Skarn Cu-Au Orebodies of the Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia - Mertig 1993

ELSEVIER Journal of Geochemical Exploration 50 (1994) 179-202

JOURNAL O[ GEOCHEMICAL EXPLORATION

Skarn Cu-Au orebodies of the Gunung Bijih (Ertsberg) district, Irian Jaya, Indonesia

Heidi J. Mertig, Jeffrey N. Rubin and J. Richard Kyle Department of Geological Sciences, The Universi~ of Texas at Austin, Austin, TX 78712, USA

(Received 17 May 1993; accepted after revision 28 October 1993)

Abstract

The major Cu-Au skarn deposits of the Gunung Bijih (Ertsberg) district in central lrian Jaya are products of hydrothermal systems that developed in association with Pliocene magma emplacement in an active continental margin. The Cu-Au skarn orebodies occur within a Cretaceous to Tertiary sedimentary sequence that was deformed as the northern Australian continental margin entered a north-dipping subduction zone at ~ 12 Ma. The intermediate-composition intrusions consist of fine- grained porphyritic stocks, dikes, and sills that have K-Ar ages ranging from 2.7 to 4.4 Ma. Most intrusions are slightly potassic, but these data could be affected by alteration.

The skarn orebodies in the Ertsberg district are hosted in deformed lower Tertiary New Guinea Group carbonate strata along the periphery of the Pliocene Ertsberg intrusion. Major skarn orebodies include the Ertsberg (GB), the Ertsberg East (GBT) complex, including the GBT, the Intermediate Ore Zone (IOZ) and the Deep Ore Zone (DOZ), and the Dom. Chalcopyrite is the dominant ore mineral in the GB and Dom orebodies, whereas bornite dominates in the GBT complex. Native Au occurs within bornite and chalcopyrite in GB and GBT ores.

The district calc-silicate alteration assemblages are characterized by high-temperature skarn minerals, including forsterite, monticellite, and minor melilite. Diopsidic clinopyroxene is common, particularly in GBT. Anhydrite and phlogopite are abundant in the GBT complex, and the anhydrite:calcite ratio increases with depth from GBT to DOZ where anhydrite is ubiquitous and calcite rare. At least three types of garnets have been identified at the Dom and show a progressive increase in ferric iron content. Garnet decreases with depth in the GBT complex. Talc, serpentine, tremolite- actinolite, and chlorite are common retrograde minerals. Copper sulfide mineralization is texturally associated with early retrograde alteration.

Differences among the skarn orebodies are related in part to variable protolith composition that affected skarn development within different stratigraphic positions. Distinctive fossil replacement textures preserved within skarn indicate that the Oligocene-Miocene Ainod Formation is the most likely protolith for the GB and Dom orebodies. The GBT and upper IOZ orebodies probably are hosted by the Eocene Faumai Formation. The DOZ and lower IOZ orebodies, dominated by magnesian skarn alteration, appear to be developed in a dolomitic unit within the lower New Guinea Limestone Group, which probably is equivalent to the Paleocene Waripi Formation.

0375-6742/94/$07.00 {3 1994 Elsevier Science B.V. All i-ighls reser,,ed SSDI0~75 6742(93) E0047-Z

180 H.Z Mertig et al. / Journal of Geochemical Exploration 50 (1994) 179-202

I. Introduction

The Gunung Bijih (Ertsberg) district of west central Irian Jaya, Indonesia, comprises a diverse group of orebodies associated with Pliocene intermediate-composition plutons intruded into a thick Mesozoic and Cenozoic sedimentary sequence. The Ertsberg district is located 120 km inland from the Arafura Sea within the Central Range of the island of New Guinea (Fig. 1 ). The Central Range is a spectacular Alpine terrain of folded and thrust faulted strata that was caused when the Australian continental margin entered the north- dipping subduction zone along the Melanesian island arc complex at ~ 12 Ma. Uplift and deformation in the Central Range of central Irian Jaya probably began in the Miocene ( Nash et al., 1993; Quarles and Cloos, 1994).

Exploration and development of the Ertsberg district has spanned over 50 years, as outlined by Van Leeuwen (1994). J.J. Dozy, a Dutch geologist employed by Shell, was a member of a mountain-climbing and geological reconnaissance expedition into the New Guinea Highlands in 1936. Dozy encountered a prominent black peak standing about 140 m above the glaciated valley floor at an elevation of about 3600 m within the Central Range. He named this exposure of Cu-stained massive Fe oxides the Ertsberg (Dutch for "ore mountain" ). Although he recognized the high Cu grade of the samples he collected, Dozy

t"~ ~t.~Oin . o,~LIMANTAN ~ f'-':'~ ~ m.ci,cOce.n

\ / / . U W: . ~ ~ . . . . IP,,,,uAM._ <-_ j ) ,6 "~..~' Jakarta /V h ~ ., ,-" S ~ 0 \ INEW "-~

' ' o , o < . ' . z . . . . . . . Ioo,• oo ~ Lake Panai ~ t ....... t

J ~ ,~, Ertsberg District ~ - J / e S, < W C3/j/USLake Tigi ~ Puncak Jaya (4884m) _ 4< ~ ' w

(J f " ~ / " Tembagapura I t q'~/'~ / (town)

50 1 Arnam;pa~re ~)/0 ~/j~ )/~/~i ~ /~-~ /~ Sydney/ "<"@.o ' " U / ( / ~ - \ ~ . . . . I

E 136 ° E 137 ° E 138 °

Fig. l . [nde× map o f Indonesia showing selected cultural and geographic features within the region, |nset map

shows selected features of south central Irian Jaya, including those associated with Freeport Indonesia's Ertsberg Contract of Work.

H.J. Mertig et al. / Journal of Geochemical Exploration 50 (1994) 179-202 181

believed that this deposit would not have any commercial value because of its remote location (Wilson, 1981).

Dozy described the findings of the expedition in a report (Dozy, 1939) that included a description of the Ertsberg. World War II prevented wide circulation of Dozy 's report. In 1959, a Dutch mining company came upon a number of Shell's reports including Dozy 's and called it to the attention of Freeport Minerals Company. Freeport's Manager of Explo- ration Forbes Wilson and staff were intrigued. Freeport mounted an overland expedition into the Highlands in 1960 to further evaluate and systematically sample the Ertsberg exposure (Wilson, 1981 ). A Contract of Work with the Indonesian government was signed in 1967 for a 100 km 2 area centered on the Ertsberg. Drilling commenced in 1967 and established reserves of over 30 million tonnes of high grade Cu ore.

Construction of production and support facilities began in 1970, and included a port facility at Amamapare, a 103-kin access road, an airport at Timika, and a town site at Tembagapura at elevation 1,800 m (Fig. 1 ). The second phase of construction included installation of a 6,750 tonnes per day mill at elevation 2,750 m, substantially below the level of the Ertsberg open pit mine at 3,600 m. A 1.5-kin aerial tramway covering a vertical extent of 760 m was constructed to transport equipment, personnel, and ore. A slurry pipeline was constructed to transport concentrates to the port, and production began in 1972.

