ARTICLE

Peter J. Pollard Æ Roger G. Taylor

Paragenesis of the Grasberg Cu–Au deposit, Irian Jaya,Indonesia: results from logging section 13

Received: 10 June 2001 /Accepted: 20 July 2001 / Published online: 20 December 2001� Springer-Verlag 2001

Abstract The Grasberg Cu–Au deposit is hosted withinthe Grasberg Igneous Complex (GIC), a Pliocene vol-canic and intrusive complex situated in the highlands ofIrian Jaya, Indonesia. The GIC is composed of intrusiveand volcanic rocks that were disrupted by formation ofthe Dalam Diatreme and intruded by later, multistagedGrasberg and Kali intrusions. Each intrusive phase isoverprinted by extensive hydrothermal infill and alter-ation. Based on drillcore logging on section 13, 35separate stages of alteration and infill have been recog-nized, and their spatial distribution mapped in 14drillholes that represent approximately 1.8 km of verti-cal section. Using intrusions as timelines, the hydro-thermal stages can be timed as post-Dalam–pre-MGI(Main Grasberg Intrusion), post-MGI–pre-Kali, andpost-Kali, and linked into seven groups that are inter-preted as separate hydrothermal systems. Pre-Kali sys-tems include ten of the recognized stages, and are mostlyhigh-temperature alteration (K-feldspar and/or biotite)devoid of sulfide mineralization. Sulfides are restrictedto post-Kali time and, excluding early quartz–anhy-drite ± sulfide and molybdenite veins, can be groupedinto three main stages: (1) Heavy Sulfide Zone (HSZ)mineralization, (2) Grasberg copper–gold stage, and (3)late copper mineralization (mixed copper sulfides,covellite–enargite–pyrite and pyrite–covellite–marca-site). The HSZ is dominated by fine-grained replacementpyrite and distributed mainly towards the periphery ofthe GIC, with only minor occurrences towards thecentral zones. It is suspected that a high proportion ofthe copper and gold content of the HSZ is due tooverprinting by the Grasberg copper–gold stage and latecopper mineralization.

The Grasberg copper–gold stage is a major chal-copyrite–bornite±pyrite±gold±hematite event occur-ring as a centrally focused fracture system that hasbeen traced from the surface to the limits of drilling(>1,800 m). It is considered to also have a minorfocus in peripheral zones, where it overprints the HSZ.The Grasberg copper–gold stage exhibits no obviousvertical or lateral changes in mineralogy. Late coppermineralization comprises several stages and is domi-nantly disseminated in character. The early stages aredominated by chalcopyrite, bornite, digenite–chalco-cite, covellite±nukundamite, and colusite, with thelater stages containing pyrite, marcasite, covellite, andenargite±minor chalcopyrite. Late copper mineraliza-tion is essentially a high sulfidation system, and isassociated with zones of mild acid leaching, develop-ment of small-scale vugs, andalusite alteration, andabundant intermediate argillic alteration (illite, kaoli-nite). Rare pyrophyllite is reported. Hydrothermalalteration/infill stages within the GIC are controlleddominantly by oriented fracture arrays and majorbrittle–ductile fracture systems. These are focusedaround pre-existing igneous or igneous–sediment con-tacts, with the margins of the Kali intrusions being aprime focus of fracturing linked to the Grasberg cop-per–gold stage. The pattern of repetitive introductionof fluid and magma from deeper levels is compatiblewith the presence of an evolving magma chamber atdepth. No significant quantities of sulfur-bearing min-erals were precipitated until formation of the purpleanhydrite–quartz veins that preceded the major sulfidestages. This suggests that the early hydrothermal fluidshad temperature and oxygen fugacity characteristicsthat precluded precipitation of sulfur-bearing phases,and/or that a model involving late-stage addition ofsulfur to a deeper level magma chamber, perhaps byintrusion of more primitive magma, may be applicableto Grasberg.

Keywords Grasberg Æ Porphyry copper–gold ÆIrian Jaya Æ Indonesia

Mineralium Deposita (2002) 37: 117–136DOI 10.1007/s00126-001-0234-7

P.J. Pollard (&) Æ R.G. TaylorSchool of Earth Sciences,James Cook University,Townsville, Queensland 4811, AustraliaE-mail: Peter.Pollard@jcu.edu.auTel.: +61-7-47815050Fax: +61-7-47251501

Introduction

The Grasberg Cu–Au deposit is situated in the Ertsbergmining district of Irian Jaya, Indonesia (Fig. 1) and isthe world’s major copper–gold mine. Grasberg wasdiscovered by P.T. Freeport Indonesia in 1988 whenthe first vertical drillhole (GRS-4) encountered 600 maveraging 1.65% Cu and 1.4 g/t Au (Van Nort et al.1991; MacDonald and Arnold 1994). Current recover-able reserves are 51 billion pounds of copper,62.4 million ounces of gold, and 135.5 million ouncesof silver (see also Table 1; Freeport McMoRan Copperand Gold Inc. 2000). The Grasberg deposit comprises

the present open pit that is being mined at a rate ofapproximately 240,000 t of ore per day, and thedownward extension of mineralization that is plannedas a future underground mine.

The Ertsberg mining district forms part of the CentralRange Mobile Belt of Irian Jaya that contains Miocene–Pleistocene magmatic rocks formed in response to thepartial subduction of the northern margin of Australiabeneath a south-facing island arc located on the south-ern edge of the Pacific Plate. Mineralization in theErtsberg district appears to be related to a suite ofhigh-K calc-alkaline to shoshonitic intrusive rocks(McMahon 1994a, 1994b) that were emplaced duringthe Pliocene (McDowell et al. 1996). The strontium

Fig. 1 Simplified geologicalmap of the Ertsberg district

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(87Sr/86Sr = 0.70626 to 0.70707), neodymium (�Nd =–13.7 to –15.3) and lead isotopic characteristics of theErtsberg district intrusions suggest that the parentalmagmas were mixtures of components derived from adepleted mantle reservoir, 2–3% from an ancient en-riched mantle reservoir, and a substantial component oflower crust of probable Proterozoic age (Housh andMcMahon 2000).

The Grasberg deposit is hosted within the GrasbergIgneous Complex (GIC; Figs. 1, 2, 3), and mineraliza-tion has been delineated from the original surface at4,200 m and is still open below 2,500 m. As part ofongoing exploration in the Ertsberg district, a detailedparagenetic study of alteration/mineralization within theGIC was initiated by P.T. Freeport Indonesia. The aimsof this study are to determine the nature and controls ofCu–Au mineralization within Grasberg and ultimatelyto correlate alteration/mineralization events at the dis-trict scale. This paper reports results of the parageneticstudy based on detailed logging of drillholes on section13 (Figs. 2 and 3), supplemented by observations and

sampling of pit exposures and drillcore from other partsof the deposit.

The alteration and mineralization history of the GIChas been previously described by MacDonald and Ar-nold (1994), who recognized three major episodes ofalteration and Cu–Au mineralization linked to succes-sive Dalam Diatreme, Main Grasberg Stock and SouthKali Dyke phases of intrusion. The results of the presentstudy (Table 3) are broadly consistent with those ofMacDonald and Arnold (1994) in that they indicate thepresence of several distinct alteration systems, some ofwhich were separated in time by intrusion of (mon-zo)dioritic magmas. However, they differ significantly indetail since we recognize numerous overprinting stagesof alteration/infill with two main episodes of Cu(–Au)mineralization, both of which post-date intrusive rockswithin the GIC. The major Grasberg chalcopyrite–bor-nite–gold vein system is best developed towards thecentre of the GIC, while the later disseminated coppermineralization (mixed copper sulfide, covellite–enargite–

Table 1 Aggregate proved andprobable mineral reserves in theErtsberg district at 31 Decem-ber 2000 (Freeport McMoRanCopper and Gold, 2000)

Tonnage (tonnes · 106) Copper (%) Gold (g/t) Silver (g/t)

GrasbergOpen pit 1,081 0.99 1.20 2.32Underground 743 1.09 0.79 2.77

Kucing Liar 320 1.41 1.41 5.3Deep Ore Zone 185 1.19 0.82 5.83Ertsberg Stockwork Zone 101 0.55 0.80 1.75Big Gossan 37 2.69 1.02 16.42Dom 31 1.67 0.42 9.63Intermediate Ore Zone 16 1.09 0.42 7.76

Total reserves 2,515 1.1 1.04 3.4

Fig. 2 Simplified geological map of the Grasberg Igneous Complexshowing the location of section 13 Fig. 3 Cross section 13 showing simplified geology and the

distribution of drillholes used in this study

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pyrite, etc.) is best developed towards the periphery ofthe GIC. Results of this study place significant con-straints on potential genetic models for Grasberg min-eralization and suggest that magmas and fluids werederived from an evolving, deeper level magma chamber.

