The Mineralogy of Gold-copper Skarn

J. SE Asian Appl. Geol., Jan–Jun 2011, Vol. 3(1), pp. 12–22

THE MINERALOGY OF GOLD-COPPER SKARNRELATED PORPHYRY AT THE BATU HIJAUDEPOSIT, SUMBAWA, INDONESIA

May Thwe Aye∗1, Subagyo Pramumijoyo1, Arifudin Idrus1, Lucas Donny Setijadji1, AkiraImai2, Naoto Araki2, and Johan Arif3

1Department of Geological Engineering, Gadjah Mada University, Indonesia2Department of Earth Resources Engineering, Kyushu University, Japan3Mine Geology Department, PT Newmont Nusa Tenggara, Indonesia

Abstract

Clacic gold–copper bearing skarn in the Batu Hijauporphyry deposit is located in the western part ofSumbawa Island, Indonesia. Skarn mineralizationswere found at the deep level of the deposit (-450m to-1050mL) by drilling program 2003. No evidencearound Batu Hijau has limestone although mostskarn are metasomatized from carbonate-rich rock aslimestone or marble. Most skarn-type metasomaticalteration and mineralization occurs at the contactof andesitic volcanic rock and intermediate tonaliteporphyry intrusion and within intermediate tonalitein some. Although both endoskarn and exoskarn canbe developed, it has no clear minerals to known theendoskarn. Exoskarn is more principle skarn zone.The formation of skarn occurred two min stages: (1)prograde and (2) retrograde. The prograde stage istemporally and spatially divided into two sub-stagesas early prograde (sub-stage I) and prograde metaso-matic (sub-stage II). Sub-stage I begin immediatelyafter the intrusion of the tonalite stock into the cal-cium rich volcanic rocks. Then, sub-stage II origi-nated with segregation and evolution of a fluid phasein the pluton and its invasion into fractures andmicro-fractures of host rocks developed during sub-stage I. The introduction of considerable amount ofFe, Si and Mg led to the large amounts of medium-to coarse-grained anhydrous calc-silicates. From the

∗Corresponding author: M.T. AYE, Department of Ge-ological Engineering, Gadjah Mada University, Indonesia.E-mail: [email protected]

texture and mineralogy, the retrograde metasomaticstage can be divided into two sub-stages: (a) earlyretrograde and (sub-stage III) and (b) late retrograde(sub-stage IV). During sub-stage III, the previouslyformed skarn zones were affected by intense multiplehydro-fracturing phases in the gold-copper bearingstocks. Therefore, the considerable amounts of hy-drous calc-silicates (epidote), sulfides (pyrite, chal-copyrite, sphalerite), oxides (magnetite, hematite)and carbonates (calcite) replaced the anhydrous calc-silicates. Sub-stage IV was coexisting with the in-trusion of relatively low temperature, more highlyoxidizing fluids into skarn system, bringing aboutpartial alteration of the early-formed calc-silicatesand developing a series of very fine-grained aggre-grates of chlorite, clay, hematite and calcite.Key words: Gold-copper skarn, metasomatic alter-ation, Batu Hijau, Indonesia

1 Introduction

Gold-copper skarns probably the world’s mostabundant among skarn types, which can bemainly produced in orogenic zones related tosubduction zone setting, both in oceanic andcontinental settings (Einaudi, 1982a and Ein-audi, 1982b; Einaudi et al., 1981). Most are asso-ciated with I-type calc-alkalic, porphyritic plu-tons and form along the contact with intrusivestocks (Meinert, 1992).