Mining and support facilities progressively expanded with ore definition and production from additional orebodies (Table 1). Drill evaluation commenced on the Ertsberg East (GBT) skarn complex in 1975 and on the Dom skarn in 1985. With the discovery of the Grasberg intrusion-hosted Cu-Au deposit in 1988 (Van Nort et al., 1991; MacDonald and Arnold, 1993 ), the district achieved world-class status ( see MacDonald and Arnold, 1994). The mill currently treats about 60,000 tonnes per day, of which about 20% comes from the skarn orebodies of the GBT complex. Freeport-McMoRan Copper and Gold Inc. has announced expansion of the Ertsberg operations to 90,000 tonnes per day by mid- 1996.

This article briefly reviews the geology of the skarn orebodies of the district based on preliminary results of investigations of the GBT complex ( Rubin, 1994), the Dom ( Mertig, 1994), and the Big Gossan (Gonzalez, 1994). General results from several complementary studies in progress are included as they pertain to the development of mineralization within the district.

Table I Tonnage and grade of the Ertsberg district skarn orebodies

Ore deposit Discovery Tonnes Grade year" ( 106)

Cu Au Ag (%) (g/t) (g/t)

Ertsberg (GB) 1967 32.6 2.3 0.8 9.1 GBT 1975 61.4 2.0 0.7 I 1.5 DOZ 1979 22.6 2.4 1.1 8.7 IOZ 1979 18.9 2.0 0.6 10.5 Dom 1976 30.9 1.5 0.4 9.6

Signifies year in which evaluation by drilling commenced; surface exposures of some of the mineralized zones were known previously, e.g. Dozy's description of the Ertsberg in 1936.

182 H.J. Mertig et al. / Journal of Geochemical Exploration 50 (1994) 179-202

2. Districtgeology

2.1. Stratigraphy

Surface exposures within the Ertsberg district are dominated by the upper Mesozoic Kembelangan Group siliciclastic strata and the Tertiary New Guinea Group carbonate strata that were deposited on the northern Australian shelf (Visser and Hermes, 1962) (Figs. 2 and 3). The stratigraphic character, age, and regional correlations of the units within the Ertsberg district currently are being reevaluated. Therefore, the nomenclature used in this article (Fig. 3) generally represents the stratigraphic terminology that has been used by Freeport and may be revised as additional information becomes available.

Quarles (1994) measured a well-exposed upper Cretaceous and Tertiary stratigraphic section within a breached anticline in the Dugan-dugan valley, located just outside the northeast corner of Fig. 2. The exposed sequence consists of about 300 m of fine-grained glauconitic sandstones assigned to the upper D-member of the Kembelangan Group (Visser and Hermes, 1962) (Fig. 3). The total thickness of the D-member is 600 m locally (K. Heron, pers. commun., 1993). In exposures near Tembagapura, Kembelangan Group strata overlies fine-grained siliciclastic strata of the Triassic Tipuma Formation. The total stratigraphic thickness of the Kembelangan Group is not well established, but appears to be on the order of 4,000 m (Martodjojo et al., 1975).

In the Dugan-dugan valley, the New Guinea Group includes about 1500 m of carbonate and siliciclastic strata, locally assigned to the Faumai and Ainod Formations (Fig. 3 ). The Faumai Formation consists of 150 m of miliolid-bearing limestone, that has been determined to be of Eocene age (Quarles, 1994). Heron (1990) identified a variable sequence of interbedded fine-grained siliciclastics and carbonates, including a "banded" dolomite sequence in the Big Gossan area between these Eocene limestones and the Cretaceous Kembelangan Group siliciclastics (Figs. 2 and 3 ). This unit may be equivalent to the Waripi Formation recognized elsewhere in Irian Jaya (Pieters et al., 1984; Dow et al., 1988). The Waripi Formation is considered to be Paleocene in age because of its stratigraphic position, but could also include strata of late Cretaceous age (Pieters et al., 1984). The presence of dolomite in the carbonate sequence is important with regard to the development of the magnesian skarns of the district.

A coarse-grained sandstone unit as much as 40 m thick separates the Faumai Formation from the overlying Ainod Formation and serves as a local stratigraphic marker; this sandstone probably is equivalent to the Sirga Formation (Pieters et al., 1984). On the basis of large foraminifera biostratigraphy, this sandstone unit probably marks the Eocene/Oligo- cene boundary, and the Oligocene/Miocene boundary occurs within the lower Ainod For- mation (Fig. 3). The lower Ainod Formation, 600 m thick, is predominantly a fusilinid limestone with less than 20% siltstone and sandstone. The upper Ainod Formation consists of about 300 + m of coral-rich limestones, including conglomeratic units. Lignitic horizons occur in the upper section, signifying shallowing of the depositional environments in the early Miocene (Quarles, 1994). The upper Tertiary limestones have been assigned many formational names within the region (Pieters et al,, 1984), and additional studies will be necessary to establish regional correlations and a consistent stratigraphic nomenclature.


O { I I Alluvium

t [ ~ - ] Intrusions ~, Te = Ertsberg, ~, Tg = G rasberg

~ Ainod Fm. Faumai Fm.

v { ~ Kembelangan Gp.

SKARN DEPOSITS

D Gunung Bijih (Ertsberg) ~ r ~ Gunung Bijih Timur (Ertsberg East)

Dom Big Gossan

Fig. 2. Geologic map of the Ertsberg district showing locations of major skarn deposits. Generalized from 1 : 10,000 mapping of Freeport Indonesia geologists from 1970 to 1993. Limited areas of glacial cover are omitted from the northeastern part of the area.

184 H.J. Mertig et al . / Journal of Geochemical Exploration 5 0 ( 1 9 9 4 ) 1 7 9 - 2 0 2

I I I

'1 ' 1 ' 1 '

i i i i

I i I i I i I t ' - i i t t Q ) , ' . I . ' l

I I ' I ' I

. 9 .. ~.'...-. :.'.....' ~ . . ~

I i I i I i I

' i , i , I '

. . . . i , i , I ,

2 ,_--,--a_L OQ1, . . . . . 13) e- ----------.--~--

~ , ~ ' , ' l ' , ' I i I i I i I i i i i

- - ~ - - I . . . . I . . . I . . . . . I . . . .

: : l : : : : : : l : : : : : : 1 ; : : = : : 1 : ~ . ~ i I i I i I i t ' - i . , , , i i ~ I I I I I I I

~ , ~ i I I I I I I

0 I I 1

i i ~ ' I ' I ' I '

i . . . . . .

a , ~ e " , i ~

- - ~ . . . . . . . . . i~_ 0 ~ " . . . . . ...

Upper Ainod Fm

Lower Ainod Fm

(Upper) • 400 m Faumai Fm

(Lower) Faumai Fm o m

(Waripi Fm ?)