Geology of the Ertsberg mining district

Cu–Au mineralization in the Ertsberg mining district isassociated with Pliocene intrusive rocks that were em-placed into Cretaceous and Tertiary siliciclastic andcarbonate rocks that belong mainly to the Kembelanganand New Guinea Limestone Groups (MacDonald andArnold 1994). The intrusive rocks are quartz dioritic toquartz monzonitic in composition (Katchan 1982;MacDonald and Arnold 1994; McMahon 1994a, 1994b;Pennington and Kavalieris 1997), and K–Ar ages for 15biotite separates range from 4.4 to 2.6 Ma (McDowellet al. 1996). The presence of remnant volcanic rocksoverlying the intrusions (Fig. 2), together with apatitefission track analysis of the intrusions, suggests emplace-ment depths of 2 km or less (Weiland and Cloos 1996).

The Cretaceous and Tertiary sediments in the Erts-berg district contain a number of Cu–Au deposits(Fig. 1), including the Ertsberg East Skarn System(Rubin and Kyle 1998; Coutts et al. 1999), Big Gossan(Meinert et al. 1997), Kucing Liar (Widodo et al. 1999),and Dom (Mertig et al. 1994). The Grasberg deposit ishosted within the GIC, a pipe-like body approximately950 m in diameter that flares above 3,400 m, reachingapproximately 2.4 by 1.7 km at surface. In its upperpart, the GIC is composed dominantly of diatremebreccia that is referred to as the Dalam Diatreme(MacDonald and Arnold 1994). MacDonald and Ar-nold (1994) noted that most fragments below 3,500 mare intrusive (diorite or monzodiorite porphyry) whilethose above 3,500 m are dominantly volcanic (andesit-ic). The two rock types were interpreted to be cogenetic(MacDonald and Arnold 1994).

The GIC is overlain by remnants of a volcanic edificethat includes volcaniclastic rocks and (trachy)andesiticflows or domes. The volcaniclastic rocks are well topoorly bedded and sorted, and include tuffaceous com-ponents with eutaxitic textures and accretionary lapilli,carbonized wood fragments and bomb and splashstructures that indicate in part, a surficial aqueous en-vironment of deposition (MacDonald and Arnold 1994).Blocks of the volcanic debris have been found in theupper parts of the diatreme breccia (MacDonald andArnold 1994), as have fragments of banded clay (Fig. 2)that are interpreted to be surficial lake sediments thatwere incorporated downwards into the diatreme breccia(Prendergast 2001).

The Dalam Diatreme is intruded by a series of quartzmonzodiorites whose precise timing and distribution areyet to be fully resolved because intense hydrothermalalteration commonly prevents recognition of the natureof the original rock. MacDonald and Arnold (1994)

divided the intrusive rocks into three main types: (1)fine-grained diorite which is a late dyke phase of theDalam intrusion, (2) early, middle, and late intrusions ofthe main Grasberg stage, and (3) early and late stages ofthe South Kali dyke system. Pennington and Kavalieris(1998) divided the intrusive rocks into: (1) Dalam, (2)Grasberg, and (3) Kali quartz monzodiorites but, whilerecognizing the multistage character of the intrusions,did not further subdivide these units.

Geology of Grasberg section 13

Seven holes drilled from the Ayam Hitam drift (uppersection) and seven holes drilled from the Amole Drift(deep Grasberg; Fig. 3) have been logged in detail inorder to determine the paragenesis and spatial distri-bution of infill and alteration stages on section 13. Alligneous rock types encountered in the drillholes areconsidered to be intrusive in origin, and possible dia-treme breccia material was observed only in GRS-37-111. The rock types proved difficult to identify due tointense hydrothermal alteration and the similar ap-pearance of some rock types. The distribution of rocktypes shown in Fig. 3 is simplified from the current mineinterpretation. In the drillholes examined, the GIC iscomposed of Dalam intrusive rocks, with MGI and Kaliintrusions located towards the centre. The rocks havebeen divided into nine types based on mineralogical/textural characteristics (Table 2) and, where possible,crosscutting relationships.

Dalam intrusive rocks

The most abundant rocks within the GIC on section 13are Dalam intrusive rocks that can be divided into fivemain textural/mineralogical types based on observationsof the least altered samples. The distribution of eachrock type is unknown because pervasive alteration hasdestroyed the texture in most places.

The dominant rock type on section 13 is porphyriticdiorite (Dalam I; Table 2, Fig. 4A) with approximately80% phenocrysts of plagioclase, amphibole, and biotite,generally around 5 mm (rarely 10 mm) in size occurringin a microcrystalline matrix. Amphibole and biotitecomprise approximately 20% of the rock. Next inabundance is fine-grained porphyritic diorite (Dalam II,Table 2, Fig. 4B) containing approximately 50% phe-nocrysts of plagioclase, amphibole, and biotite in amicrocrystalline matrix. A few per cent of larger phe-nocrysts of amphibole (to 1 cm) and plagioclase (to5 mm) occur with the predominant phenocryst popula-tion (1–2 mm). The texture is very commonly obscuredby intense biotite and/or K-feldspar alteration. Amphi-bole and biotite comprise approximately 10–15% of therock. A third type of intrusive rock (Dalam III; Table 2,Fig. 4C) was observed in AM96-43-05 from approx-imately 270 m. This is distinguished by a relatively low

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abundance of mafic minerals compared with the por-phyritic diorite and consists of approximately 40–50%of sub-equant plagioclase phenocrysts to 5 mm (rarely10 mm), together with 5–10% primary biotite generally<1 mm in size, and approximately 40% microcrystal-line matrix. Thin section examination reveals thatoriginal amphibole (�5%) has been entirely replaced byfine-grained, pale-brown secondary biotite. Dykes ofporphyritic microdiorite (Dalam IV; Table 2) wereobserved in AM96-40-07 (393.3–393.5 m and 394.3–399.0 m), where they crosscut Dalam I. The dyke rocksare composed of approximately 60% microcrystallinematrix and 40% phenocrysts of amphibole and plagio-clase to 10 mm. The amphibole content is around 10%.The Dalam diorite (Dalam V, Table 2, Figs. 2, 3, 4D)has a relatively coarse porphyritic texture with approx-imately 30% plagioclase crystals. Characteristic featuresinclude the presence of pale-green plagioclase pheno-crysts up to 5 mm, and small hexagonal plates of biotite(1 mm). The Dalam diorite is noted as being alwaysstrongly K-silicate altered and mineralized (Kavalieris,personal communication 2000). The Dalam diorite hasbeen logged in several holes in the upper part of section13 (e.g. GRS-37-39, GRS-37-161 and GRS-37-128,Fig. 8), but is difficult to distinguish from other Dalamintrusive rocks with confidence.

Main Grasberg Intrusion (MGI)

The MGI is present in several of the holes on section 13,but also proved difficult to identify with certainty due tosuccessive stages of intense K-feldspar, biotite andmagnetite alteration. MacDonald and Arnold (1994)described the Main Grasberg stock as a medium- tocoarse-grained monzodiorite porphyry containing 35–50% plagioclase, 2% hornblende, and 3% biotite phe-nocrysts. The example shown in Fig. 4E contains ahigher proportion of ferromagnesian minerals.