The Batu Hijau deposit, the gold-copperskarn related porphyry deposit is located in the

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THE MINERALOGY OF GOLD-COPPER SKARN RELATED PORPHYRY AT THE BATU HIJAU DEPOSIT

southwestern part of Sumbawa Island, Indone-sia (Figure 1a). The skarn was formed in deeplevel of the deposit from -450m to -1050mL. an-desisitc volcaniclastic rock. Ore grade in skarnmineralizaion (1.7g/tAu and >4% Cu) whichmainly occurred in the magnetite broken zone.Proffett (2003) firstly introduced and Steyankhaet al. (2008) reported about skarn mineraliza-tion by microscopic examination and Idrus et al.(2009b) carried out some preliminary investi-gations on the alteration and mineralization inthis skarn. In this study, focuses principally onmore detailed geologic aspects, such as skarnmineralogy, texture and zonation, considersmetamorphic and metasomatic alteration andmineralization during prograde and retrogradestages of skarn forming processes and finallydeals with mineralizing fluids which controlledthe skarn mineral assemblages.

2 Background geology of skarn

The skarn mineralization at the Batu Hijaudeposit is closely related to the intermedi-ate tonalite hosting a porphyry copper-typedeposit. The Early to Middle Pliocene por-phyry mineralization associated with tonaliteporphyry stock intruded the Early to MiddleMiocene andesitic volcanic rocks which hostthe skarn rocks. Figure 1b shows the geologicalmap of Batu Hijau illustrating the bore hole lo-cation of skarn distribution. The skarn ores areused to be exploited in the skarn zone and thecontact of cacic rich volcanic rock and the adja-cent of intermediate tonalite porphyry stock inFigure 1c. The size of skarn can only estimateapproximately 400m thick by drill hole datacaused by most skarn occurrences from -500 to-900mL by drill hole. The associated contactmetasomatic alteration and skarn mineraliza-tion are well developed in places where thefracture density in the host rocks is relativelyhigh. Metasomatic effects are well prominentat the contact of intermediate tonalite porphyrystock with andesitic volcanic rocks, and reducegradually outwards from the contact.

3 Method of investigation

Some representative samples from the skarnrocks were thin-sectioned and examined micro-scopically. The XRD technique was employedwhen microscopic identification for mineralswas uncertain. Scanning Electron Microscope(SEM/EDX) was used for determining the com-position of anhydrous silicates, such as pyrox-enes and garnets, accurately. These analyseswere carried out at the Department of EarthResources Engineering, Faculty of engineering,Kyushu University, Japan.

4 Petrography

4.1 Andesisitc volcaniclastic rocks

The andesisitc volcaniclastic rocks consist oftwo majoir units: a lower sequence of fine-grained volcaniclastic and an upper sequence ofvolcanic lithic breccia (McPhie et al., 1993). Thelater is the most wide spread unit of the studyarea and also the protolith for the skarn (Fig-ure 1b). Megascopically, they are greenish grayto dark gray, massive to stratified, crystal andlithic-rich volcanic mudstone, sandstone, brec-cias and conglomerates (Gerteisen, 1998). Un-der the microscope these rocks contains grainsof 10–20% of broken plagioclase and mafic sil-icate minerals (amphibole>clinopyroxene) aswell as minor volcanic lithic fragments. The-ses clasts are contained in a plagioclase grain-rich, very fine sand ot mud matrix. the clast isvaries from rounded to angular, but is mostlysubrounded to subangular, with width rainingfrom 2–64mm (granule to pebble) and locallyexceed 100mm. Chlorite and calcite partiallyto completely replace the mafic silicate min-erals. Plagioclase is locally replaced by epi-doe. Magnetite, chlorite, epidote, calcite andminor pyrite are also present. The composi-tion of clasts and matrix are probably derivedfrom plagioclase-rich andesitic volcanic sourceclosed to calc-silicate rock (Figure 2A).