Kembelangan Group

Tipuma Fm

limestone

~1 dolomite

sandstone

siltstone

mudstone

I undifferentiated siliciclastics

Fig. 3. Generalized stratigraphic column for the Ertsberg district. Generalized from Hefton and Pennington ( 1993 ),

who compiled local measured sections by Hefton, Pennington, and Quarles, and utilized regional stratigraphic nomenclature o fDow et al. ( 1 9 8 8 ) , Pigram and Panggabean ( 1 9 8 3 ) , and Visser and Hermes ( 1 9 6 2 ) .

2.2. Structure

Aspects of the complex tectonic framework of the Indonesian region has been studied by many researchers; a useful regional review is provided by Hamilton (1979). Nash et al. (1993) have recently evaluated the structural development of central Irian Jaya, based on information generated during Freeport Indonesia' s regional exploration program. The major structural features in central Irian Jaya formed as the result of the Central Ranges Orogeny which resulted when the Australian continental margin entered a north-dipping subduction zone along the Melanesian island arc at ~ 12 Ma (Quarles and Cloos, 1994).

Two styles of deformation have been recognized within the Ertsberg district. Folding is the most obvious mechanism of shortening, and the km-scale folds trending about 120 ° represent the largest structural features mapped in the district (Fig. 2). A series of dip-slip faults have traces parallel to the fold axes. Most of the faults are steeply dipping and intraformational, and appear to have reverse motion with displacements probably less than


a few hundred meters (McDowell et al., 1994) The folds and vertical faults are offset by high-angle faults that have a bimodal distribution, trending 030 ° to 070 ° and 170 ° to 180 ° ( Fig. 2). Preliminary structural interpretation suggests that the NE-trending faults have left- lateral offset and the N-trending faults have right-lateral offset (Quarles, 1994). The apparent displacement along these faults ranges from a few meters to as much as several hundred meters.

2.3. Magmat ic activity

Intrusions in the Ertsberg district consist of fine-grained porphyritic stocks, dikes, and sills, all of which have been hydrothermally altered to some degree. Local exposures of altered volcanic strata have been identified along the periphery and overlying the upper portions of the Grasberg intrusive complex (MacDonald and Arnold, 1993, 1994; Sapiie, 1994). The plutons represent shallowly emplaced magmas, as suggested by the remnants of volcanic cover rocks on the periphery of the Grasberg complex. Fission track analysis (Weiland, 1993; Weiland and Cloos, 1994) and reconnaissance fluid inclusion data (J.R. Kyle, unpublished data) suggest emplacement depths of 2 km or less.

SiO2 contents of intrusive rocks range from 54 to 62 wt % with most falling between 59 and 62 wt % ( McDowell et al., 1994). Most intrusions are slightly potas sic with Na20 + K20 between 5.6 and 8.6% and K 2 0 / ( N a 2 0 - 2 ) ranging from 1.3 to 3.8 (McMahon, unpubl. data), but these data could be affected by alteration. Although alteration makes classification of the igneous rocks difficult, most of the intrusions plot in or near the trachyandesite and trachydacite/trachyte field on a Total Alkali-Silica plot (Le Maitre et al., 1988). On a QAP plot (Le Maitre et al., 1988), three samples from the Ertsberg stock range from quartz monzodiorite to monzogranite (McDowell et al., 1994). All of the analyzed Ertsberg district intrusions are silica-oversaturated and would be classified as alkalic-calcic to calc-alkalic using the index of Peacock ( 1931 ) (McMahon, 1994).

Plagioclase is the most abundant phenocryst in all intrusive phases. Biotite, amphibole, clinopyroxene, magnetite, and (rarely) potassium feldspar also occur as phenocrysts. In general, groundmass mineralogy is dominated by quartz and potassium feldspar, although plagioclase is also abundant. Apatite and sphene are common accessory phases (McMahon, 1994).

K-Ar ages have been determined for 15 biotite separates from intrusive phases within the Ertsberg District. These data indicate emplacement of the Ertsberg intrusions within the range of 4.4 to 2.6 Ma, with the majority of ages falling between 3.0 and 3.3 Ma. Three analyses of the Ertsberg intrusion, with which all of the currently producing skarns of the Ertsberg district are spatially associated, indicate ages ranging from 2.6 to 3.1 Ma (McDowell et al., 1994).

Trace element characteristics of the intrusions are suggestive of an arc-like magma source and are similar to those of intrusions associated with some Cu-Au deposits in Papua New Guinea (McMahon, 1994). However, McDowell et al. (1994) conclude that the Plio- Pleistocene magmatism of the Ertsberg district was not directly related to subduction, but to an episode of asthenospheric upwelling resulting from arc-continent collision.


3. Ertsberg district skarns

3.1. General

The major skarn orebodies of the Ertsberg District are the Ertsberg (Gunung Bijih or GB), the Ertsberg East (Gunung Bijih Timur or GBT) complex, including the GBT, IOZ (Intermediate Ore Zone) and DOZ ( Deep Ore Zone), and the Dom (Fig. 2; Table 1 ). Most of the skarns average less than l g / t Au and thus would be categorized as "byproduct Au skarns" (Theodore et al., 1990). Although the Ertsberg skarn orebodies contain near l g/ t Au, they are not enriched in Au in comparison with their Cu contents and are typical of skarns associated with porphyry Cu deposits worldwide (Meinert, 1989).

The Ertsberg calc-silicate alteration assemblages are characterized by high-temperature skarn minerals, including forsterite, monticellite, and minor melilite. The major ore minerals are chalcopyrite and bornite. Chalcopyrite is the dominant ore mineral in the GB and Dom orebodies, whereas bornite dominates in the GBT complex. Native Au occurs in association with bornite, and probably with chalcopyrite, in GBT complex and GB ore.

Producing skarn orebodies are hosted in the lower Tertiary carbonate units (Figs. 2 and 3). The GBT and Dom orebodies occur along the periphery of the Ertsberg intrusion, whereas the GB orebody is entirely surrounded by the pluton. The GB and Dom skarns show distinctive replacement textures within skarn of the abundant foraminifera of the Ainod Formation (Fig. 4). The GBT complex skarns show only local preservation of fossil textures.

The presently mined Grasberg Cu-Au orebody occurs in a potassic alteration zone within the interior of the Grasberg intrusive complex (Fig. 2). However, deep drilling has inter- sected an irregular zone of sulfides concentrated at the contact with the Tertiary limestones (MacDonald and Arnold, 1993). Also, recent exploration along the Big Gossan structure (Fig. 2) has encountered major Cu-Au concentrations associated with calc-silicate alteration zones. These calc-silicate zones are developed at least in part within the basal 80 m of the New Guinea carbonate sequence (K. Hefton, pers. commun.), within a poorly dated dolomitic sequence that is probably equivalent to the Waripi Formation (Fig. 3). Results from the Big Gossan drilling program suggest the presence of structurally controlled skarn zones with higher Cu and Au concentrations than the currently mined skarn orebodies (Anon., 1992).