Kali intrusive rocks

Intrusive rocks in the centre of the GIC on section 13form part of the South Kali dykes and have been dividedinto three types. Kali I is by far the most abundant, and isa fine-grained porphyritic rock containing approximately40–50% phenocrysts that are commonly partially aligned(Fig. 4F). The phenocryst content is variable, andincludes plagioclase, amphibole, biotite, and clinopy-roxene. Kali II is a medium-grained, seriate-textured andweakly porphyritic rock (Fig. 4G). The grain size isgenerally <2 mm, but plagioclase phenocrysts range upto 10 mm. Amphibole, biotite, and magnetite are the

Table 2 Textural and mineralogical characteristics of intrusive rocks from Grasberg section 13

Rock type Texture Phenocrysts Comments

Dalam I Crowded porphyritic texture withapproximately 80% phenocrystsin a microcrystalline matrix

Amphibole (5%) mostly<5 mm, rarely to 10 mm.Plagioclase (75%) up to 5 mm

Dominant intrusive rock type onsection 13. Invariably strongly altered

Dalam II Porphyritic, with approximately50% phenocrysts in amicrocrystalline matrix

Plagioclase (45%) to 8 mm,commonly tabular to rounded.Amphibole (5%) to 5 mm

Essentially a less crowded and slightlyfiner grained version of Dalam 1.Invariably strongly altered

Dalam III Porphyritic, with approximately80% phenocrysts in a fine-grainedto microcrystalline matrix

Plagioclase mostly <5 mm,rarely to 8 mm. Mafic phenocrystsless than 2 mm and interstitial toplagioclase

Only observed in AM96-43-05 towardsthe edge of the GIC. Invariablystrongly altered

Dalam IV Porphyritic, with approximately50% phenocrysts in a fine-grainedto microcrystalline matrix

Amphibole (10%) 1–8 mm,commonly euhedral. Plagioclase(40%) 1–5 mm, commonly elongateto tabular

Dykes intruding DalamI in AM96-40-07. Invariably strongly altered

Dalam V Porphyritic with approximately30% phenocrysts in afine-grained matrix

Plagioclase (30%) up to 5 mm,commonly euhedral

Part of Dalam intrusive suite,invariably strongly altered

MGI Crowded porphyritic texture withapproximately 50% phenocrystsin a microcrystalline matrix

Plagioclase (40%) up to 2 mm,amphibole, biotite, andmagnetite (10%) to 5 mm

Invariably strongly altered

Kali I Porphyritic, with 40–50%phenocrysts in a microcrystallinematrix. Phenocryst alignmentcommonly observed

Phenocrysts typically 5 mm orless. Phenocryst assemblage isvariable, includes clinopyroxene,amphibole, biotite, magnetite,and plagioclase. Rare quartzphenocrysts (0.5 mm) in somesamples

Contains mafic enclaves,generally <1 cm diameter

Kali II Seriate texture, with crystalsranging from 1 to 8 mm, butgenerally 1–2 mm. Rarely porphyritic

Rare plagioclase phenocrysts to1 cm

Contains mafic enclaves,generally <1 cm diameter

Kali III Microcrystalline matrix,commonly porphyritic, withphenocrysts of plagioclase andK-feldspar to 2 mm

Phenocryst content variableincludes plagioclase and K-feldsparto 2 mm and magnetite to 0.5 mm

Associated with pegmatiticK-feldspar–quartz veins

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main ferromagnesian minerals and comprise 10–15% ofthe rock. Kali II occurs as metre-scale dykes that crosscutKali I and its associated fine-grained magnetite alteration(see below).

Kali III (Fig. 4H) is an aplitic granite phase composeddominantly of quartz (30–35%), K-feldspar (30–35%),and plagioclase (30–35%), with 1–2% of dark mineralsthat include biotite, amphibole, and magnetite. Kali IIItypically occurs as centimetre to decimetre-scale dykesthat crosscut Kali I and Kali II and extend into the MGIand Dalam rocks. Kali III dykes are commonly closelyassociated with the coarse-grained K-feldspar veins de-scribed below. Kali III is the youngest intrusive rock typerecognized on section 13 and is overprinted by all the

hydrothermal stages of Packages 4–7 (except the mag-netite veins with magnetite alteration borders, see below).

Alteration and mineralization

The paragenesis of alteration and mineralization onsection 13 (Table 3) has been established through carefulobservation of crosscutting and overprinting relation-ships in drillcore (e.g. Figs. 5, 6), and each 3-m assayinterval has been logged for the presence and intensity ofdevelopment of each stage of the paragenesis (e.g.Fig. 7). For the purposes of description and discussion,the 35 alteration/infill stages (Figs. 8 and 9) have been

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separated into seven packages (Table 3) whose overallmineralogical associations suggest that they may repre-sent individual, evolving fluid systems. Packages 3 and 6probably represent several different fluid systems, butadditional data from elsewhere in the GIC are requiredbefore these can be subdivided with any confidence. Twovein types whose timing is poorly constrained, togetherwith the late gypsum veins, have not been assigned toany package of stages.

Package 1

InAM96-43-5 fragments of alteredDalam intrusive rocksare contained in an igneous matrix interpreted to be MGI(Fig. 5A), and this allows the distinction of K-feldsparand biotite alterations within the Dalam intrusive rocksprior to intrusion of the MGI. This is the earliest packagerecognized and is restricted to the Dalam intrusive rocks,although overprinting by numerous later stagesmakes theoverall distribution difficult to evaluate.

Dalam K-feldspar alteration

Dalam K-feldspar (Fig. 4A) alteration is fracture con-trolled and texturally very destructive, resulting in fine-grained, granular pale-brown alteration. This obscuresoriginal rock textures, and at hand specimen level theprominent feldspar phenocrysts are obliterated. Petro-graphically, the alteration can be observed as the de-velopment of very fine grained K-feldspar in the matrix

and feldspar phenocrysts. Narrow, linear zones ofcoarser-grained feldspar may represent infill channel-ways. Although not totally clear, the original maficcomponents seem diminished or absent.

The alteration has been traced from the lowest pointof section 13 (approximately 2,500 m) to around4,160 m, a vertical distance of 1,700 m. It was not re-corded in the mid to upper regions of the southwesternflank (Fig. 8b), although the very strong MGI K-feld-spar, MGI magnetite, MGI biotite, black spot biotite-K-feldspar, and late K-feldspar stages occurring as lateroverprints would effectively mask earlier stages in thisarea. Dalam K-feldspar alteration is also present infragments within the Dalam Diatreme in GRS-37-111.

Dalam biotite alteration

The distribution of Dalam K-feldspar alteration isparalleled by an overprint of dark, fine-grained biotite(Fig. 8c). This varies considerably in intensity, and athand specimen level ranges from crack-style veinlets toirregular blotches and zones of fine-grained dark rocks.Thin section observations suggest that the fine grainsize relates to the preceding fine-grained K-feldsparalteration. The overprinting biotite variably alters thefine-grained K-feldspar grains, creating even finergrained biotite flakes. A few areas of slightly coarserbiotite may represent remnants of original maficminerals. The biotite ranges from pale to deep yellow-brown in plane polarized light, with some of the smallergrains exhibiting tinges of green. Dalam biotite altera-tion has been traced vertically for 1,500 m on section 13(Fig. 8c).

Package 2

MGI K-feldspar alteration

MGI K-feldspar alteration is best observed within theshort sections of MGI intrusion that occur adjacent tothe Kali system. It occurs as irregularly distributed palebrown-pink K-feldspar alteration of the igneous matrix.In more intensely altered samples, K-feldspar has alsoreplaced the phenocrysts and destroyed the originaltexture of the rocks. In thin section, very fine grained,granular K-feldspar can be seen as an alteration of thematrix, and extending into the plagioclase phenocrystsvia small cracks and cleavage planes. No infill could bediscriminated and it is surmised that the alteration iscontrolled by systems of small cracks, grain boundaries,and mineral cleavages/dislocations.