4.2 Tonalite porphyry intrusion

This intruded rock which hosts the porphyrycopper ores at the Batu Hijau has undergone

© 2010 Department of Geological Engineering, Gadjah Mada University 13

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Figure 1: (a) Map of Indonesia showing the location of the Batu Hijau deposit; (b) geological mapof the Batu Hijau deposit illustrating the bore hole location of skarn distribution (c) a profile oflithologic cross-section of porphyry and skarn contact along the direction A–B

14 © 2010 Department of Geological Engineering, Gadjah Mada University


intense multiple hydro-fracturing and perva-sive hydrothermal alteration processes. Al-though three phase of tonalte porphyry intru-sions, intermediate tonalite porphyry intrusionis closely associated with skarn mineralizationbecause skarn mineralization occurred alongthe contact of host rock, volcanic lithic brecciasand intermediate tonalite porphyry intrusion.Under the microscope these rocks is character-ized by light gray color and show typical por-phyritic texture, containing phenocrysts of pla-gioclase (Figure 2B). Plagioclase is the predomi-nant phenocrysts up to 10-25 vol.% of the rocks,and generally euhedral-subhedral and uniformin size from 2–3mm. The secondary biotitealso was partially, to completely alter to chloriteand opaques (sulfides). Pyrite and chalcopyriteare the most abundant sulfide minerals and themost important copper ore in the intermediatetonalite stock.

Figure 2: Photomicrographs of (A) the chlorite-epidote-altered volcaniclastic rocks: Chloritetogether with an aggregate of epidote, replacingplagioclase-containing clast in the rocks, and(B) the intermediate tonalite (plain polarizedlight)

5 Skarn mineralogy

Various types of anhyrous and hydrous calc-silicates (garnet, diopside, amphibole, epidote),silicates (chlorite, quartz, clay minerals), sul-fides (pyrite, chalcopyrite, sphalerite, galena,bornite), oxides (magnetite, hematite) and car-bonates (calcite) were developed during se-quential stages of skarn forming processes inthe skarn zones along the contact between in-termediate tonalite porphyry stock and the vol-canic rocks. Figure 3 showing the paragenetic

sequence of minerals and ore in skarn at theBatu Hijau deposit.

Figure 3: Paragenetic sequence of mineralspresent in skarn in the Batu Hijau deposit

Garnets occur ubiquitously in the skarn zoneas fine to coarse-grained, anhedral to euhedralcrystals. Most crystals display concentric zon-ing (Figure 4a). Textural examination showsthat two types of garnets, belonging to differ-ent generations, (Figure 4b). are distinguish-able as (1) fine-to medium-grained, anhedralto euhedral, reddish brown, anisotropic garnetswhich are mainly present in the outer parts ofthe skarn zone (proximal skarn), near the vol-canic contact. Most garnet may be the productof metamorphic processes during which diffu-sion of elements between clay minerals and cal-cium rich in host rocks took place. (2) Medium-to coarse-grained, subhedral to euhedral, darkbrown garnets, occurring principally in thevicinity of the contact. They occur as patchyaggregates with conspicuous growth zoning,and are isotropic. Coarse-grained anisotropic


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garnets have being altered to epidote and cal-cite (Figure 4c). Overall the SEM/EDX datashow that the garnets are mainly andraditic gar-net and most are partially, to entirely alter toepidote, calcite, quartz, ± sulfides, ± hematite,± chlorite and ± clay minerals mainly alongfractures and micro-fractures.

Clinopyroxenes occur as fine- to medium-grained, anhedral to subhedral crystals show-ing decussate texture (Figure 4d). XRD andSEM/EDX data shows that the clinopyroxeneare mainly diopside. They are cutting cross bymagnetite veinlets and altered to chlorite (Fig-ure 4f).

They are partially to entirely alter to amphi-bole (tremolite–actinolite) (Figure 4d), quartz,calcite, ± chlorite, ± clay and ± opaques (sul-fides and oxides). The alteration is more pro-nounced along calcite micro-veinlets. Althoughcompositional variations of the pluton and pro-tolith exert a strong control on the mineralogyof the skarn zones (Meinert, 1995 and Ray et al.,1995), generally the higher ratios of garnet topyroxene in the study area may be related tothe oxidation state of the pluton and the wallrock (Meinert, 1997). Clinopyroxene occurs asvein which cut cross fine-grained garnet alteredby chlorite (Figure 5a) and also cut crossed byquartz veinlets and magnetite veinlets in some(Figure 5b).