3.2. GB/Ertsberg Skarn

The Ertsberg deposit was the original discovery in the Ertsberg District. The GB (Gunung Bijih is "ore mountain" in Bahasa Indonesia) Cu-skarn orebody had original reserves of 33 million tonnes of ore averaging 2.5% Cu and 0.8 g/ t of Au. The GB reserves were largely depleted by the mid-1980s, and open pit production ceased in 1989. Available geologic information on the Ertsberg deposit consists of studies by Katchan (1982) and Soeparman and Budijono (1989) on which most of the following discussion is based.

The GB deposit was surrounded by the Ertsberg intrusion and generally is thought to have been a roof pendant (Figs. 2 and 5A,B). The bulk of the orebody was a plug-like mass of magnetite-rich skarn surrounded by a zone of calc-silicate alteration as much as 100 m


Fig. 4. Replacement textures in the Ertsberg skarn ore. A. Ainod Formation limestone with abundant large foraminifera. B. Magnetite-rich skarn showing chalcopyrite "wisps" relict after large foraminifera.

wide. Katchan (1982) divided the calc-silicate alteration into 10 assemblages presumed to have replaced dolomitic limestone wall rocks and one assemblage that replaced intrusive rock. Magnetite and calc-silicates dominate the more abundant exoskarn. Monticellite is the most abundant mineral in the calc-silicate envelope, although diopside-rich skarn dominates in the lower levels of GB (Soeparman and Budijono, 1989). A common ore texture within both calc-silicate and magnetite skarns preserves original limestone textures, including replaced foraminifera (Fig. 4). Copper sulfides preferentially replaced the foraminifera, relative to matrix, in both.

According to Katchan (1982), initial skarn formation resulted from high-temperature silica metasomatism of dolomitic wall rocks that produced a diverse suite of magnesian calc-silicates, including monticellite, diopside, forsterite, melilite, spinel, and fassaitic clinopyroxene. Later Fe-A1-Mn metasomatism, associated with decreasing temperatures, produced glaucochroite, garnet (grandite, Zr-ugrandite, hydrogarnet), vesuvianite, clintonite, Ca amphibole, and harkerite. The Ertsberg intrusion is the most likely source of the hydrothermal fluids that initiated development of the skarn orebodies, and structural controls probably played a major role in providing access for mineralizing fluids as the intrusion cooled (Katchan, 1982).

Soeparman and Budijono (1989) observed that magnetite concentrations crosscut both calc-silicate skarn and intrusive rocks in the eastern part of the orebody and interpreted this


relationship to indicate that magnetite skarn was younger than calc-silicate skarn. However, fragments of magnetite within extensive breccias are rimmed by calc-silicates, sulfides, and magnetite, suggesting that there are multiple generations of magnetite (and other skarn- related minerals). Sulfide deposition was both contemporaneous with and younger than

(A)

+ + + + + + + + + + + ~

+ + ÷ + + + + + +

+÷÷+++*,+0+: +++ ÷++ ++~+" + +++ +++ ~ .

+ + cuc~t:z:,-+t ~ ..-: + ÷ 4tl~+.~ + + +

+ + + + + + + +++ ++ . ~ + '~:':F,~'.'., ++ + + + + + + + + + • + : "+ +

. . . . . . . . . % + + } . + + + . + . + , + e * + + + + + + + +

++++ + + + + + ÷ + + + * * ++++ : ' + + + + +

+ + + + + + + + + + + + + + + + + + .=,

+ + + + + + + + + + + ":::!~ ' . + + + ",- ¢t...~ji + + + + + + ÷ + + + + 4~..+.,.~r.,,+,~l.

. . . . . . t/++++,,, , ~ + + + +

] ~ + + ÷ + + + ~ . . : . : - . : . - ) - ~ + ÷ + ÷ # + + . : ' - . . . . . ' ' ' /

. + + + + ~ + . " . - . . . ' . ' ) + ÷ + ~ + + + + : . , : ' .

+ + + ÷ + + + + ~ ' . . - . / + ÷ + + + + + + + ' ' . . : " /

++++++++++'~:/+÷÷÷++~ I ~ + + + + + + + ~ + + # * + + + + + + + + + + + + + + + + + + ÷ + ÷ /

+ + + + + + ÷ + + + ~ 1 / + + ÷ + + + + ~ + ÷ + ~

I + ÷ + + + + + + + + + X

~ + + + + + + + ÷ + ÷ +

+ + + + + + + + + + + + ~ I + * + + + + + + + + + + N ~ + + + + + + + + + + + + + ~

+ + + + + + + + + + + + + ~

~ + + + + + + + + + + + + ~ + + ÷ + ÷ + + ÷ + + + + + ~ + ~ + ~ + + + ~ + + + + + + + + + + + + + + + + + + *

~ . + + ÷ ~ + + + + + + + + + + + + ÷ + + + ~ + + + + + + + * + + + + + + ÷ + ÷ ~ + +

÷ + + 1

+ + + + + + + + + + + + + ÷ + + + + + + + + + + + + * + + + ÷ + ÷ + +

+ + + + + + + + + + + + + + + + + + + + + * + + + + + ~ + + + + + + + + + + + * ' ÷ * " + + ÷ + + + + + + ~ + + + + + + +

+ + + + + + ~ + + + + + + + + + + + + + - ÷ + + + + + + + + + + + + + + + + + + + + + + 4 + + 4

+ + + + + + + + 4 + + + + + + + + + + + + + ÷ + + + + + ÷ ÷ + + + + + + + + + + + ÷ + + + + + + +

+ * + + + + * + + ~ + + ÷ + + * + + + + + + + +

+ + + + + + + + + + ÷ + + + + + + + + + + + + + + + + + ~ + + + + + + + + ÷ + + + + + + + + + + + * + + + + + + + . + ~

+ + + + + + + + + + + + + 4 ÷ + ~ + + + + + + + + + + + + ~ + + + + + + + ~ + + + + + + + +

+ + + + + + + + + + ÷ + + + + + . . . . . . + + + + + + ~ + + + + ÷ + + + + + . : :

+ • + + + + + + + + + + + + + ÷ + + . + + + ÷ + + + ~ + + + + + + + + + + + ' :

+ + + + + t ÷ ÷ + ÷ ~ + + + + + + + i ~ . . . . + + + + + ÷ ÷ + + + + + + + + ~ +

+ + + + ~ + + + + + + + + + + + + i : " ' + ~ + + + + + + + + + + + ~ + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + ~ + * ~ + s ~ |

+ + + ~ + + + + + + + + + + + + + ~ + / ~ - + + + + + + + + + + + + + + + ÷ + + +

+ + + + + + + + + + + ~ + + + ~ ÷ ~ *

+ + + + + + + + + + + + + ÷ + ÷ - - + + - + ~ + + + ÷ + + * + + + * Y + + + h + ~

' + + + + + + + + + ÷ + + ~ + + + # ~ + t + ~ + + + + + + ~ + + ÷ + + + * f + + ~ + + ÷ + + + + ~ + ÷ ~ + + + + + + + ~ . . ~ + + + + + ~ + + ~ + + + + * + + ÷ : ' L + + + + + + + + + + + + + ~

