar trends in their hydrothermal alteration signatures. The trends, as represented by the arrows in Figure 6a, b, reflect an increase in the alteration intensity, from the least-altered rock, through early proximal actinolite (inner propylitic)-distal chlorite-epidote (outer propylitic), central biotite (potassic) and transitional chloritesericite (intermediate argillic), to late sericite-paragonite (argillic) and late pyrophyllite-andalusite (advanced argillic) alteration zones. The replacement of primary hornblende by secondary chlorite, actinolite and biotite, depicts a trend in the distal chlorite-epidote (outer propylitic), proximal actinolite (inner propylitic) and central biotite (potassic) alteration zones, respectively. The destruction and/or replacement of the plagioclase by sericite-paragonite and pyrophyllite-andalusite portray a trend in the late argillic and advanced argillic alteration zones, successively. Chemically, the alteration trend may be attributed to the general decrease in Ca, Mg, Na and K, as a consequence of the destruction and/or replacement of the mafic minerals and plagioclase. The chemical discrimination also indicates that R 1 increases and R 2 decreases with increasing alteration intensity, from the leastaltered (shaded area; Fig. 5a, b), through early, transitional, to late alteration zones. The increase of R 1 and the decrease of R 2 correspond to the general decrease of mass and volume of the altered-rocks associated with the subsequent alteration zones REE geochemical discrimination Rare earth elements can potentially be remobilised during alteration, particularly K-silicate (potassic), sericitic, argillic and propylitic alterations (Alderton et al., 1980; Schneider et al., 1988; Ward et al., 1992; Poitrasson et al., 1995). The geochemical analyses of the rocks at the Batu Hijau deposit, i.e. tonalite porphyries (a) (b) Fig. 6 R1-R2 diagrams (after De La Roche et al., 1980) discriminating various alteration zones within the Batu Hijau deposit with respect to the host rocks: (a) andesitic volcaniclastic rocks (N = 51), and (b) equigranular quartz diorite (N = 49). The chemical discrimination indicates that R1 increases and R2 decreases with increasing the alteration intensity, from the leastaltered (shaded area), through early, transitional, to late alteration zones. Major mineral compositions help distinguish the differences between the alteration assemblages (Refer to Table 1 for the abbreviations). and equigranular quartz diorite as well as the andesitic volcaniclastic rocks reveal that the light REE (LREE) and middle REE (MREE) concentrations are slightly changed during hydrothermal alteration (cf. Fig. 6a, b). In general, the absolute REE concentration in the rocks tends to decrease with increasing alteration intensity from the least-altered rock, through the proximal actinolite-(distal chlorite-epidote), the transitional Journal compilation 2009 The Society of Resource Geology 225

12 A. Idrus et al. (a) (b) Fig. 7 REE-chondrite (C1) normalised patterns discriminating various alteration zones within the Batu Hijau deposit with respect to the host rocks: (a) andesitic volcaniclastic rocks (N = 14), and (b) equigranular quartz diorite (N = 12). The patterns show that the REE are depleted with increasing alteration intensity from the least-altered rock (shaded area), through early proximal actinolite (Act)- distal chlorite-epidote (Chl-Ep), central biotite (Bt) and transitional chlorite-sericite (Chl-Ser), to the late pyrophyllite-andalusite (Prl- And) alteration zones. C1 chondrite normalization values from Sun and McDonough (1989). chlorite-sericite and the early central biotite, to the late pyrophyllite-andalusite alteration zones. The systematic decrease in REE concentrations are also illustrated by their chondrite (C 1)-normalised patterns (Fig. 7a, b). The general depletion and similar pattern of the REE in the altered-rock types, particularly the equigranular quartz diorite (Fig. 7b), corresponds to mass and volume losses of the rock, which increases with increasing alteration intensity, from the earlytransitional to the late hydrothermal alteration zones as outlined in the previous section. The overall depletion of REE is also closely related to the stability of hydrothermal minerals during alteration. The LREE in the altered volcaniclastic rocks of the distal chlorite-epidote and transitional chloritesericite alteration zones, however, are slightly increased (Fig. 7a), which may indicate that epidote and chlorite accomodate the released REE during the breakdown of the primary phases. The REE in the actinolitic-altered volcaniclastic rock (sample P35) are anomalously depleted in the LREE. The sericiteparagonite altered rocks are commonly enriched in the 226 Journal compilation 2009 The Society of Resource Geology

