Gold Paragenesis and Chemistry at Batu Hijau

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A R T IC L E

J. Arif T. Baker

Gold paragenesis and chemistry at Batu Hijau, Indoneisa: implications

for gold-rich porphyry copper deposits

Received: 15 January 2004 / Accepted: 27 July 2004 / Published online: 10 September 2004 Springer-Verlag 2004

Abstract Gold is an important by-product in manyporphyry-type deposits but the distribution andchemistry of gold in such systems remains poorlyunderstood. Here we report the results of petrographic,

electron microprobe, laser ablation inductively coupledplasma mass spectrometry (LA-ICP-MS), and flotationtest studies of gold and associated copper sulfideswithin a paragenetic framework from the world-classBatu Hijau (914 mt @ 0.53% Cu, 0.40 g/t Au) por-phyry coppergold deposit, Indonesia. Unlike manyother porphyry coppergold deposits, early copperminerals (bornitedigenitechalcocite) are well pre-served at Batu Hijau and the chalcopyritepyriteoverprint is less developed. Hence, it provides anexcellent opportunity to study the entire gold para-genesis of the porphyry system. In 105 polished thinsections, 699 native gold grains were identified. Almost

all of the native gold grains occurred either withinquartz veins, attached to sulfide, or as free gold alongquartz or silicate grain boundaries. The native goldgrains are dominantly round in shape and mostly 112 lm in size. The majority of gold was depositedduring the formation of early A veins and is domi-nantly associated with bornite rather than chalcopyrite.The petrographic and LA-ICP-MS study results indi-cate that in bornite-rich ores gold mostly occurs withincopper sulfide grains as invisible gold (i.e., within thesulfide structure) or as native gold grains. In chalco-pyrite-rich ores gold mostly occurs as native goldgrains with lesser invisible gold. Petrographic obser-

vations also indicate a higher proportion of freegold (native gold not attached to any sulfide) in chal-copyrite-rich ores compared to bornite rich ores. Thepattern of free gold distribution appears to correlate

with the flotation test data, where the average goldrecovery value from chalcopyrite-rich ores is consis-tently lower than bornite-rich ores. Our data suggestthat porphyry copper-gold deposits with chalcopyrite-rich ores are more likely to have a higher proportionof free gold and may require different ore processingstrategies.

Keywords Gold Porphyry Copper Batu Hijau,Indonesia

Introduction

Gold-rich porphyry copper deposits (defined as thosewith bulk Cu/Au atomic ratios of


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chalcopyrite in chalcopyrite-rich porphyry copper ores,particularly where there is a later overprint by chalco-pyrite-pyrite-bearing phyllic alteration. Kesler et al.(2002) also showed that bornite contains one order ofmagnitude more gold in solid solution than chalcopyrite(1 ppm and


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the emplacement of the tonalite porphyry intrusions wasrapid and that the three intrusions were emplaced within90 ky (3.760.123.670.10 Ma).

Alteration and mineralization

Gold and copper grade are not uniform in the deposit(Fig. 3a, b). In order to evaluate how sulfide mineralogyinfluences the variation in copper-gold ratios (Fig. 4a)block models of chalcopyrite, bornite and pyrite wereconstructed (Fig. 4b, c, d). Mineralogical studies con-ducted by Brosnahan (2002) have shown that theabundance of copper sulfide showed a good correlationto sulfur and copper assays. Microprobe analysis indi-cated that copper sulfide in Batu Hijau is stoichiometric,and hence the sulfur and copper ratios (S/Cu ratio) forchalcocite, digenite, bornite, and chalcopyrite are

0.2522, 0.2803, 0.4036, and 1.0089, respectively. Theo-retically, if the S/Cu grade ratio of the ore is >1.0089,the sulfide assemblage should consist of pyritechalco-pyrite. This does not mean that bornite cannot bepresent in relicts of non-overprinted rock, but if present,they would have to be compensated for by a significantamount of pyrite. A S/Cu grade ratio chalcopyrite mineralogy inthe upper part to chalcopyrite>bornite in the deeperpart (Fig. 4b and c). Furthermore, results from recentdeep drilling programs indicate that the sulfide assem-blages below the current ultimate pit boundary aredominantly chalcopyrite and pyrite rather than bornite(Fig. 4d).