. . . ' - - - ~ . ~ < ~ ' . 1 + ~ + + + + ÷ ~ + +

~ _~~:" . .~- + ~ + + + ÷ + ~ ~ • ' ' ' ~ - + + + ÷ + ~ + , : + + ÷ + + + +

+ ~ . . . . Z + " ÷ " ~88:: ': ~ + + ÷ * +~ ÷ ÷ i " ) " f + + + + + + + * ~

+ i : : . . . . . . ~ . . . . . . • : + + ÷ ~ + ÷ + * - + ÷

+++ + + +

• ., / • • , + ÷ . , + + + + . + ÷ + + + + ~ "

+ ~ + + + ÷ . : .;..~ . ' . ' i : ~ + + + + + + + ÷ ÷ +++

+ + + + + + + * + + ~ + ..~.~c ~ " + + + * + * + + + + + * + ÷ + ÷ + + ÷ ÷ + ÷

: ÷ + + + + + + ÷ ÷ + ÷ , 3 + ÷ + + + + + + + ~ * + + + + + + + + * ÷ + + + ÷ ~ ÷ ÷ \ + + + ÷ + ~ + + + + + + + + + + + + +

\ + 8 6 + + + + + + + ÷ + + ÷ + + + + + + + + + ~ ~ + + + + + + + + + + + + + ÷ + + + + - | ÷ + + ÷ + + + + + + + + + + + + + + + + ~

~ + + + + + + * + + + + ÷ + + + ~ + + . i + + + + + + + + + ~ + + + + + + + + + + ÷ + + + ÷ + ~ + + + + ÷ + + + + +

#+++.++*+++++++++÷+"0 100 m / ,~ - - + + + ÷ + + ÷ + + + + + + + ÷ + + + + + ~ J / + + ÷ + + + + + + + + + + + + + + + + ÷ +

I ~ ,+ ,+ ,+ ,+ ,+ ,+ , t , + ,+ ,+ ,+ ,+ ,+ ,+ ,+ ,+ ,+ ,+ ,+ ,+1+ ,

l - - ] Covered ~ Quartzi te [ ] M a g n e t i t e - chalcopyrite

~ Er tsberg int rus ion ~ Monticell ite, garnet, ~ - ~ Specu la r i t e - diopside skarn

chalcopyrite Marble ~ Epidote skarn

• Pit outline at 3500m

Fig. 5A. Geologic map of the Ertsberg (Gunung Bijih) skarn orebody. Generalized from 1:2,000 bench level mapping by Freeport Indonesia geologic staff.

H.J. Mertig et al. /Journal of Geochemical Exploration 50 (I 994) 179-202 189

S W

(B) F I N A L P I T

NE 1

++++++

4~++4 + ~ +++++ +4 ~+ ++++++++

÷ + - - 4 + -- + + + +

i

+ + + + + + + \ ~- . -~ + + + + +++ + + + + + + + + ~ " + + + + + +

+ * + ++++++++ +4~+ ++

+ 4 + +,+*++++++++. + + ~ + ++ +*+*++++++43~0+ ~ , + + + + + + + + + + .

. + + + ÷ + + + + ~ + ~ 4 + + + + +

-- + + + -- ~ + 4 + + , + . . ++ ++ +

+ + + 4 + + ~ * + 4 ~ + + + + + 4 + + ~ -

7+ + + + + + + + + ~ + + ~ + +

• + + + + + • + + ~ + + ~ + ~ +

+ + + + + , + + + + 4 ; + :" ÷ + + + + + + * + + + + ~ + +

+ + + + + + ÷ + 4 + * + + + + + + + + + + • + + + + + ~ ,.

+ + + - - + + + ~ * + + + + ÷

+ + + -" + + • + + • ÷ 4 + + + + + + + + + ~ + + * + 4 + ~ +

+ + + ~ + + + , + + ~ * , +

+ ÷ + + + + + + + ~ + + 4 +

+ + + + + ~ + ~ + + ~ - , + + . . . . . . . . '~3400 ~ + + + + + + + + + 4 * 4 +

• + + + * + + + + + + 4 + + * ~ + + + + + + + + + + + + + e + + + + + + + + + + ~ + ~ +

+ + + + + + + , + + ~ + 4

+ * + + + + + + + I ?

. . . . . . . . . . . : i i + + + + + + + + + + +

+ + + + + + + + - . 4 + 4 + + + + + ~ +

+ + + + + + + + + + + + + + + + ~ + + .

~ + ÷ + + ~ ÷ + + + + ~ + 4 + + + + + + + + +

+ + + 4 + + + + + + + . + ~ + + + + + + + + +

+ 4 + + + + + + + + + , + + + + + + + + + + +

~ + + + + * + + + + + ' I + + + + + + + + + + +

+ - + + + ~ + + + + + ~ + + + + + + + + + +

+ + + + + + + + + + + . 4 + + + + + + + + + +

+ ~ + + + ÷ + + + + + + + ~ + + + + + + + + +

+ ~ + + + + + + + + + + + + + + + + + + + + -

+ ~ + + + + + + + + + + + + + + + + + + + + +

÷ + + + + ÷ + + + + + +

+ + + + + + + + + + + + + + + + + +

. . . . . . . . . . + + + . . . . . . ~ Garnet -mont ice l l i te s k a m -% 0 1 0 0 m + + " + % + + + ÷ + + + + L

+ i + + + + + + + + + . . . . . . + . . . . ~ Ertsberg intrusion + + + + + + + + + + + + + + + + + + + +

. + + + + + + + + + + + + + + + + + + + +

f* . . . . . . . . . . . . . . + + < . + + + + + + + + + + ÷ 4 + +

+ + ' + + + + + + + + + + + + + +

+ + + + + + + + + + + ~ + ~ 2 ~ + + + + + + ~ + + + + + • ~ V V *

f+ + + ÷ ~ . . . . . . . . . . . . . . + + + + + + + + + ÷ + + + + + t + + +

+ + + + + + + 4 + + + + ~ + + + + 4 • ÷ + + + + + + + + 4 + + + ~ *

+ + + + + + + ÷ + + + + t + ÷ + + + + ÷ ~ + + + + + + + + + + * + • + + ~ + + +

+ + + + + + + + + + + + + ~ + + + * ~ + + + + + + + + + + + + + + + + + ~

+ ~ + ÷ + + + + + + + + + + + + 4 +

~ ~ M a g n e t i t e - c h a l c o p y d t ~ k a r n

Fig. 5B. Geologic cross-section of the Ertsberg (Gunung Bijih) skarn orebody. Generalized from a 1:2,000 cross- section prepared by Freeport Indonesia geologic staff.

magnetite, and the Cu-Au mineralization is concentrated within the magnetite mass. Other metallic minerals include bornite, chalcopyrite, pyrite, chalcocite, digenite, molybdenite, sphalerite, galena, bismuthinite, native Au, electrum, and native Ag. The most abundant ore mineral is chalcopyrite with lesser bornite. Native Au and electrum rarely are completely enclosed within any single sulfide phase, but grains up to 100/zm typically occur along boundaries between Cu sulfides (generally bornite) and quartz or calcite.