Epidote occurs as fine to medium-grained;anhedral to subhedral crystal aggregates show-ing granoblastic to decussate texture (Figure5c) and are present mainly associated with gar-nets. The majority of epidote is the product ofretrograde alteration by metasomatizing fluids.Most of garnets are replaced by epidote.

Amphibole (tremolite–actinolite) occurs asfine- to medium-grained fibrous and feltedaggregates (Figure 5d) associated with the py-roxenes. Textural relationships suggest thatthey are the product of retrograde alteration ofpyroxenes.

Chlorite is present as fine flakes within theanhydrous and hydrous calc-silicates (Figures4f,5a). It appears to be a late-alteration product.

Quartz occurs as very fine- to medium-grained patchy aggregates, dispersed and soli-

Figure 4: Photomicrographs of skarn samples:(a) concentric zoning in coarse-grained gar-net, XPL; (b) two generation of gernet yel-lowish coarse-grained and and reddish brownmedium-grained which replacement by garnet,XPL; (c) coarse-grained garnet being alteredto epidote and calcite, XPL; (d) decussate tex-ture in medium- to coarse-grained garnet andclinopyroxenes (Cpx), XPL; (e) decussate tex-ture in medium- to coarse-grained clinopyrox-enes (Cpx), XPL; (f) replacement of clinopyrox-ene by chlorite, XPL

tary crystals, and as veinlets and micro-veinletswithin the calc-silicates.

Chalcopyrite is relatively abundant andminute blebs of native gold generated in chal-copyrite associated with sphalerite (Figures6a–d) . It partially to fully replaced by pyrite,and magnetite and occurs as intergranular ag-gregates as solitary crystals within magnetite(Figure 6e–f,7a). In some samples, sphaleriteoccurs as inclusion in chalcopyrite (Figure 7b).Occasionally it has been replaced partially byhematite.

Pyrite occurs mainly within calc-silicatesas solitary and dispersed patchy aggregates



Figure 5: Photomicrographs of skarn sam-ples: (a) Clinopyroxene occurs as vein whichcut cross fine-grained garnet altered by chlo-rite, XPL; (b) clinopyroxene are cut crossed byquartz veinlets and magnetite veinlets, PPL; (c)Epidote showing granoblastic to decussate tex-ture and mainly associated with garnets, XPL;(d) partial alteration of clinopyroxenes by am-phiboles (tremolite–actinolite). XPL; (e) gernetassociated opaque minerals are cut crossed byquartz veinlets and replacement of pyrite bychalcopyrite along the grain margin and frac-ture planes, PPL; (f) zoning plagioclase associ-ated with garnets, XPL

and/or as veinlets and micro-veinlets. It re-places at the margin of chalcopyrite associatedwith sphalerite (Figure 7c).

Clay minerals are commonly accompaniedby aggregates of very fine-grained calcite andhematite and appear to be a late alteration prod-uct.

Magnetite shows as patchy and aggregratetexture and contains lamellae of hematite (Fig-ure 7a,f), and replaces pyrite (Figure 7a) andis replaced partially by chalcopyrite and spha-lerite (Figure 7c). Garnet is mainly commonwithin magnetite.

Sphalerite is observed associated with chal-

Figure 6: Photomicrographs of skarn ore sam-ples. Polarized reflected light: (a,b,c) blebsof native gold (Au) generated in chalcopyrite(Ccp) associated with sphalerite (d) replace-ment of pyrite (Py) by chalcopyrite associatedwith native gold and sphalerite (Sp) (e) replace-ment of pyrite by magnetite and chalcopyrite (f)replacement of magnetite (Mag) by pyrite andchalcopyrite

copyrite, minute blebs of native gold and as in-clusion within gold in some (Figure 6c). Spha-lerite is partially replaced by magnetite andpyrite and chalcopyrite (Figure 7e–f).