MGI K-feldspar alteration can be traced for over1,100 m vertically on section 13. At depth it is relativelyminor and virtually confined to the small section of MGIthat is present on the southwest side of the Kali. How-ever, in the upper sector (Fig. 8d) it is much more ex-tensive and occurs in 200–300 m core intercepts affecting

Fig. 4A–H Intrusive rocks within the Grasberg Igneous Complex.A Dalam I intrusive rock composed dominantly of phenocrysts ofplagioclase and mafic minerals (black). Dark K-feldspar alteration(centre) is overprinted by secondary biotite which forms a networklinking original mafic phenocrysts now composed dominantly ofsecondary biotite. Late quartz veins (grey at centre) are cut andoffset by minor sulfide veins. AM96-43-05 166.5 m. Width of field7.5 cm. B Dalam II intrusive rock. Probably a finer grained variantof Dalam I, contains prominent phenocrysts of plagioclase maficminerals. Crosscut by MGI quartz vein (white, lower left) which isoverprinted by minor sulfides. AM96-43-05 182.6 m. Width of field7.5 cm. C Dalam III intrusive rock. Note the fine network of darkbiotite in the matrix. Crosscutting quartz-anhydrite vein (centre),and sulfide spot alteration controlled by microfractures throughoutthe matrix. AM96-43-05 316.4 m. Width of field 6 cm. D Dalam V(Dalam diorite). Porphyritic diorite with elongate white plagioclasephenocrysts and interstitial dark mafic minerals (amphibole andbiotite). GRS-37-128 7.0 m. Width of field 5 cm. EMain GrasbergIntrusion (MGI). Porphyritic to seriate-textured diorite withamphibole (upper right and lower left) and plagioclase (white,centre) phenocrysts. AM96-56-01 375.0 m. Width of field 7 cm.F Kali I intrusive rock (quartz monzodiorite) containing prominentphenocrysts (amphibole, biotite) with a weak flow foliation. AM96-43-05 57.3 m. Width of field 7.5 cm. G Kali II intrusive rock(quartz monzodiorite). Equigranular to seriate texture with rareplagioclase phenocrysts (e.g. centre right). Note contrasting textureand higher abundance of mafic minerals compared with Kali I.AM96-40-05 40.3 m. Width of field 6.5 cm. H Kali III intrusiverock. Porphyritic to seriate-textured monzogranite with pheno-crysts up to 3 mm in size of quartz, plagioclase, and alkali feldspar.Contains rare biotite and magnetite. AM96-43-07 15.3 m. Width offield 6.5 cm

b

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a range of pre-Kali stage intrusive rocks. Although notlogged to the northwest, the rocks in this region havebeen largely obliterated by succeeding magnetite, lateK-feldspar, and quartz vein overprinting.

MGI biotite alteration

MGI biotite comprises some 10–15% of the alteredrocks, and occurs as discontinuous, ragged veins, andas relatively coarse-grained, diffuse edged clustersinterpreted to be alteration of original mafic pheno-crysts. Within the altered porphyry matrix, MGI biotiteoccurs as micrometre-scale crystals that occur as vaguely

spotted zones and selectively within feldspar (plagio-clase?) phenocrysts. The biotite is inclusion free, andvaries from colourless to dark orange-brown in planepolarized light.

The distribution of MGI biotite alteration (Fig. 8e) isdifficult to discern accurately, as it commonly mergeswith very similar, but earlier alteration within Dalamunits. However, a distinction can be made in deepGrasberg, where fragments of biotite-altered Dalam ig-neous rocks were found within MGI (igneous breccia).The distribution at depth is thus judged to be restrictedto within the MGI and the immediate surrounds, whereit overprints MGI K-feldspar alteration. The latter cri-terion was utilized to ascertain the distribution in the

Timing Package Number Stage Style Distribution

Lateral Vertical

Post-Kali 35 Gypsum veins v c, m, o t34 Pyrite veins with clay borders* v, a m l33 Sphalerite–galena veins* v o l

7 32 Sulfur veins v c, m, o u31 Covellite–pyrite–marcasite–

sericite–chalcopyrite veins+v, sp m, o u?

30 Covellite–enargite–pyriteveins/alteration

v, sp m, o l

29 Quartz–carbonate veins* v m l?28 Mixed copper sulfide stage v, sp, l m, o l27 White clay–illite alteration sp, l c, m, o l26 Vuggy quartz veins v m, o l25 Andalusite alteration m t

Leaching event6 24 Grasberg copper–gold veins v c, m, o t

23 Heavy Sulfide pyrite v, a, sp, m c,o l22 Dark sericite–silica alteration v, sp o l21 Quartz–sericite alteration

(silicification)v, a, m o l

5 20 Molybdenite veins v c, m, o l19 Anhydrite–quartz veins

(minor sulfides)v, sp c, m l

4 18 Chlorite veins and alteration* v, sp c t17 Amphibole veins v c l16 Coarse-grained magnetite veins v c, m u15 Biotite veins v c l14 Quartz veins v c t13 Quartz veins–K-feldspar

alteration bordersv, a c l

12 Coarse-grained K-feldspar veins v c l11 Magnetite veins (with

magnetite alteration borders)v, a c t

Post-MGI –pre-Kali 3 10 MGI quartz veins v c, m, o t9 Late K-feldspar (creamy

white) alterationsp c, m, o t

8 Green sericite alteration sp m, o t7 Brown biotite alteration sp m, o l6 Biotite–K-feldspar (black

spot) alterationv, a, sp c, m, o t

2 5 MGI magnetite alteration v, a, sp, m c, m u4 MGI biotite alteration v, sp c, m t3 MGI K-feldspar alteration sp c, m u

Post-Dalam –pre-MGI 1 2 Dalam biotite alteration v, sp c, m l1 Dalam K-feldspar alteration sp c, m l

Table 3 Paragenetic sequence of alteration/infill stages in theGrasberg copper–gold deposit from the early post-Dalam/pre-MGIstage to the post-Kali stage. v veins, a alteration, sp semi-pervasive,m massive. c central, m middle, o outer parts of the GIC. u increase

in upper Grasberg (4,100–4,300 m), l increase in lower Grasberg(3,300–2,500 m), t no obvious vertical change, present throughout.* Paragenetic position not well-constrained, + could involve morethan one stage

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Fig. 5A–H Infill and alteration stages. A Fragments of biotite-altered Dalam intrusive rock (dark, lower left, centre right)contained within an igneous matrix (MGI) and crosscut by anMGI quartz vein (white, top) with later infilling sulfides. AM96-43-05 146.7 m. Width of field 6 cm. B Magnetite-altered MGI (right)with crosscutting MGI quartz veins intruded by Kali I (left).AM96-43-06 75.0 m. Width of field 7 cm. C MGI with pervasiveK-feldspar alteration (pale) of the matrix overprinted by fine-grained MGI magnetite alteration (black matrix), MGI quartzveins (grey–white), coarse-grained magnetite (veins and black spotsin MGI quartz), and chalcopyrite vein (dark grey, centre). Grasbergpit sample. Width of field 6 cm. D An original Dalam intrusiverock overprinted by late K-feldspar now converted to illite (white,lower right), silica–sericite alteration (dark grey to black), and thinHSZ pyrite veins (pale). Width of field 7 cm. E Vuggy quartz veinscontaining coarsely crystalline quartz crosscut fine-grained, mas-sive quartz which is interpreted as MGI quartz stockwork

material. The vuggy quartz veins are interpreted as quartz–anhydrite veins from which the anhydrite has been leached. AM96-40-07 469.2 m. Width of field 7 cm. F Altered Dalam intrusiverock from a zone of sericite–illite alteration and anhydriteleaching. Dark areas are andalusite–sericite alteration, while palerareas contain relict K-feldspar and muscovite. Crosscut by a veinwith a white alteration halo composed mainly of quartz andcarbonate (right) and overprinted by sulfides which are dominatedby chalcopyrite of the late copper system. AM96-43-05 275 m.Width of field 7 cm. G Molybdenite-filled veins and cracks (darkgrey and black) in a silica-altered Dalam intrusive rock, crosscut byHSZ pyrite vein (centre). AM96-43-08 243.5 m. Width of field6 cm. H Vuggy quartz vein which is interpreted to be a quartz–anhydrite vein from which the anhydrite has been leached. Thevoid space has been partially infilled by very fine-grained quartzand tiny sulfide grains (black). AM96-43-07 83.0 m. Width of field5 cm

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upper Grasberg sector (southwest flank, Fig. 8e), wherethe biotite extends over widths of some 100–200 m, asopposed to 20–50 m at depth.

No MGI biotite was recorded on the upper north-western flank, although it may have been obscured byintense magnetite and quartz vein development. Thisimpression is supported by the lack of MGI biotite inGRS-37-128 (central southwestern flank) where majormagnetite has eliminated previous alteration styles, butbiotite alteration can be seen in the drillhole above (GRS-37-162), particularly where magnetite intensity weakens.Some of this distribution may be explained by the loggingtechnique, where small amounts of magnetite can easilybe detected with a magnet, while small amounts of rem-nant biotite are difficult to see in hand specimen.