Locally intense retrograde alteration produced phlogopite, talc, chlorite, serpentine, mont- morillonite and clintonite. Phlogopite occurs both intergrown with magnetite and as an

3500 m

H.J. Mertig et al. / Journal of Geochemical Exploration 50 (1994) 179-202

SW

4000 m

NE

3000 m

Ertsberg intrus

Forsterite ± m(

Chalcopyr i te-

~ Marble

0 100 200 m I

190

Fig. 6. Geologic cross-section of the GBT complex skarn orebody. HWF = Hanging Wall Fault. Modified after Soeparman and Budijono (1989)

H.J. Mertig et aL / Journal of Geochemical Exploration 50 (1994) 179-202 191

a l te ra t ion p roduc t of ca lc - s i l i ca tes , a l t hough c l in ton i te is typ ica l ly res t r ic ted to ca l c - s i l i ca t e

skarn.

3.3. GBT Skarn Complex

T h e G B T [ G u n u n g Bi j ih T i m u r in B a h a s a I n d o n e s i a ( " o r e m o u n t a i n e a s t " ) ] c o m p l e x

is loca ted app rox ima te ly 1.5 k m east of G B a long the no r the rn m a r g i n o f the Er t sberg

in t rus ion (Fig . 2 ) , and cons is t s o f three ver t ica l ly s tacked o rebod ies [ G B T , In t e rmed ia t e

Ore Z o n e ( I O Z ) , and Deep Ore Z o n e ( D O Z ) ] , tha t toge the r fo rm one o f the la rges t Cu-

bea r ing m a g n e s i a n skarns in the wor ld ( Rub i n , 1994) . P roduc t ion plus reserves (as o f

1990) for the G B T c o m p l e x totals 100.2 mi l l ion tonnes ave rag ing 2 .1% Cu, 0.8 g / t Au,

and 10.6 g / t Ag ( T a b l e 1 ). The re is poo r overa l l cor re la t ion b e t w e e n Cu and A u wi th in the

orebodies . A l t h o u g h m a n y areas o f h igh Cu grades have h igh Au grades as well , Au seems

to cor re la te be t t e r wi th zones of f rac tur ing than wi th Cu grade (A. Schapper t , pers. c o m m u n . ,

1990) .

The G B T c o m p l e x skarns ex t end f rom the surface exposures at app rox ima te ly 4000 m

to app rox ima te ly 2700 m above sea level , a l t hough the b o t t o m of the D O Z mine ra l i zed

zone is poor ly def ined (Fig . 6 ) . G B T , IOZ, and D O Z are m i n e d t h rough separa te m i n e

work ings ; D O Z is t empora r i ly inact ive, I O Z recent ly b e g a n produc t ion , and G B T is near ing

Table 2 Geologic Features of the Major Divisions of the GBT Complex. Mineral identification is based on petrography, X-ray diffraction studies, and electron microprobe analysis of more than 200 petrographic sections

GBT IOZ DOZ

Prograde: gar, cpx > fo; gar, cpx in upper levels minor mnt, vsv, wo fo dominates lower level

Retrograde: chl, ep, srp tic, srp, trm-act, chl, ep

Accessory: ca = an; trace gy; an > ca; minor gy, qz; common mica ( clt ) common mica (phi)

Opaques: bn -~ mt; common cp; mt >> bn; minor cp minor py, mc, id, cb minor py, mc, id

Ore texture: bn, cp hosted by "BAS"; intergrown bn-mt; "BAS"; much ore BR-hosted also bn-, cp-an veins

Other: trace ga, sl, ar trace ga,sl with py, gy trace mo in intrusion trace electrum in bn

fo >> cpx; minor spinel: locally abundant trm

tic, srp, trm-act, chl

an >> ca; minor gy, qz; abundant mica (phi)

mt >> bn; minor cp rare py

intergrown bn-mt; also bn-, cp-an veins

Au avg. > 1 ppm (incl. inbn) trace Bi, bs, te in bn, cp, phi; trace mo in bn; trace ga, sl

Abbreviations: act=actinolite; an=anhydrite; ar=argentite; BAS="black amorphous silica" (see text); Bi = native bismuth; bn = bornite; bs = bismuthinite; BR = breccia; cb = cubanite; ca = calcite; chl = chlorite; clt = clintonite; cp = chalcopyrite; cpx = clinopyroxene; fo = forsterite; ga = galena; gar = garnet (grandite); gy = gypsum; hdl = hedleyite; id = idaite; mc = marcasite; mnt = monticellite; mo = molybdenite; mt = magnetite; phi = phlogopite; py = pyrite; qz = quartz; srp = serpentine; sl = sphalerite; te = tetradymite; tic = talc; trm = tremolite; vsv = vesuvianite; wo = wollastonite.

192 H.J. Merti g et al. /Journal of Geochemical Exploration 50 (1994) 179-202

Fig. 7. GBT complex skarn mineralization. (A) Photomicrograph of forsterite-magnetite-clintonite skarn with pore-filling calcite from the DOZ. Transmitted light, crossed polars; long dimension = 4.0 mm. (B) Photomicro- graph of monticellite skarn with pore-filling calcite from the GBT. Transmitted light, crossed polars; long dimension = 1.2 ram. (C) Photomicrograph of euhedral and subhedral magnetite within bornite from the DOZ skarn. Intergrowths, principally digenite, are common in bornite. Non-reflective matrix is quartz and talc, non-reflective elongate grains are phlogopite. Reflected light, plane-polarized; long dimension = 0.4 mm. (D) Massive bornite and bornite veinlets within gray anhydrite from the DOZ. (E) Photomicrograph from same sample as in (D), showing opaque bornite filling center of anhydrite vein. Transmitted light, crossed polars; long dimension = 2.5 mm. (F) Native Au inclusions in bornite from the DOZ. All Au in this sample is very high-fineness (0.9g--0.99 Au). Reflected light, plane-polarized (in oil); long dimension = 100/~m. Abbreviations: a = anhydrite; Au = gold; bn = bornite; c = clintonite; ca = calcite; cp = chalcopyrite; dg = digenite; f = forsterite; g = garnet; m = monticellite; mt = magnetite; p = phlogopite.


the depletion of its reserves. The GBT complex is bounded by the Ertsberg intrusion on the south and by the near-vertical Hanging Wall Fault (HWF) on the north, which places skam against barren marble. The marble zone varies in width from less than 10 m to approximately 50 m. The orebodies and the HWF are offset by as much as 15 m by a series of low-angle faults that are found throughout the GBT complex. Chalcopyrite and bornite commonly occur in fault gouge (locally termed "Black Amorphous Silica" or " B A S " ) and breccia matrices with clay, calcite, anhydrite, and pyrite in GBT and upper IOZ. This material is related to deformation along the HWF and probably to solution-induced collapse concurrent with skarn formation. The breccias in GBT and upper IOZ, described in detail by Katchan (1982), represent a variety of lithologies and mineral assemblages in both clasts and matrices.