Hypogene hydrothermal alteration, veins and sulfidemineralization developed in five temporally and spa-tially overlapping events termed Early, Transitional,Late, Very Late and Zeolite alteration stages (Mitchell

et al. 1998). The early alteration consists of biotitereplacement of mafic phenocrysts and groundmass, andthe development of magnetitebiotitequartz stringersand EDM-like (early dark micaceous) biotitesericiteveinlets (cf. Meyer 1965). Secondary plagioclase occursalong the selvages of early quartz veinlets. Early alter-ation is pervasive within and proximal to the tonaliteporphyries, and although the fracture density andalteration intensity rapidly decrease away from themineralizing intrusions, secondary biotite extends out-ward for more than 500 m from the porphyry centre

Fig. 2 Lithology distribution on section 9080 N (modified afterClode et al. 1999)

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(Mitchell et al. 1998). Transitional alteration consists ofchlorite and vermiculite that replaced early-formedbiotite, and replaced oligoclasealbite by sericitecal-cite. Magnetite is converted to hematite and/or chal-copyrite (proximal) and pyrite (distal) (Mitchell et al.1998; Clode et al. 1999). Late alteration consists offeldspar replaced by sericite, and locally by andalusite

and pyrophyllitekaolinite, and the development of Dsulfide veinlets and veins. The veinlets consist of pyriteand quartzchalcopyrite locally with sphalerite andtennantite. Very late hydrothermal alteration is alsocharacterized by feldspar destruction, but differs fromlate alteration in that feldspar is replaced by smectite inassociation with sericite and chlorite, and the sulfideminerals consist of sphalerite, galena, tennantite, pyrite,chalcopyrite and locally bornite (Clode et al. 1999). Thelast stage of hydrothermal alteration is recognized aslow temperature open space filling commonly along

vein centerlines and small open spaces in the wall rocks,and consists of stilbitelaumontitecalcite (Mitchellet al. 1998).

Copper and gold grades are positively correlatedwith the density (volume percent) of quartz veins, withearly A veins comprising about 80% of the total vol-ume of quartz veins and a similar proportion of the

copper (Mitchell et al. 1998). The A veinlets are thin(less than 10 mm), wispy and discontinuous, and arecharacterized by wavy to diffuse wall-rock contacts(Fig. 5a and b). The veins commonly contain feldspar,magnetite, and abundant void space (up to 25%) thatprobably reflects original anhydrite subsequently lea-ched during later hydrothermal and/or weatheringevents. Hypogene sulfides include chalcocite, digeniteand bornite, typically averaging 0.255 vol%. Thedigenite typically occurs as exsolution lamellae withinbornite whereas chalcocite commonly occurs as a rim

Fig. 3 a Gold gradedistribution on section 9080 N(modified after Clode et al.1999). The area of the drill coresamples ( dashed ovals) is acompilation of all samplelocations from other sections(9020 N and 9120 N), projectedto section 9080 N. The graycolor in the center of the goldshell is the Young Tonalite

body, which has weak goldmineralization. b Copper gradedistribution in section 9080 N(modified after Clode et al.1999)

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surrounding the bornitedigenite grains suggesting thechalcocite was deposited later (Fig. 5c). Generally, theA veins are composed of granular quartz grains1 g/t, whilst thewhite line is the outline of ultimate open pit. The gold grade datafrom below the ultimate pit is limited, but suggests that gold gradeis open at depth. Block models of b chalcopyrite, c bornite andd pyrite distribution and abundance on section 9080 N based onS/Cu ratios (see text for details)

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and are likely trapping temperatures based on thecoexistence with vapor-rich inclusions. Early A veinswere not sampled for the study but Garwin (2000) sug-gested that these veins likely formed at >500700 Cbased on the coexistence of magnetitebornitechal-cocite (cf. Simon et al. 2000). The fluid inclusion resultsfrom B and C veins are also consistent with phaseequilibria temperature estimates based on the mineral-ogy of the veins chalcopyritebornite (450500 C;Simon et al 2000). Late D veins formed at temperaturesof