13 Alteration geochemistry of Batu Hijau deposit HREE, due to the ability of sericite-paragonite and possibly chlorite to absorb the released elements. The pyrophyllite-andalusite (advanced argillic) altered rocks display the strongest REE depletion, which is explained by the complete breakdown of the primary feldspar and mafic minerals during the alteration, and the inability of the aluminosilicates and coexisting clay minerals to capture all the released elements (Fig. 7a, b). 6.2 Origin of hydrothermal alteration zones The early central biotite (potassic) alteration is intimately associated with the highest copper and gold grade within the deposit. This alteration zone is predominantly originated by magmatic hydrothermal fluid (Idrus, 2006). The lack of K-feldspar related to the alteration zone may, in part, reflect the low-k calcalkaline nature of the tonalitic magmas that led to the development of the hydrothermal system. Mitchell et al. (1998) attributed the deficiency of potassium to the lack of magma contamination by crustal rocks or sea floor sediments. Mass balance calculations indicate that the potassic-altered rocks are generally enriched in K, Na, minor Fe, Si as well as CO 2, S and Cu. During the consumption of primary hornblende and calcic plagioclase, Ca and probably Mg, Sr and Ba are removed. In solution, the aqueous sulphur species may be represented by sulphuric acid (H 2SO 4) and hydrogen sulphide (H 2S). Mitchell et al. (1998) outlined the conversion of hornblende to biotite, anhydrite and minor magnetite by addition of acid and sulphate in the form of H 2SO 4 as well as the removal of Mg. This is supported by the experimental data of Brimhall et al. (1985) which indicates that the biotitization of hornblende was produced by a generalised reaction: hornblende + K 2SO 4 + H 2SO 4 biotite + anhydrite + quartz, over a temperature range of C. The development of discontinuous pattern (lengths less than a few centimeters) of EDM-like stringers and A -veins/ veinlets family in this alteration zone suggests a plastic rather than a brittle fracturing (cf. Gustafson & Hunt, 1975; Muntean & Einaudi, 2001). The plastic nature of the veinlets may indicate their formation under lithostatic regime (cf. Fournier, 1999). Copper concentration is considerably enriched, about 5 10 times the leastaltered rocks. The early distal chlorite-epidote (outer propylitic) alteration zone is typically characterised by a high CaO content (up to 11.3 wt%). The relatively high CaO is clearly influenced by the presence of the secondary Ca-rich minerals, i.e. epidote and calcite. The epidote replacing plagioclase is represented by the following chemical reactions (cf. Meyer & Hemley, 1967; Bowman et al., 1987): + 2NaCaAl3Si5O16+ 2SiO2+ Na + H2O andesine = Ca Al Si O ( OH)+ 3NaAlSi O clinozoisite H The veins/veinlets associated with this alteration zone commonly show a continuous pattern, regular wallrock contact and drusy texture, which may suggest their formation by open-space filling in brittle rock under hydrostatic regime (cf. Fournier, 1999). The transitional chlorite-sericite (intermediate argillic) alteration zone is characterised by chloritisation and the breakdown of plagioclase, which are expressed by a general depletion of ferromagnesian oxides and alkalis. Some K 2O liberated through chloritisation may, in part, be fixed into sericite. Chloritisation and plagioclase breakdown are consistent with the moderate amounts of the major oxides with respect to those in the potassic-altered rocks. The chloritisation of biotite is represented by the following chemical reaction (cf. Meyer & Hemley, 1967; Mitchell et al., 1998): + 3KMgFeAlSiO ( 2 ) 3 10( OH) 2 + 4H biotite = Mg FeAl Si O ( OH) + 3SiO + Fe chlorite 2 quartz + 2 2K + Chalcopyrite is the typical sulphide present. It occurs either in the C veins (chalcopyrite bornite) or as disseminated grains. The chalcopyrite, in part, is inferred to be a product of sulphidation process of the early bornite solid solution. The late sericite-paragonite (argillic) alteration zone is typified by variably low content of ferromagnesium oxides (1 11 wt% Fe 2O 3 and <6 wt% MgO). The low concentration of these elements is consistent with the complete destruction and replacement of pre-existing mafic minerals (e.g. hornblende, biotite) and plagioclase by fine- to medium-grained white micas (sericite and paragonite). The complete destruction of the preexisting minerals is consistent with the overall decrease of volume and mass ( and %, respectively) of the late argillic-rocks with respect to their precursors (transitional altered-rocks). These results are similar to the mass balance calculations for rocks related to the feldspar-destructive zones Journal compilation 2009 The Society of Resource Geology 227