MGI magnetite alteration

The MGI magnetite stage occurs principally as fine-grained alteration of various igneous host rocks andearlier alteration styles. Fluid ingress was controlled bynumerous millimetre-scale cracks that are generally wellmasked by the accompanying ragged edged alteration.Magnetite grains also appear as disseminated spotswithin the matrix of the intrusive rocks. The alterationseems to be focused within feldspars and is noted pro-lifically as alteration of the K-feldspar veins and felds-pathized matrix of the MGI. The magnetite is variablyaltered to hematite that occurs along fractures and grainboundaries.

The MGI magnetite veining/alteration (Fig. 8f) ex-hibits an upwardly expanding pattern. At depth it occursover an area of some 50 m adjacent to the Kali–MGIcontact. However, in mid to upper levels (Fig. 8f) thearea occupied by the maximum development is com-monly at the 50 m scale, with the zone of magnetitedevelopment extending some 200–250 m away from thepresent Kali contact (Kali not present when MGImagnetite alteration formed).

Package 3

Black spot biotite-K-feldspar alteration

A biotite–K-feldspar infill/alteration combination iswidespread throughout the central to inner zone of theGIC (Fig. 8g), and is very distinctive owing to thepresence of coarse-grained flakes (0.2–0.8 cm) of darkbiotite. These occur sporadically with finer grained, palepink to cream feldspar and give a very characteristic‘black spot’ effect. Intense, fine-grained alteration asso-ciated with a network of cracks/vein channelways iscommonly three to ten times wider than the infillchannel. In thin section, the infill zones contain coarsergrained, semi-equant feldspar crystals, while the altera-tion feldspar is finer grained. The alteration is texturallydestructive, and the prominent feldspar phenocrysts of

the Dalam host rocks have been converted to fine-grained K-feldspar. The biotite crystals are distinctive inthin section with birefringence ranging from colourlessto very pale green.

Brown biotite alteration

Brown biotite alteration is one the most distinctivestages and occurs over extensive zones within the centralto outer sections of the GIC, particularly in deepGrasberg (Fig. 8h). The lower southwestern sector has azone of almost 300 m in AM96-40-05, with several 100-m-scale intercepts recorded to the northeast. The effectof the alteration is to give the rocks a very distinct palebrown colour where all the original mafic minerals (in-cluding primary and secondary biotite) have been con-verted to brown biotite. At hand specimen level it isdifficult to see any controlling channelways for this ex-tensive alteration, with very few indications of veins, oreven the discontinuous, small-scale cracks/biotite trailsnormally associated with widespread biotite alteration.In thin section, small biotite trails and vague discon-tinuous networks of cracks can be observed. The biotiteis inclusion free and ranges in colour from very pale tomedium orange in plane polarized light.

Green sericite alteration

Green-grey, fine-grained sericite alteration (Fig. 8i) iscommonly subtle and difficult to recognize in hand

Fig. 6A–H Ore minerals. A Grasberg copper–gold stage. Chal-copyrite (white) and bornite (grey) infill cutting earlier vein quartz(dark). Sample RP6, 3740 bench, Grasberg pit. Width of field1.4 mm. B Grasberg copper–gold stage. Chalcopyrite (white) andhematite (grey) infill, with earlier infill quartz (black). Sample RP2,3835 bench, Grasberg pit. Width of field 1.4 mm. C Grasbergcopper–gold stage. Chalcopyrite (grey) and gold (white) infill inearlier quartz (dark). Sample RP6, 3740 bench, Grasberg pit. Goldgrain approximately 125 lm across. D Mixed copper sulfide stage.Texture of a high-grade zone. All the sulfides (dark) are infill,reflecting a crack and dissolution pattern prior to deposition. Theporphyritic wall rock is heavily feldspathized and sericitized. GRS-37-162 764.0 m. Width of field 4 cm. E Mixed copper sulfide stagein vugs. The grey infill minerals are quartz (Qtz) and elongatemuscovite (Musc). The sulfides include pyrite (Py), chalcopyrite(Cpy), molybdenite (Mo), and covellite (Cov). GRS-37-162764.0 m. Width of field 2.8 mm. F Covellite–enargite–pyrite stage.Covellite (dark and pale grey) and enargite (grey, centre top andbottom) infill in earlier (Heavy Sulfide Zone) pyrite. AM96-43-7365.8 m. Width of field 1.4 mm. G Pyrite (white) occurs in variablegrain sizes, successively infilling a cavity with associated covellite(grey) which is also of variable grain size. Note development ofcrustiform layers, and framboid-like clusters. Some fine-grainedwhite material may be marcasite. GRS-37-162 340.1 m. Width offield 1.4 mm. H Covellite–pyrite–marcasite–sericite–chalcopyritestage. Large crystals of marcasite (pale grey–white) with interstitialcovellite (grey), minor chalcopyrite, and bornite. The texture isinterpreted as vein infill. Sample ES3745-02, Grasberg pit. Width offield 1.4 mm

c

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specimen, but is most noticeable in Dalam intrusiverocks where it overprints rocks that were previously al-tered to fine-grained K-feldspar and mildly overprintedby biotite alteration. The rocks take on a pale greentinge, and it is this feature that has been logged. Withinrocks that are brown biotite-dominated the alterationcolour is pale grey, and in thin section the brown biotitecan be seen to be partially altered to fine-grained sericite.

Late K-feldspar alteration

Late K-feldspar alteration is widespread (Fig. 8j) andextremely texturally destructive. The alteration is com-monly pale grey-white to cream, and may have a pink

tinge. In thin section the rocks are mixtures of coarserand finer grained domains of untwinned K-feldspar.Some of the coarser grained K-feldspar occurs in lineardomains (veins?), and the finer grained zones are prob-ably matrix replacement. The structural controls areonly observable in areas of less intense developmentwhere fine-scale crackle networks can be discerned. LateK-feldspar overprints the black spot K-feldsparalteration and is hence post-MGI in timing. The alter-ation can be traced over a vertical interval of 1,500 m(Fig. 8j). It occurs in both inner and outer segments ofthe GIC in zones that range from around 30 to 200 m.At mid-Grasberg level, the removal of the later Kaliigneous system suggests an original width of some350 m. The distribution of white clay–illite should also

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Fig. 7 Log of AM96-43-07showing rock types, assaydata, and the distributionand intensity of severalparagenetic stages. Theintensity of each stage islogged from 0 to 3, and is arelative scale for each stage,with 3 being the most intensedevelopment

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be inspected carefully as much of this material is analteration of pre-existing feldspathized rocks, especiallynear the margin of the GIC.

MGI quartz veins

The MGI quartz veins (Fig. 8k) are one of the mostspectacular features of the Grasberg system, and,

although barren, are strongly represented over some1,600–2,000 m vertically. The veins are commonly lessthan 10 cm wide, although examples >1 m have beenobserved on the southeastern side of the GIC. Theveins appear monomineralic in hand specimen, beingcomposed solely of quartz infill, mostly with no visiblealteration. The veins are grey-white, normally com-pletely infilled, and with little sign of euhedral crys-tals. This contrasts with the later anhydrite–quartz

Fig. 8 Distribution of rocktypes and alteration/infill stageson section 13. Solid fill showsdistribution extrapolated be-tween logged drillholes, dotsrepresent extrapolation beyondlogged drillholes

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veins, in which euhedral quartz crystals are morecommon.

The MGI quartz vein zone is some 300–400 m acrossin deep Grasberg, and slightly wider (400–500 m) in themid to upper levels. The veins are major sites of multiplegenerations of hydrothermal vein overprinting, and havecommonly been broken along their margins, centralzones, and/or at random and may contain magnetite,biotite, anhydrite, bornite, chalcopyrite, pyrite, etc. Thiscommonly gives a false impression that minerals otherthan quartz are an integral part of their infilling.

Package 4

Package 4 comprises several vein±alteration stages thatoccur mainly in and around the Kali intrusive rocks.