The Faumai Formation is mapped along strike to the southeast of the GBT complex ( Fig. 2), and drilling from the DOZ production level has intercepted altered Kembelangan Group strata. The DOZ and lower IOZ, both dominated by magnesian skarn alteration, appear to be hosted by the basal Tertiary dolomitic sequence (Waripi Formation), and it is likely that GBT and upper IOZ are hosted by the Faumai Formation. Fossil-replacement textures, common in the GB and Dom ores (Fig. 4), are not well developed in the GBT complex skarns.

Skarn mineralogy in the DOZ (Table 2) is dominated by forsterite (Fig. 7A), generally in contact with massive, fine-grained magnetite-bornite-anhydrite ore. Principal alteration products of skarn include talc, serpentine, tremolite-actinolite, and chlorite. Grandite garnet and diopsidic clinopyroxene are the dominant prograde phases in the GBT and upper IOZ skarns, although forsterite is present (and can be locally abundant) there as well. Garnet also is a minor endoskarn phase throughout the GBT complex, but decreases in exoskarn below GBT and is restricted to endoskarn in the DOZ. Clinopyroxene is present in all three divisions of the complex but is most abundant in GBT. Spinel is a minor phase in the GBT and DOZ skarns. Zircon, sphene, and rare zirkelite-zirconolite are trace minerals in the DOZ. Tremolite may represent both retrograde and late prograde alteration; more actinolitic amphibole occurs primarily in veins. Amphibole is common in the DOZ skam but is found only in trace quantities in GBT. Wollastonite and vesuvianite are rare and appear restricted to GBT, as is monticellite (Fig. 7B), which forms coarse masses in some areas.

The GBT skarn is considerably more Ca-rich than the skams of the DOZ. The GBT mineral assemblage, including grandite gamet, clinopyroxene, monticellite, clintonite, wollastonite, and vesuvianite are all calcic with varying amounts of Mg, A1, and to a much lesser extent, Fe. The DOZ mineralogy is predominantly Mg-rich, i.e., forsterite, spinel, serpentine, talc, and phlogopite. The only significant Ca-beating silicates in the DOZ are clinopyroxene and tremolite-actinolite, both of which are subordinate to forsterite; the major Ca-bearing phase is anhydrite. Forsterite compositions throughout the GBT complex range from FO89 to Foloo (avg.=Fo96) and skarn clinopyroxenes from Di76 to Di,~ (avg. = Di95 ). Garnets in exoskarn range from Ad62 to Advl (avg.--= Ad68), whereas those in endoskarn have a range of Ado4 to Ad98 (avg. = Ad67 ) (Fig. 8). Most garnets contain traces of Mg and Mn; some have minor Ti (Rubin, 1994).

Magnetite appears to predate sulfides (Fig. 7C), and clearly postdates forsterite, clinopyroxene, and possibly garnet, but is rimmed by spinel. Some magnetite is zoned compo- sitionally with respect to Mg and Mn. Magnetite generally occurs as euhedral grains, some


GBT-Complex Dora

Pyralspite Pyralspite

/ ~ • GBTintrusion (n=70) / ~ • Late (n=13) A

• G BT ska~i(::~ ~ / "~k ÷ Intermediat0e (n=9)

../... / /

Gr

\ / . . . . \ / _ \

AO Gr AO Fig. 8. Compositions of garnets from the GBT complex and Dom skarns based on electron microprobe analysis ( Rubin, 1994 and Mertig, 1994 respectively). See text for discussion.

of which are concentrically zoned and many of which are corroded and partially replaced by sulfides. Magnetite locally has been altered to hematite along rims and fractures.

The abundance of anhydrite in the GBT complex is unusual in comparison with skarns worldwide. Anhydrite generally occurs as cement of prograde calc-silicates and fills vugs and veins in barren and mineralized skarn. Although anhydrite is present in GBT, it generally is subordinate to calcite. With increasing depth, the anhydrite:calcite ratio increases from GBT, through the IOZ, and into the DOZ, where anhydrite is ubiquitous and calcite rare. Most anhydrite is paragenetically later than prograde calc-silicates and some magnetite, but predates the bulk of sulfide deposition ( Figs. 7D, 7E). However, some anhydrite veinlets cut bornite. There probably are multiple generations of sulfide minerals, and likely multiple generations of anhydrite as well. Petrographic and isotopic studies indicate the presence of sedimentary anhydrite preserved within the calc-silicated sequence (Kyle and Dworkin, unpublished data).

Phlogopite, with variable Fe content, is abundant in the DOZ but very rare in GBT; it is earlier than most retrograde phases, and its paragenetic relation to anhydrite is ambiguous. The phlogopite in the DOZ contains 1-4 wt% F and may be the source of the F anomaly in the skarn-ore concentrates. Phlogopite grain size varies from ~< 50/xm to > 5 mm, and coarse ( ~> 1 mm) phlogopite is commonly kinked and hosts Cu sulfides and, more rarely, hedleyite along cleavage traces. Phlogopite replaces early calc-silicates and locally appears to replace anhydrite. Mica occurs in GBT dominantly as clintonite.

Bornite, and less commonly chalcopyrite, form massive intergrowths with subhedral

H.J. Mertig et al. /Journal of Geochemical Exploration 50 (1994) 179-202 195

magnetite_+ anhydrite in the DOZ and the lower IOZ (Fig. 7C). The chalcopyrite:bornite ratio decreases with depth, although chalcopyrite can dominate locally in the DOZ. Bornite commonly occurs as fine-grained intergrowths with clay, calcite, anhydrite, and pyrite in the GBT and the upper IOZ, and with magnetite and anhydrite in the DOZ and the lower IOZ. In addition, bornite_+ anhydrite form veins several cm wide at and near the intrusive contact in DOZ (Fig. 7D,E). Bornite and chalcopyrite also occur in anhydrite veins, with the sulfides filling vein centers, and chalcopyrite-bornite intergrowths are common. Pyrite, common in altered intrusive rock, also is present in exoskarn, whereas marcasite is present as an alteration product of pyrite. Digenite and chalcocite/djurleite are common alteration phases in bornite; covellite is less common, and appears to be an alteration product of chalcopyrite. Idaite, and more rarely, cubanite, are found in small quantities in GBT and IOZ. Covellite and idaite are most abundant in and near breccias and fault zones. Native Au, with average fineness ~860, generally forms inclusions in bornite (Fig. 7F), but locally occurs within quartz veins in the absence of Cu sulfides. Electrum has been identified in a few samples. Argentite has been identified in one sample each from GBT and DOZ. Galena and sphalerite, both Ag-poor, are sparsely disseminated as veinlets in and rims on bornite throughout GBT. Other trace phases in DOZ include native bismuth, bismuthinite, tetradymite, and molybdenite, all occurring as < 10-/xm inclusions in bornite, and two unidentified Bi( ± Ag) tellurides, forming < 5-/xm inclusions in chalcopyrite.