14 A. Idrus et al. 7. Conclusions Fig. 8 Mineral stability expressed in terms of cation activity ratios of K + /H + and Mg +2 /H + with a coexisting aqueous phase at 300 C and 500 bars. The schematic locations of the major hydrothermal alteration zones from early potassic and propylitic, through transitional intermediate argillic, to late argillic and advanced argillic observed in the Batu Hijau deposit are shown by solid and grey circles. The grey arrow represents changes in assemblages from early, transitional to late alterations. The dashed thin line represents talc saturation. The diagram is modified from Beane and Titley (1981), on the basis of the data from Bowers et al. (1984). in other porphyry copper deposits (e.g. Sungun-Iran, Hezarkhani, 2002; Alumbrera-Argentina, Ulrich & Heinrich, 2002). The complete destruction of the preexisting mafic minerals and plagioclase during the formation of the late argillic alterations is chemically expressed by the decrease of K + and Mg +2 activities relative to the H +, as shown in the mineral stability diagram (Fig. 8). In general, a decrease of cations from the early, transitional to late alteration zones may imply a general decrease of element activities in the hydrothermal fluids during alteration (Fig. 8). The late pyritic D quartz veins/veinlets associated with this alteration zone typically show drusy texture, continuous pattern (lengths greater than tens of centimeters) and regular wall-rock contacts, which may suggest that the veins/veinlets formed by open-space filling under hydrostatic conditions (cf. Fournier, 1999; Muntean & Einaudi, 2001). Four stages of ore-related hydrothermal alteration (early, transitional, late and very late) are recognised at the Batu Hijau porphyry copper-gold deposit. The early central biotite (potassic) zone is enriched in Si, Fe, K, Rb, S, Cu and Au with a decrease of mass and volume decrease of % and %, respectively. Copper and Au enrichments represent the abundance of copper-gold-bearing sulphides. The early proximal actinolite (inner propylitic) zone shows a general enrichment in Ca, which indicates the presence of Ca-bearing silicates. The early distal chlorite-epidote (outer propylitic) zone is relatively unchanged or slightly depleted in mass and volume of % and %, respectively. The transitional chlorite-sericite (intermediate argillic) zone shows a general depletion of ferromagnesian oxides and alkalis, which is consistent with a decrease of mass and volume of and %, respectively. The late sericite-paragonite (argillic) zone is characterised by a complete destruction of pre-existing mafic minerals and plagioclase, expressed by an overall decrease of mass and volume ( and %, respectively). In general, major elements (Ca, Mg, Na and K) decrease from least altered rocks towards the late alteration zones as a consequence of breakdown of hornblende, biotite and plagioclase. The chemical discrimination also indicates that R 1 [4Si - 11(Na + K) - 2(Fe + Ti)] increases as well as R 2 [6Ca + 2Mg + Al] and REE decrease with increasing alteration intensity, from least-altered, through early, transitional, to late alteration zones. Degree of mass and volume losses increases during the alteration stages. A decrease of the elements from early, transitional to late alteration zones implies a general decrease of the element activities in hydrothermal fluids during the alteration. Acknowledgments This paper is a section of the first author s PhD thesis completed at the Institute of Mineralogy and Economic Geology, RWTH Aachen University, Germany. The authors are very thankful to the management of the Newmont Nusa Tenggara Company, which has given permission to do investigations at the Batu Hijau deposit and its vicinity. The authors also wish to express honest gratitude to Thomas Derichs as well as Dr Annemarie Wiechowski and Roman Klinghardt for the sample preparation and assistances in electron 228 Journal compilation 2009 The Society of Resource Geology