Magnetite veins (with magnetite alteration borders)

Fine-grained magnetite (Fig. 8l) occurs sporadicallywithin and adjacent to the fine-grained Kali I intrusionas oriented crackle zones. The veins are associated withintense, but patchy and irregular magnetite alteration ofthe wall rocks. The finer grained rock matrix is prefer-entially altered, although some fine-grained magnetitespots also form in the feldspar phenocrysts of Kali I.Fine-grained magnetite is crosscut by Kali II and KaliIII intrusions.

Coarse-grained K-feldspar veins

Coarse-grained K-feldspar veins (Fig. 8m) are relativelyrare, with a tendency to occur in the vicinity of thefine-grained leucocratic dykes (Kali III, aplite–pegma-tite association). The veins range up to 4 cm in width,and in hand specimen are sharp bordered and com-posed of flesh to creamy pink K-feldspar infill withminor associated quartz. Individual feldspar crystalsrange up to 2 cm in length, and this creates a peg-matitic appearance. In thin section, patches of grano-phyric quartz–feldspar intergrowths are observed, andsome veins have narrow halos of K-feldspar alterationof plagioclase.

Quartz veins±K-feldspar alteration borders

Quartz veins with white borders are common in andadjacent to the Kali intrusive rocks and form an errat-ically distributed array/network of narrow (0.25–1 cm)veins composed of grey quartz with minor white K-feldspar. In hand specimen, the veins are easily recog-nized owing to narrow, white alteration halos dominatedby K-feldspar. The alteration halos are very erratic, andrange from absent to >5 mm in width. In thin section,the alteration occurs as replacement of the plagioclase-

dominated rock matrix, and partial replacement ofplagioclase phenocrysts. The quartz veins with K-feld-spar alteration borders were recorded only in deepGrasberg (Fig. 8n).

Quartz veins

Quartz veins (Fig. 8o) within the Kali intrusions are ir-regularly distributed and rare. The quartz occurs asglassy grey infill without obvious visible crystals, andthere is no attendant alteration. The veins rarely exceed1 cm in width.

Biotite veins

Black biotite veins (Fig. 9a) occur throughout the Kalizone and are conspicuous within the core, which com-monly breaks along the 1–2-mm-thick veins. In thinsection the biotite is russet red-brown to pale brown inplane polarized light. The veins are associated with mi-nor biotite alteration where they intersect igneous maficminerals (especially hornblende).

Coarse-grained magnetite veins

A second magnetite stage is distinctive in that it occurs asmagnetite-only veins (0.1–0.5 cm wide) commonly withlittle or no visible alteration. The magnetite is generallyabout twice the grain size of the earlier fine-grainedmagnetite, and tends to have an obvious granular nature.The grains reflect the light and hence appear brighter andblacker than the dull fine-grained magnetite. When welldeveloped and closely spaced, there is a minor alterationeffect with magnetite spotting in the host rocks. Themagnetite veins are tightly constrained within the Kaliand adjacent MGI and/or Dalam host rocks (Fig. 9b).

Amphibole veins

Amphibole occurs as narrow (0.2–1 cm) veins that areeasily visible in the core trays as the core commonlybreaks along them. The amphibole is pale to dark greenand occurs as infill with no apparent associated alteration.The veins are commonly overprinted and altered to palergreen chlorite. The veins tend to overprint and be over-printed by other veins, and hence are closely associatedwith pre-existing biotite and magnetite veins, producingmixed assemblages. The distribution is tightly con-strained in and around the Kali contact zones (Fig. 9c).

Chlorite alteration

The porphyritic Kali I is very rarely completely freshand some 85–95% of the unit has been subjected to

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semi-pervasive ‘background’ alteration which results inthe white/cream colour turning pale grey, with anattendant loss of sharpness of the phenocryst bound-aries. In thin section this ‘blurring’ is due to selectivecarbonate and chlorite alteration focused within andaround the original amphiboles, mineral cleavages,grain boundaries, and cracks. The feldspars are lessaffected and carbonate predominates with a clear

microcrack structural control. Biotite remains essen-tially unaffected, with small patches of chlorite rarelydeveloped along the crystal margins. There are alsochlorite veins (Fig. 9d) in 5–20-m-lengths of corewhere previously formed biotite and amphibole veinshave been chloritized. It is presumed that this chlori-tization is related to the more pervasive backgroundstyle.

Fig. 9 Distribution of altera-tion/infill stages on section 13continued. Symbols as in Fig. 8

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Package 5

Anhydrite–quartz veins (± minor sulfides)

A series of anhydrite–quartz veins±K-feldspar, biotite,apatite, chalcopyrite, bornite, pyrite are very prominentin deep Grasberg, but much less common in the upperlevels (Fig. 9e). They extend from the central Kali zonetowards the outer edges of the GIC (Fig. 9e). The den-sity of veining is variable, and generally low (one to fourper metre). Vein widths are similarly variable (0.5–4 cm)and in general the veins are easily identified by thepresence of lilac/pale purple anhydrite. The veins displaya distinct lateral variation in composition. In the centralKali region they contain variable amounts of coarse-grained black biotite, pink K-feldspar, and chalcopy-rite–bornite–pyrite along with quartz–anhydrite.Beyond approximately 100–150 m from the centralzone, the veins are composed only of quartz and anhy-drite. The proportion of quartz versus anhydrite alsovaries considerably in the outer 200–300 m of thesystem, with a major increase in quartz, and the lilacanhydrite becoming white to pale green. The quartzcomponent is distinctive as coarsely granular to combtextured and white in colour, and can generally bedistinguished from the finer, grey coloured quartz of theMGI quartz veins.

Molybdenite veins

Molybdenite veins are characteristically narrow (�1 mm)and composed solely of molybdenite. The veins com-monly occur preferentially along the margins of anhy-drite–quartz veins and are also much more prominent indeep Grasberg (Fig. 9f). The molybdenite veins arecrosscut by HSZ pyrite veins (Fig. 5G; see below).

Package 6

Quartz±sericite alteration (silicification)

The deep Grasberg outer Dalam zone to the northeastcontains a prominent zone of quartz veining linked tointense silica alteration of pre-existing wall rocks(Fig. 9g). Core intercepts of 100 m or more have beenconverted to very fine grained grey-white quartz, whichis overprinted and replaced by later, dark sericite andmassive pyrite of the HSZ. The veins are generally dif-fuse bordered owing to the accompanying silica altera-tion, but it is apparent that veins of up to 5 cm wide arepresent. In marginal zones, the alteration adjacent tomore isolated veins can be seen as dominantly siliceousgrading outwards to zones of silica–sericite. Withinzones of intense veining, the intervening rocks arecomposed essentially of fine-grained grey alteration sil-ica with very minor sericite. This quartz vein style, withits intense silica alteration, is significantly different to the

MGI quartz veins, which are essentially quartz veining(infill only) with little or no alteration.

Dark sericite–silica alteration

Dark, fine-grained sericite–silica alteration is prominentin the outer parts of deep Grasberg (Fig. 9h). Thesericitization post-dates previous feldspathization andsilicification of original porphyritic igneous rocks, and isin turn overprinted by fine-grained pyrite of the HSZ.

Heavy Sulfide Zone pyrite

The Heavy Sulfide Zone (HSZ) occurs near the marginof the GIC (Fig. 3), and is well represented on section 13(Fig. 9i). This consists of zones of massive, fine-grainedreplacement pyrite grading into veins with less well-de-veloped alteration in peripheral zones. The veins areespecially well developed on the Dalam side of thecontact zone. Occurrences on the southwestern flank aredominated by veins, with no massive zones recorded.The pyrite occurs as an alteration of carbonate, andsilicified-, sericitized-, feldspathized- and magnetite-altered intrusive rocks. The pyrite contains minoramounts of intergranular chalcopyrite and one goldgrain has been observed in the pyrite in polished section.

Grasberg copper–gold veins

Chalcopyrite–bornite veins contribute the bulk of thecopper and gold grades which constitute the Grasbergdeposit. On section 13 the Grasberg sulfide stage exhibitsa concentration of veins and copper and gold valuesadjacent to the Kali (Fig. 9j), with a general decrease invein intensity towards the perimeter of the GIC (e.g.Fig. 7). Chalcopyrite and bornite predominate, and thereare hints of more pyrite towards the perimeter. However,this occurs in areas where the Heavy Sulfide Zone pyriteveins extend inward from the perimeter of the GIC(Fig. 7), and detailed study is required for clarification.