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ranges from 330 m above sea level to 400 m below sealevel; Fig. 3a). The gold grades of the samples rangefrom 0.4 to 4.2 ppm. Polished thin section petrographywas carried out on 105 polished thin sections and 699gold grains were identified (Table 1). Visible gold is lo-cated in two sites: (1) gold grains in direct contact with

sulfide, and (2) gold grains hosted by non-sulfide min-erals (quartz). The latter was classified as free gold

whereas the former had a wide variety of associatedsulfide minerals (Fig. 6). From a total of 699 gold grains,

almost 65% are identified within or along grainboundaries of bornitedigenite and bornite, approxi-

Fig. 5 ab A veins (quartzbornitedigenitechalcocite) cut by aD vein (chalcopyritepyrite) in a fine-grained volcanic rock,strongly altered by biotite and feldspar (coin diameter is 2 cm).The feldspars in the selvage of the D veins have been replaced bysericite and smectite. The yellow box indicates the area of A veinswhere D vein mineralization has replaced early sulfides (bornitedigenitechalcocite) to chalcopyrite and pyrite. The light blue boxis where D vein mineralization weakly overprints A veins(transition zone). Chalcopyrite content decreases away from theD vein. The red box is where the A vein is unaffected by D veinmineralization overprint, thus bornitedigenitechalcocite arepreserved. c Digenite exsolution lamellar in bornite rimmed bylater chalcocite. d Two parallel B veins, with the centerline andorthogonal fracture set infill of chalcopyrite and bornite. e B(quartzchalcopyritebornite) and C (chalcopyritebornite) veins.fPhotomicrograph showing the rim of a bornite grain within an Avein (from Fig. 5b) partially replaced by chalcopyrite

b

Table 1 Summary of gold deportment at Batu Hijau in relation to vein paragenesis

Quartzvein type

Common sulfidesin quartz vein

Number ofveins studied

Numberof gold grains

Mean sizeof gold grains(microns)

Ratio of freeAu/Au in sulfide

A Bornite, digenite, chalcocite 75 556 6.9 0.23B Bornite, chalcopyrite 54 123 7.8 0.41C Chalcopyrite, pyrite 24 20 10.2 0.54D Pyrite, chalcopyrite 21 0 N/a N/a

Fig. 6 Pie chart illustratingnative gold grain occurrencesand copper sulfide association.Native gold in Batu Hijau isclosely related to bornite andthe location of native goldoccurrence in the copper sulfideis equally distributed asinclusions or along grainboundaries of copper sulfides

Fig. 7 Native gold grain sizes at Batu Hijau deposit. Mean valueexcludes the outliers (>50 lm)

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mately 14% occur within or along grain boundaries ofchalcopyrite, and 21% occur as free gold in quartz. Theoccurrence of gold as inclusions within sulfide and asgold grains along sulfide grain boundaries is almostequal. Free gold becomes proportionally more abundantin later vein stages (B and C; Table 1). LA-ICP-MSidentified a third category of gold occurring as solid

solution within sulfide.The occurrence and distribution of native gold inBatu Hijau is closely related to quartz veins and theirparagenesis, whereby the early quartz veins (A veins)contain almost 80% of total native gold observed andthe rest occurred in B veins and C veins respectively(Table 1). Rare native gold in wall rock is located


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concentration vary, and are dependent on copper sulfideassociation and mineralization assemblages. Gold inbornitedigenite and bornite (within A veins) containsup to 8% copper (mean = 3.1%), significantly morethan gold in chalcopyrite (within B and C veins; mean =

1.9%) and free gold (0.2%). The copper values areinterpreted to be real rather than contamination of thesignal by the host sulfide due to careful selection of largegold grains and because the results are consistent withprevious microprobe studies reported by Mitchell et al.(1998) that noted up to 6% Cu. Silver contents arehighest in gold in chalcopyrite (mean = 10.9%) whereasthe silver content of gold in bornite has a mean averageof 4.0% and free gold has a mean average of 5.2%.Thus, as the vein paragenesis evolved early bornitecontained gold with higher copper and lower silvercontent (mean Cu/Ag = 1.2), and later chalcopyritecontained gold with lower copper and higher silver

content (mean Cu/Ag = 0.2).