Galena partially to completely replace bypyrite associated with sphalerite within chal-copyrite is also present (Figure 7d). Galenaseems to have been the first sulfide mineralthat was precipitated and was then replaced bypyrite.

Hematite occurs as bladed lamellae withinmagnetite (Figure 7f, 8a) and as fine-grainedaggregates filling the micro-fractures in anhy-drous calc-silicates and calcite. Hematite maypartially replace pyrite and chalcopyrite.

Calcite occurs as very fine to coarse (up to5 mm) anhedral to subhedral crystals withinthe skarn zones. Medium- to coarse-grainedcrystals are also present in veinlets cutting calc-silicate aggregates, where they show decussate


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Figure 7: Photomicrographs of skarn ore sam-ples. Polarized reflected light: (a) chalcopy-rite (Ccp) replaced by pyrite (Py) and magnetite(Mag) (b) replacement of magnetite (Mag) bypyrite (Py), chalcopyrite (Ccp) and sphalerite(Sp); (c) pyrite (Py) replace at the margin ofchalcopyrite (Ccp); (d) replacement of galena(Gn) by pyrite (Py) associated with sphalerite(Sp); (e) pyrite and sphalerite replace chalcopy-rite chalcopyrite (f) lamellae of hematite (Hem)within magnetite (Mag)

texture (4 mm). Some calcites along with quartzand opaques replace calc-silicates.

Sphene can be identified by SEM/EDX as in-clusion in other minerals, euhedral crystals isoccasionally present in the skarn zone (Figure8c).

6 Skarn forming processes

Skarn-type metasomatic alteration occurred inthe Batu Hijau as both endoskarn as exoskarn.Although endoskarn was developed only insome place within the intermediate tonaliteintrusion, it is not clearly characterized bythe presence of minerals to know endoskarn.But alteration minerals such as epidote tremo-lite–actinolite chlorite and calcite can be found.

Exoskarn is the major skarn zone in this area,

Figure 8: Photomicrographs of minerals andore in skarn by SEM/EDX (Scanning ElectronMicroscope). Adr: andradite, Di: diopside,Hem: hematite, Bi-Te: bisumith-telluride, Spn:sphene, Kln: kaolinite, Wo: wollastonite

and contains copper and minor zinc sulfides(e.g. chalcopyrite and sphalerite). The thick-ness of this zone varies along the intrusive con-tact and locally reaches up to 400 m (by the borehole). Garnet is the important anhydrous calc-silicate that is ubiquitously present within thiszone. Pyroxene is not present in all samples ofthis zone and shows intense modal variations,and is locally more abundant than garnet. Ingeneral, the skarn rocks in this zone containprincipally fine-grained calcite, garnet, pyrox-ene, epidote , chlorite, clays , and minor sul-fides.

The reactions and the mineral assemblagesformed in skarns usually depend upon thecharacter of invaded rocks and the compositionof the metasomatizing fluids and the overallpressure and temperature regime (Titley, 1973and Guilbert and Lowell, 1974).

Based upon field evidence, along with miner-alogic and petrographic data from intermediatetonalite porphyry intrusion and volcanic rocksand the skarn mineralization zone, the metaso-matic alteration zones in the Batu Hijau can beclassified as porphry-related calcic skarns. Bytaking mineralogical and textural criteria intoconsideration, the process of skarn can be cat-



egorized into two discrete stages: (1) progradeand (2) retrograde.

Prograde: Temporally and spatially this stageis divided into two distinct sub-stages: 1. meta-morphic and 2. prograde metasomatism. Sub-stage I was coeval with the initial emplacementof the tonalite porphyry intruded into volcanicrock, host rocks. The effect of heat flow fromthe pluton caused the enclosing rocks to be-come isochemically metamorphosed in the hostrocks. However, in the clay-rich interlayers,in addition to recrystallized calcites, a seriesof fine-grained, Fe-poor calc-silicates (grossu-laritic garnet, diopsidic pyroxene, tremoliticamphibole, Fe-poor (epidote) and silicates(chlorite and quartz) were developed. Dueto the high thermal effects, (near contact zone),a series of anhydrous and hydrous calc-silicateswere developed within the rocks. No opaqueminerals (oxides and/or sulfides) were formedduring this sub-stage. In substage II, the crys-tallization of magmas is associated with thedevelopment of a volatile-rich phase (Candelaand Piccoli, 1995). This sub-stage probablycommenced with the consolidation and crys-tallization of the magma of porphyry intrusion.As crystallization progressed the volume of hy-drothermal fluid is generated in the porphyryintrusion.