The chalcopyrite–bornite stage occurs predominantlyas infill within veins that range up to 2 cm wide (Fig. 5C,D), although the vast majority are <0.5 cm in width.Sulfide alteration spots are also widely developed, par-ticularly in original mafic minerals in the intrusive rocks,and within zones of earlier biotite and K-feldsparalteration. Polished thin section examination indicatesthat the veins are composed of chalcopyrite, bornite, andgold, with minor, early hematite and possibly rarequartz (Fig. 6A, B, C). The veins are essentially brittlefractures, and exhibit little or no evidence of movement.They overprint numerous pre-existing vein and altera-tion stages and the fractures commonly occur along thedirection of pre-existing veins (biotite, magnetite,anhydrite, quartz). This is especially true of the pre-existing MGI quartz veins which formed prior to the

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Kali igneous rocks. This gives a superficially misleadingimpression that the quartz veins and sulfide depositionoccurred at the same time.

Package 7

Andalusite alteration

Andalusite is not recognizable in hand specimen, andhas only been documented via thin section observations.Several areas with a thinly distributed petrographiccover reveal occurrences of andalusite over drillcoreintervals in excess of 100 m in GRS-36-162 (225 m),AM96-43-08 (111 m) and AM96-43-05 (107 m). Recentinvestigation (Imants Kavalieris, personal communica-tion 2001) has also confirmed the presence of andalusitein the southeastern part of the Grasberg pit. Andalusitehas been observed as an alteration of a fine-grained tu-ffisite dyke that cuts and brecciates quartz–anhydriteveins, suggesting a very late timing.

The major andalusite occurrences (deep Grasberg,GRS-37-162, and in the Grasberg pit) have a strongspatial association with the development of white seri-citic–argillic alteration zones (see below). In deepGrasberg, these white zones also coincide with quartz–anhydrite veins, which have been leached of their an-hydrite content to produce vuggy quartz veins (Fig. 9k).This association was not observed in the upper Grasbergholes where anhydrite of the quartz–anhydrite stage isabundant both in vein and spot alteration formats.

Vuggy quartz veins

The sericite–clay zones are characterized by two vari-eties of vuggy quartz veins. The most prominent are 1–2-cm-wide quartz veins composed of coarse-grained,crustiform quartz crystals (to 0.5 cm long) projectinginto open central voids. Coarse-grained, more granularquartz is also present closer to the vein walls, and thereis no quartz alteration of the wall rocks. The quartz iswhite to pale grey and appears identical to that of thequartz–anhydrite veins. The latter are present on bothsides of the vuggy quartz vein zones, and it appearsthat removal of anhydrite created the vuggy centralvoids.

Smaller quartz veins (0.2–0.5 cm wide) are also vuggyalong their central zones, but contain much finergrained, clear quartz, and rare white mica and clay. Thefine-grained, clear quartz also overprints the coarse-grained quartz described above. The veins are particu-larly noticeable where they cut the MGI quartz veins inthe perimeter zone of AM96-40-5, appearing as whitezones with tiny vugs traversing the grey quartz. Nu-merous examples were observed where tiny sulfidecrystals (bornite, chalcopyrite, pyrite, and molybdenite)had nucleated on the faces of the quartz, and grown intoopen void space.

White clay–illite alteration

Hypogene sericitic–argillic alteration is widespread onsection 13 as 100–200 m zones of grey-white muscovite–illite–kaolinite alteration (Fig. 9l). Two main styles ofalteration are recognized. One occurs as large-scale zonesdominated by rubble in the core trays, which displaymajor signs of both pre- and post-sericite brecciation,and contains the two types of vuggy quartz describedabove. This style is particularly dominant in deep Gras-berg, especially to the northeast, where earlier K-feldsparalteration(s) are largely obliterated by intense alterationto sericite. The other alteration style is similar, but lacksmajor signs of pre- and post-alteration disruption, andwithout significant vuggy quartz or signs of majorleaching of anhydrite–quartz veins. These zones are pe-trographically similar to the above, being dominated bywhite sericite (presumably illite), minor kaolinite, car-bonate, and quartz. The host rocks are mostly verystrongly feldspathized. Pyrophyllite has been reported inthe southwestern part of the GIC and in the adjacentKembelangan Group sandstone by Gibbins et al. (2000).

The distribution of the white clay–illite alteration(Fig. 9l) suggests the presence of two broad-scale frac-ture/fault zones: one to the northeast which is wellrepresented in deep Grasberg, the other to the south-west, towards the GIC margin. The alteration material isalso weakly represented in near-surface domains (GRS-37-39) within more central sections of the GIC. Similarmaterial forms a significant part of the current southernpit wall, where a zone some 50–100 m wide trendsnorthwest. The zone is steeply dipping, and appears tobe a major fault zone, with evidence of multiple dis-ruption both pre- and post-mineralization events.

Mixed copper sulfide stage

The mixed copper sulfide stage (Fig. 9m) is composedlargely of chalcopyrite, bornite, nukundamite, digenite–chalcocite, covellite, and pyrite. It is difficult to see all ofthe components within any single sample, and this stageis best defined by the appearance of bornite-dominatedand/or black sulfide spots within white/cream clay–illitealtered rocks. The sulfides occur in discontinuous veinsand cavities (Fig. 6D). The cavities are partially lined bycrystalline quartz that is succeeded by late muscovitecrystals (Fig. 6E). Molybdenite (rare), chalcopyrite,nukundamite, pyrite, valleriite, and covellite mixturesoccur in cavities (e.g. Fig. 6E), with further late covellitein cracks traversing earlier-formed minerals. The earlypyrite contains ovoid domains composed of bornite anddiginite, with minor chalcopyrite.

Nukundamite (Cu3.37, Fe0.66 S3.97) (Rice et al. 1979)occurs as pale coloured, extremely bireflectant (palepink–pale yellow-grey), subhedral crystals which arespectacularly anisotropic (green-red). Nukundamite canoccur in complex arrangements with chalcopyrite, cov-ellite, and valleriite. Chalcopyrite is present as a major

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component associated with both vein and vug infillnukundamite and occurs in sharp contact with complexcombinations of chalcopyrite, covellite, and a pale redextremely bireflectant and anisotropic mineraltentatively identified as valleriite. Covellite occurs ascoarse-grained infill crystals, and as intimate fine-grained mixtures of covellite and chalcopyrite inter-preted to have formed by simultaneous precipitation.Minute veins of fine-grained chalcopyrite also crosscutthe coarse-grained covellite.

Nukundamite and covellite exhibit relationshipsbroadly similar to those of chalcopyrite and covellite.Nukundamite and covellite coexist as discrete infill stagesand also in complex assemblages where covellite appearsto occur on cleavage or fracture controls within nukun-damite. A very blue, isotropic, mica-like sulfide mineral isa rare accessory within this stage (e.g. GRS-37-162786.5 m) and has been identified (Honea 1992) asspionkopite.

A pale-blue sulfide mineral that is present in associ-ation with bornite is referred to in previous reports asdigenite (e.g. Honea 1992), and is present in majoramounts within GRS-37-162. It is not clear how digenitehas been distinguished from the blue end members ofchalcocite, and this awaits microprobe/XRD confirma-tion. The bornite–digenite combination is present asmillimetre-scale veins, isolated spots, and within cavitieswhich also contain chalcopyrite, covellite, and colusite.The sulfide spot format is spatially associated withthe nukundamite, chalcopyrite, covellite, valleriite±molybdenite, bornite assemblage described above, andthe entire package is presently considered as one as-semblage. However, there is a possibility that there is aseparate bornite–digenite stage.

Quartz–carbonate veins

Rare quartz–carbonate veins composed of clear crys-talline quartz that is partially infilled by granular car-bonate crystals were observed in AM96-43-05. Thesemillimetre-scale veins are presently poorly timed, butappear to post-date the mixed sulfide assemblage de-scribed above.