LA-ICP-MS analysis of gold in bornite, chalcopyrite,and pyrite

Analytical setup

LA-ICP-MS analysis was used to determine the invisiblegold concentration of bornite in A veins ( n =17),bornite in B veins ( n =15), chalcopyrite in B veins

(n=18), chalcopyrite in C and D veins ( n =18), and inpyrite in D veins ( n =3). A total of 70 LA-ICP-MSanalyses were performed on 12 polished thin sectionsfrom samples with grades >1 g/t Au. LA-ICP-MS wascarried out at the Geochemical Analysis Unit, ARC

National Key Centre GEMOC, Macquarie University.The system used was a Merchantek LUV266 lasermicroprobe connected to an Agilent 7500 s ICPMS.Typical laser operating conditions included a repetitionrate of 4 or 5 Hz and an output power of 0.50.6 mJ/pulse. These conditions produced a spot-size of 4050 lm in sulfide and an ablation rate of 1 lm/s.Ablation was carried out in a mixture of He (0.250.3 L/min) + Ar (1.11.15 L/min). The ICPMS was tuned togive an oxide production


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(Van Achterbergh et al. 2001; http://www.es.mq.edu.au/gemoc/glitter). A combination of the NIST 610 glassand a synthetic nickel sulfide standard (PGE-A) wasused for the external standards, and Fe and S concen-trations in the sulfides were used as the internal standardfor the NIST and PGE-A respectively. The Cu, S and Feconcentrations for bornite, chalcopyrite and pyrite wereassumed to be in stoichiometric proportions. Anuncertainty of 3% relative on the concentration of theinternal standard was incorporated into the errorpropagation routines in GLITTER. The effect of thisuncertainty is to change absolute trace element abun-dances but not relative abundances.

Results

Gold in bornite showed distinctly higher concentrationscompared to chalcopyrite (Table 2 and Fig. 10). Bornitegrains in A veins are typically smaller than bornite in Bveins, commonly 50 lm in diameter) and all grains contained signifi-cant gold ranging from 0.53738.370 ppm (Fig. 10a).The latter is anomalously high and likely reflects abla-tion of a buried native gold grain. The average goldcontent of the B vein bornite with this removed is3.832 ppm (0.5377.990 ppm). Gold content of chalco-pyrite in B veins was commonly below detection limits

and ranged from 0.0300.258 ppm for those valuesabove detection limits (average = 0.110 ppm; Fig. 10b).Only three chalcopyrite grains contained gold abovedetection in C and D veins (Table 2 and Fig. 10b).Bornite also contained significantly more silver thanchalcopyrite (Fig. 10); average content of silver inbornite in A veins was 134.358 ppm (25.990339.950 ppm), in bornite in B veins was 306.318 ppm(192.740617.130 ppm), in chalcopyrite in B veins was10.854 ppm (0.65035.830 ppm) and in chalcopyrite in Cand D veins was 18.762 ppm (1.200130.510 ppm).Pyrite in D veins contained no detectable gold or silver.

Discussion

Kesler et al. (2002) used SIMS (ion probe) to measurethe gold content of chalcopyrite and bornite from con-centrates at Batu Hijau (Fig. 11). LA-ICP-MS resultsfrom this study show similar, although not identical,results and confirm that bornite contains approximatelyone order of magnitude more gold than chalcopyrite.Experimental work by Simon et al. (2000) is also con-sistent with this relationship with bornite accommodat-

Fig. 10 Histograms illustrating a the gold content and b the silvercontent of bornite and chalcopyrite in A, B and C/D veinsmeasured by LA-ICP-MS. Only values above detection limits areshown. Bornite consistently contains higher concentrations of goldand silver than chalcopyrite

Fig. 11 Summary of gold concentrations (above detection limits)measured by LA-ICP-MS in bornite and chalcopyrite in differentvein stages compared with the SIMS results from Kesler et al.(2002) that were carried out on Batu Hijau concentrates (thereforenot paragenetically constrained). The ranges from the SIMS resultswere obtained from histograms plotted by Kesler et al. (2002),however, the lower limits can only be estimated to