During sub-stage I, metamorphic alterationwas accompanied by decarbonation reactionsthat cause slight volume decrease and lead tothe formation of fractures. This type of frac-turing, with those resulting from upward pres-sure exerted by the ascending magma, and theevolving fluid phase (hydro-fracturing), devel-oped possibility for the infiltration fluids intonear the host rocks. It can be formed duringthe sub-stage I. Evidence of the metasomatic al-teration which occurred during this sub-stage isprovided by medium to coarse-grained Fe-richanhydrous calc-silicate minerals which over-printing almost entirely the sub-stage I assem-blages. During sub-stage II, coarse-grained gar-nets with a granoblastic texture were devel-oped. The replacement of sub-stage I assem-blages by andradite-grossularite may suggeststrongly that substantial quantities of dissolved

components such as Fe, SiO2, and Mg wastransferred into the skarn system by metasom-atizing hydrothermal fluids.

These prograde anhydrous calc-silicate as-semblages in skarn can be correlated with thecharacteristic potassic alteration in the mineral-ized pluton Meinert (1992). By accepting thispremise, the fluid inclusion data from the in-termediate tonalite porphyry intrusion indicatethat the temperature of these magma-derivedfluids was possibly >515 °C (by fluid inclusiondata) and caused the prograde metasomatic al-teration, particularly in the proximity of the in-trusive contact.

Retrograde stage: In some porphyry-relateddeposits, intense retrograde alteration is com-mon in copper skarns (Meinert, 1992) maydestroy most of the prograde anhydrous calc-silicates, (e.g. Ely, Nevada; James, 1976). Thepresence of numerous cross-cutting veinletsand micro-veinlets of quartz, quartz-sulfide,sulfide, calcite, magnetite in anhydrous calc-silicates, which are extent in the adjacent tothe porphyry intrusion, may suggest a coinci-dence of the hydro-fracturing phases duringphyllic alteration in the prograde skarn zonein tonalite porphyry intrusion. generally thereis a correlation between characteristic phyllicalteration in mineralized pluton in PCD andretrograde hydrous calc-silicate assemblagesin skarn (Einaudi, 1982a and Meinert, 1992).Such a correlation between Porphyry Stock andthe associated skarn zone in Batu Hijau can bereasonably proposed.

Mineralogical and textural studies show thatthe retrograde stage can also be divided intotwo distinct (1) early and (2) late sub-stages.During the early of retrograde stage (sub-stage III), large amounts of anhydrous calc-silicates were replaced by a series of hydrouscalc-silicates (epidote, ± tremolite–actinolite),sulfides (pyrite, chalcopyrite and sphalerite),oxides (magnetite, hematite) and carbonates(calcite), mainly along the fractures and micro-fractures. Epidote is the most common al-teration mineral. In the late retrograde stage(sub-stage IV), the development of retrogradeassemblage may characteristically continue


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hydrofracturing processes and prolonged hy-drothermal activity operating in both the min-eralized porphyry intrusion and in the adjacentskarn zone. During this sub-stage both anhy-drous and hydrous calc-silicates were alteredto low temperature fine aggregates mineralsas calcite, clay, chlorite and hematite alongfractures and micro-fractures.