Covellite±enargite–pyrite alteration/veining

Covellite occurs commonly in the outer perimeter zonesof the GIC, but is difficult to log because the mostcommon occurrence is as millimetre-scale veins andminute alteration and/or infill spots. Veins were re-corded only within deep Grasberg (Fig. 9n), and some ofthese also contain enargite (Fig. 6F). The veins occur inand around zones dominated by pyrite veins and mas-sive pyrite of the HSZ. No covellite veins were noted inthe upper Grasberg holes, but petrographic examinationdemonstrates abundant covellite in spot and discontin-uous crack mode in GRS-37-162, with other occurrences

in isolated thin sections from GRS-37-173, GRS-37-33,and GRS-37-128.

Covellite–pyrite–marcasite–sericite–chalcopyrite veins

This stage is loosely defined and embraces a series ofisolated occurrences that post-date the mixed coppersulfide stage. Several stages could be present, and/orsome examples may link with the earlier mixed coppersulfide and covellite–enargite–pyrite stages. Examplesinclude fine-grained pyrite–covellite±marcasite, quartz,enargite, sphalerite, chalcopyrite, and molybdeniteoccurring as veins, with widely variable pyrite grain size(Fig. 6G). Similar materials are abundant in the adja-cent Kucing Liar deposit.

Also included here are a range of covellite–pyrite–marcasite associations in small vugs and in tiny veinsthat also contain chalcopyrite. One large vug (Fig. 6H)has millimetre-scale marcasite crystals with interstitialcovellite and chalcopyrite. It seems clear that a covel-lite–chalcopyrite association precipitated at the samegeneral time as pyrite–marcasite, and the same situa-tion was recorded within the Ertsberg East SkarnSystem by Katchan (1982) who showed curving layersof alternating covellite and fine-grained chalcopyriteinfill.

Another variant included with this stage is a pyrite–marcasite–covellite assemblage that appears to replaceGrasberg copper–gold stage chalcopyrite. Clearly, fur-ther work is required to unravel the details of this late-stage copper mineralization.

Sulfur veins

Rare occurrences of yellow sulfur (Fig. 9n) wererecorded as infill within discontinuous narrow zones(1–2 cm), and as late-stage infill within vugs of finepyrite–marcasite–covellite. The occurrence as isolatedveins without sulfides suggests that sulfur is a separatestage, and the post-covellite timing suggests it is verylate.

Other stages

Sphalerite±galena veins were noted in a couple ofplaces on the northeast side of the GIC where theyoverprint carbonate rocks (Fig. 9o). These may repre-sent the base metal veins which cut massive pyrite of theHSZ elsewhere in perimeter zones.

Pyrite veins with clay borders (Fig. 9o) are rare, butextremely distinctive. The veins have white alterationenvelopes (kaolinite?) with hair-line central infill crackscontaining fine-grained pyrite. The veins have not beenexamined petrographically. This stage is very visible inthe Grasberg pit as late-stage, vertical white veins in thepit walls.

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Gypsum veins ranging from millimetre to centimetrescales are ubiquitous in the GIC, but were not loggedduring this study.

Discussion

Detailed paragenetic study of section 13 supports themultistage character of intrusion and alteration withinthe Grasberg Cu–Au deposit proposed by MacDonaldand Arnold (1994). More than 30 stages of fracturingaccompanied by hydrothermal alteration and infill havebeen identified, and more can be anticipated as othersections are examined. Further work may also serve toclarify some of the timing relationships, particularly thetiming and extent of the diatreme breccia in relation toother breccia types and to the alteration stages.

The hydrothermal fluids responsible for alterationand mineralization on section 13 were apparentlysourced from below the current level of investigation asmost stages are still present in the deepest levelspenetrated by drilling. The time lines provided by theintrusive rocks allow the identification of severaldifferent fluid systems including post-Dalam–pre-MGI,post-MGI–pre-Kali, and post-Kali phases. Copper–gold mineralization has no direct connection in timewith the prominent MGI quartz vein system (post-MGI–pre-Kali; Fig. 3), even though fluid inclusions inthese veins contain abundant Cu, Fe, and Au (Kyleet al. 1996; Ulrich et al. 1999). The two major episodesof Cu–Au mineralization recognized here are exclu-sively post-Kali in age. The major Grasberg chalcopy-rite–bornite–gold vein system is strongly localized bythe subvertical contacts of the central Kali intrusions,and diminishes in intensity away from this zone(Fig. 7). Material that was previously considered as anouter phyllic zone linked to Grasberg Cu–Au miner-alization (MacDonald and Arnold 1994; Rubin andKyle 1997) is composed dominantly of illite–muscovitethat is interpreted here to be linked to the late coppersystem that overprints Grasberg Cu–Au mineralization(Table 3). Chalcopyrite–bornite–nukundamite–chalco-cite–covellite–enargite mineralization occurs within thiszone, and is also concentrated in the adjacent KucingLiar deposit where it overprints earlier, massive pyriteas well as chalcopyrite mineralization.

The multiple-staged alteration systems separated byigneous intrusions at Grasberg have created a complexpattern of overprinting alteration zones that are con-trolled by different fracture configurations. Some arefocused near the edge of the GIC, while others arecentrally focused (e.g. Figs. 3, 7). This is a very differentsituation to models of porphyry systems that envisage acentral zone of potassic alteration surrounded by suc-cessive shells of phyllic and propylitic alteration asproposed by Lowell and Guilbert (1970). Both the in-trusive rocks and fluids responsible for alteration andmineralization at Grasberg appear to have been sourcedin an underlying batholith, as most components are still

present at the deepest levels intersected by drilling. Pbisotope data for the Ertsberg East Skarn System havebeen interpreted to indicate that the ore-forming fluidswere derived from a deeper source with less crustalcontamination than the Ertsberg intrusion (James andHoush 1995). A deeper source for the Ertsberg EastSkarn System skarn-forming fluids was also proposed byRubin and Kyle (1998) based on the distribution andcomposition of skarn minerals. This overall dispositionof intrusive rocks and mineralization in the Ertsbergdistrict appears similar to the situation in the YeringtonBatholith described by Dilles (1987).

The early, high-temperature fluid systems repre-sented by the K-feldspar, biotite, magnetite, and quartzvein stages lack significant sulfide or sulfate compo-nents. The first sign of sulfur as recorded by precipi-tation of sulfide and/or sulfate minerals in the GICoccurred in the quartz–anhydrite veins that overprintthe Kali intrusive rocks and the associated vein system(Package 4). Sulfur behaves as an incompatible elementin oxidized granitoid magmas not saturated in anhy-drite, and would be anticipated to partition stronglyinto the earliest hydrothermal fluids evolved duringcrystallization. Its absence in the early vein and alter-ation stages suggests that the fluids were undersatu-rated with respect to sulfides and sulfates, possibly dueto high temperatures and/or low sulfur concentrations.In contrast, the post-Kali evolution is characterized bymassive amounts of sulfur in the form of purple an-hydrite and Cu-, Fe-, and Mo-sulfides. This suggeststhe fluids were derived from a different source to thatof the earlier alteration episodes, or that the nature ofthe source changed prior to generation of the GrasbergCu–Au system.

Anhydrite from the Ertsberg East Skarn System has Sand Sr isotopic characteristics consistent with a mag-matic source (Rubin and Kyle 1998), and models in-volving incorporation of host rock derived sulfur intothe hydrothermal fluids prior to mineralization thereforeappear unlikely. The final intrusive rocks in the GIC arethe Kali intrusions, and Kali I contains numerous small(<2 cm) basic xenoliths composed mainly of plagio-clase, biotite, magnetite, and apatite. Similar mafic xe-noliths are also abundant in the Ertsberg intrusion. Themafic xenoliths appear to reflect intrusion of more basicmagma into more felsic Kali/Ertsberg magma(s). Such aprocess can potentially supply abundant sulfur andother volatiles to the magmatic–hydrothermal system(Keith et al. 1998), and may explain the late appearanceof sulfur at Grasberg.

Acknowledgements The authors acknowledge with thanks the en-thusiastic cooperation and assistance received from P.T. FreeportIndonesia. Special thanks are due to Chuck Brannon, Al Edwards,and Larry Segerstrom for managing administrative and organiza-tional aspects of the project. Leo Wowiling, Uttu Mekel, and thecore shed crews provided generous assistance in transporting andlaying out drillcore. We thank George MacDonald and Jay Pen-nington for sharing their knowledge of the Grasberg system and fornumerous discussions. P.T Freeport Indonesia is thanked forpermission to publish this work.

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