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ing approximately one order of magnitude more goldthan chalcopyrite. However, much higher concentra-tions of gold occurred in bornite and chalcopyrite in theexperimental studies than those measured by Kesleret al. (2002) and this study. Kesler et al. (2002) calcu-lated that bornite and chalcopyrite saturated at 300and 250 C at Batu Hijau respectively, and our resultsare in broad agreement with their study. The saturationtemperatures are significantly lower than the depositiontemperatures of bornite (500700 C) and chalcopyrite(450500 C) at Batu Hijau (Garwin 2000) further sup-porting Kesler et al.s (2002) suggestion that gold is ex-solved during cooling of the deposit. They concludedthat the average gold grades of bulk ore and Cu/Auratios (40,000) could not be accounted for by theinvisible gold content of copper sulfides alone and thatthe exsolved gold must be account for the remainder. Wehave shown that the additional gold in the deposit oc-curs as small (112 lm), round native grains dominantlyassociated with early bornite from which it was likelyexsolved. The high copper content of gold associatedwith early bornite (Fig. 9) may also be used to support

the theory that gold was exsolved from bornite ratherthan deposited directly from a hydrothermal fluid.Furthermore, later free gold (presumably precipitateddirectly from a hydrothermal fluid) has low to belowdetection copper content.

Kesler et al. (2002) argued that the endowment ofgold in porphyry systems is likely fixed by the amount ofgold that will enter copperiron sulfides, and suggestedthat the gold was subsequently exsolved as native grainswithin or adjacent to the sulfide and/or redistributedduring cooling or later alteration. We have shown thatapproximately one fifth of the native gold occurs as freegold in quartz (Fig. 6). It is unclear as to whether the

free gold was deposited as an independent phase orwhether it was locally redistributed but its distinctchemistry (copper-poor) suggests it was deposited asan independent phase. In addition, the proportionalincrease in free gold with later chalcopyritepyritedominant assemblages indicates that a significantamount was precipitated during cooling and lateralteration (Table 1). However, there is evidence to sug-gest that some of the later chalcopyrite replaced earlierborniterich mineralization, which may have resulted inmore native gold (and free gold?) because chalcopyritecan not accommodate as much gold as solid solution inits structure as bornite.

The observation that there is a higher amount of freegold in the paragenetically later chalcopyrite-rich min-eralization and lesser invisible gold is critical for pro-cessing issues at Batu Hijau, and significant for gold-richporphyry deposits in general. Modelling of S/Cu ratiosand drilling indicate that the deeper portions of the de-posit are dominated by chalcopyrite and pyrite, andthere is an increase in Au/Cu ratios (Figs. 3 and 4).Results of flotation tests from 2002 and 2003 drillingindicate that the average gold recovery from chalcopy-ritepyrite ore is almost 5% lower than average bornite

chalcopyrite ore (Arif 2002; Dadang Prananta, pers.comm. 2004) whereas copper recovery is consistent forall types of sulfide assemblages. We suggest that the lowgold recovery in the flotation cells from chalcopyritepyrite assemblage ores relates to the decrease in invisiblegold and increase in free gold. This finding may have acrucial impact concerning ore processing design at theBatu Hijau deposit as the mine gets deeper in the futureand into the chalcopyritepyrite dominated ore. Thepotential loss of gold may be rectified through installa-tion of a gravity circuit to recover the free gold prior tofloatation. Installation of additional gravity circuits totrap free gold has improved the gold recovery in severalsignificant porphyry copper-gold deposits includingAlumbrera and Cadia (Keran et al. 1998; Dunne et al.1999). Interestingly the gold-rich Alumbrera and Cadiadeposits both contain dominantly chalcopyrite-rich ores(Ulrich and Heinrich 2001) suggesting that free gold ismore abundant in chalcopyrite dominant porphyrysystems than bornite-rich examples.

Acknowledgements PT Newmont Nusa Tenggara, an Indonesian

subsidiary of Newmont Mining Corporation, supported all of thefinancial expenses related to this study. We are grateful to ChrisClode and Bruce Harlan, geology managers Batu Hijau, for mak-ing the research project possible. Batu Hijau geologists are thankedfor their support and stimulating discussions. John Proffett isthanked for his insight into Au mineralization in porphyrydeposits. Thanks go to Norman Pearce at the Geochemical Anal-ysis Unit, ARC National Key Centre GEMOC, Macquarie Uni-versity for the LA-ICP-MS work. Reviews by Steve Kesler, WernerHalter, Jeremy Richards and Larry Meinert significantly improvedthe manuscript.

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Gold Paragenesis and Chemistry at Batu Hijau