7 Mineralization

Mineralogical and textural studies suggestthat no opaque minerals (oxides and/or sul-fides), were formed in the marmorized andskarnoid–hornfelsic rocks during sub-stage I.Textural evidence, such as lack of intergrowthand non-replacive crystal boundaries betweenopaques and anhydrous calc-silicates, indi-cates that opaques did not form, even duringprograde metasomatism. The existence of re-placement textures between opaques and an-hydrous calc-silicates and open-space fillingtextures in fractures within anhydrous calc-silicates, and of intergrowths between opaquesand hydrous calc-silicates (epidote, tremo-lite–actinolite), provide compelling evidencethat most opaques and hydrous calc-silicateswere developed during early retrograde al-teration (sub-stage III). Textural relationshipsamong hypogene opaque minerals also indi-cate that paragenetically their temporal orderof deposition is as follows: galena → pyrite →

magnetite + primary hematite → chalcopyrite+ sphalerite.

The crytallization of opaques seems to becontrolled chiefly by fractures, open spaces, andthe composition of anhydrous calc-silicates.The concentration of opaque minerals is mainlyin the metasomatic skarn zone. The abundanceand type of opaques varies spatially, depend-ing upon their locality and distance from theintrusive contact. Sulfides are predominantlypresent in the skarn zone.

In many porphyry related skarns, the flu-ids involved in potassic and phyllic alterationsof the mineralized pluton are responsible forprograde and retrograde alteration processes inthe skarn zone respectively (Einaudi, 1982a andMeinert, 1992). Temporal and spatial variations

of physico-chemical conditions such as pres-sure, temperature and some important volatilecomponents in the hydrothermal fluids duringskarnification processes were analyzed throughmineral paragenesis, textural relationships, sta-bility field of the dominant calc-silicates, andfluid inclusion data from the adjacent mineral-ized intermediate tonalite porphyry intrusion.

Sulfur isotopes data from 12 sulfide samples(e.g. pyrite, chalcopyrite and bornite) mainlyfrom porphyry mineralization and from foursulfide samples (e.g. pyrite, chalcopyrite) inskarn show that their d34S values range from−0.1 to 1.7‰. These values indicate a magmaticsource for the sulfur (Ohmoto and Rye, 1979)in both mineralized porphyry and the associ-ated skarn zone. Therefore, it may be suggestedthat the sulfur within the skarn zone originatedfrom a magmatic source and was transferredinto the skarn system by the metasomatizingfluids which in most likelihood was responsiblefor the alteration and associated sulfide miner-alization in the porphyry mineralization.

Depth of skarn formation has strong con-trol in the extent of the skarn, its geometry,and style of alteration (Meinert, 1992). Fluidinclusions can be used for quantitative geo-barometric studies. Fluid inclusion data fromquartz samples collected from the samples as-sociated with pyroxene abundant skarn, garnetabundant skarn and near mineralization vein inthe bore hole SBD-252/1031, 226/694, 273/930,286/814 near the porphyry contact was used tocalculate the approximate depth of skarn for-mation. The estimated depth is ∼1800 m whichwas calculated on the basis of hydrostatic pres-sure head obtained from halite-bearing inclu-sions having TS(NaCl) ≈ TH (L–V) (Calagari,1997). Therefore, the skarn zone in the presentof the study area can be postulated to havebeen developed in hypabyssal environment at adepth of ∼1800 m. The fluids contained Fe, Si,and Mg with high activity, and were relativelyat oxidizing state causing decarbonation reac-tions and development of Fe-rich anhydrouscalc-silicates. Andradite thus formed was sta-ble with the fluid in equilibrium with porphyryintrusion

The development of low-temperature min-



eral assemblages such as chlorite and clayminerals, along with very fine aggregates ofsecondary hematite and calcite, within theearly-formed assemblages may suggest that thelate metasomatizing fluids (sub-stage IV) wereprobably at a higher oxidation state and as aresult that the porphyry system with sulfur-bearing magmatic fluid probably causing theoxidation of sulfur.

8 Discussion and conclusions

Skarn mineralization in the Batu Hijau were de-veloped along the contact of the andesitic vol-canic rocks with the intermediate tonalite por-phyry intrusion. Overall data shows

Field evidence, analytical data, and miner-alogical and textural criteria show that in thestudy area probably the skarn in the Batu Hi-jau derived from the volcanic rock as it is calcicin composition. Furthermore, genetically theformation of skarn processes and their evolu-tionary trend can be categorized into two dis-crete stages: (1) prograde and (2) retrograde.The prograde stage can be divided temporallyinto two distinct sub-stages: (a) early prograde,metamorphism and (b) prograde metasoma-tism.

Metamorphic alteration (sub-stage I) origi-nated shortly after the intrusion of the interme-diate tonalite porphyry into calcium rich hostrock. The alteration progressed isochemicallyby the effects of heat flow from the pluton intothe surrounding cacic rich rocks. The thermaleffects caused layers to recrystallize metamor-phically into calcic rich rocks. In general, someanhydrous calc-silicates in proximity to the con-tact (higher temperature), and some hydrousones at distances farther away, were developedby metasomatic processes from clay-rich inter-layers during this sub-stage. The developmentof these calc-silicates is usually accompaniedby decarbonation reactions which cause a slightvolume decrease and the subsequently forma-tion of fractures and micro-fractures in the hostrock. Since, the protolith was lacking in Fe,the calc-silicates formed during this sub-stagewere Fe-poor garnets and pyroxenes. Almost

no opaque minerals (oxides and sulfides) wereformed during sub-stage I.

Prograde metasomatic alteration (sub-stageII) commenced with the beginning of crystal-lization and consolidation of porphyry intru-sion have concurrent separation of high tem-perature fluids. The invasion and infiltrationof such fluids into secondary fractures, resultedfrom both decarbonation reactions and hydro-fracturing processes, transferred to Fe, Si andMg to the contact zones. The introduction ofthese elements caused the further evolution ofthe decarbonation processes and the formationof medium to coarse-grained anhydrous calc-silicates within a fine-grained matrix. The in-tensity of alteration during sub-stage II was sohigh that in proximal zone to become obscuredor completely obliterated. Two kinds of gen-erations of garnets were also developed duringsub-stage II.

Garnets and pyroxenes formed during sub-stage II are mainly andraditic and hedenbergiticin composition, respectively, which may be dueto the introduction of considerable amounts ofFe into the skarn system by the invading flu-ids. Textural relationships indicate that opaqueminerals were not formed during sub-stage II.

Based upon mineralogical and textural data,the retrograde metsomatic alteration can bedivided into two distinct sub-stages: (1) earlyretrograde (sub-stage III) and (2) late retrograde(sub-stage IV). During sub-stage III, the pro-grade metasomatic alteration zones were prob-ably affected by hydro-fracturing, producednumerous channels for the infiltrating hy-drothermal fluids. As long as the temperatureof such fluids was >515 °C by fluid inclusiondata, the calc-silicates in skarn zones remainedstable. The considerable amounts of hydrouscalc-silicates (epidote, tremolite–actinolite),sulfides (sphalerite, chacopyrite, and pyrite),oxides (magnetite, hematite), and carbonates(calcite) replaced the early-formed anhydrouscalc-silicates. Additionally, copper sulfide min-eralization also can be occurred mainly duringsub-stage III.

The early-formed mineral assemblages werealtered to a series of fine-grained aggregates ofchlorite, hematite, calcite and clay chiefly along


AYE et al.

fractures and micro-fractures by relatively lowtemperature and low pH fluids may have beena relatively higher oxidation state during lateretrograde alteration (sub-stage IV).

Acknowledgements

This study has been supported financially byAUN/SEED-Net, JICA. The authors wouldlike to acknowledge and express their sincerethanks and appreciations to Prof. KorichioWatanabe. Our gratitude is further expressedto Mine Geology Department, Newmont, In-donesia for their kind supporting and kindhelps during the field work.

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The Mineralogy of Gold-copper Skarn

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