Cl-rich apatite occurs as a minor or trace mineral in several samples. Apatite grains host numerous solid inclusions, including phlogopite, zircon, anhydrite, calcite, Fe oxide, and an Ag-rich phase with minor and varying amounts of Cu and S (Ag-rich stromeyerite?).

3.4. Dom Skarn

The Dom (Dutch for "cathedral" in reference to the appearance of the nearby peak) orebody crops out at 4200 m in elevation and is located 0.5 km south of GBT ( Fig. 2). The orebody is wedge-shaped in cross section and roughly circular in plan view (Fig. 9). Estimated reserves include 31 million tonnes of ore averaging 1.5% Cu and 0.4 g/t Au (Table 1 ). The deposit is surrounded by the Ertsberg intrusion to the north and is fault- bounded on the west, east, and south. The south-bounding fault, the Dora Fault, places barren marble adjacent to skarn, but movement on the fault is uncertain. The east- and west- bounding faults are north-trending and essentially vertical, with right-lateral movement implied by the positioning of marble to the west of the skarn and intrusive rock to the east (Fig. 9).

The skarn is laterally zoned from intrusive rock in the north to monticellite skam, garnet + magnetite skaru (Fig. 10A, B), a "block zone", and a specular hematite zone immediately north of the Dom Fault (Fig. 9). The "block zone" consists of large blocks of Ainod Formation limestone and marble (Fig. 10C) and skarn (dominantly garnet) within a matrix of specular hematite + quartz + magnetite. It is separated from the massive magnetite _+ garnet skarn by a WNW-trending fault that trends through the center of the deposit (Fig. 9), parallel to the Dora Fault. The specular hematite zone immediately adjacent to the Dom Fault consists of coarse hematite + quartz with magnetite + chalcopyrite, and may represent alteration related to fluid movement along the fault zone.

Fossil replacement textures, similar to those of GB, are common within the Dom skarn,

196 H.J. Mertig et al./ Journal of Geochemical Exploration 50 (1994) 179-202

0 50 I O0 m

I ~ - ] Covered ~ Garnet + magnetite, chalcopyrite

r ~ Ertsberg intrusion ~ Magnetite - chalcopyrite

Marble ~ Marble - limestone - skarn block zone

Dolomitic marble ~--~ Specularite - quartz + magnetite, chalcopyrite

Monticellite skarn zx zx z~ Brecciated

Fig. 9. Geologic map of the surface exposures of the Dom skarn. Modified by Mertig (1994) from original 1:2,000 mapping by Freeport Indonesia geologic staff.

implying the same host rock affinity. Fragments of Ainod Formation limestone within the "block zone" contain abundant foraminifera of the same type preserved within the skarn (Figs. 10C, 10D). Chalcopyrite and carbonate preferentially replace foraminifera in magnetite skarn, whereas late-stage garnets, calcite, and chalcopyrite replace foraminifera in


D Gill

Fig. 10. Dom skarn mineralization. (A) View of the surface exposures of the Dom skarn deposit. The bulk of the orebody consists of magnetite-garnet skarn which is the massive dark outcrop in the right half of the photograph. (B) Surface exposure of garnet skarn surrounded by magnetite skarn. (C) Recrystallized fossiliferous limestone of the Ainod Formation showing abundant large foraminifera, Operculina cf. complanata. (D) Foraminifera replaced by late-stage "red" garnet within a finer-grained "green" garnet skarn matrix. The fossil replacement texture preserved in the skarn provides evidence that the Ainod is the host for the bulk of the Dora orebody. (E) Photomicrograph of large foraminifera (coarse blocky calcite) from the Ainod Formation limestone. Transmitted light, plane polarized; long dimension = 9.7 mm. (F) Photomicrograph of replaced foraminifera within garnet skarn. The " red" garnets replacing the foraminifera are generally isotropic with zoning along the rims, whereas the matrix garnets are zoned and birefringent throughout the grains. Transmitted light, crossed polars; long dimension = 9.7 mm. For abbreviations, see Fig. 7 caption.

198 H.,L Mertig et al. / Journal of Geochemical Exploration 50 (1994) 179-202

garnet skarn (Figs. 10E, 1 OF). An Ainod Formation protolith is confirmed by the similarity between the textures of the Ainod Formation limestone and the Dom skarn.

Magnetite and garnet form the bulk of the skarn orebody (Figs. 10A, 10B ). Chalcopyrite is the dominant ore mineral and occurs with both garnet and magnetite, although Cu values tend to be higher in the magnetite-dominant zones. Covellite and chalcocite/digenite occur along rims, fractures, or grain boundaries as alteration products ofchalcopyrite. Chalcopyrite occurs as fine to coarse pore-fillings, as fossil replacement, and in veinlets generally with early retrograde phases.

Several stages of magnetite occur in the Dora, best exemplified within "skarn breccia". The breccia consists of clasts of garnet, garnet + diopside, and magnetite, within a massive magnetite matrix. Clasts commonly are rimmed by altered pyroxene or other calc silicates and magnetite _+ chalcopyrite, quite similar to breccias described from the GB. The majority of the magnetite skarn formed during an early stage, contemporaneous with or subsequent to the bulk of garnet skarn formation. Under reflected light, two types of magnetite are evident: ( 1 ) coarse, euhedral zoned grains, and (2) more massive, granular grains. Chap copyrite occurs with both textural types.

Three stages of garnet have been identified based on hand sample occurrence, optical properties, and composition (Fig. 8 ). The bulk of the garnet skarn is "green" garnet, which is relatively fine-grained, massive, and green to yellowish in color. In transmitted light, the green garnets are concentrically zoned and birefringent. Microprobe analysis indicates that their composition ranges from Ads~o. Late stage, euhedral, " red" garnets occur as coarse vug filling and fossil replacement with calcite + quartz__+ chalcopyrite (Fig. 10D). These tend to have isotropic cores that are extremely iron-rich, averaging Ad98, although the rims of the red garnets are likewise zoned and birefringent. Zoning within the garnets is apparent on microprobe backscatter images and represents variations in the A13 + to Fe 3 + content. The earliest stage of garnet occurs as fragments or clasts within green garnet skarn. They are generally brown in color, isotropic, and form anhedral masses. These early garnets are more Al-rich, roughly Adso (Fig. 8). The progressive increase in ferric iron content of the garnets with time indicates a general increase in the oxidation state during skarn development.

Like the other skarns in the Ertsberg district, diopside (avg. = Di93) and monticellite are among the higher temperature skarn minerals formed at the Dom. The bulk of garnet formed paragenetically later, as it surrounds and partially replaces pyroxene laths. Retrograde alteration of higher temperature calc-silicates is prevalent throughout the deposit. Chlorite, talc, calcite, micas and clays partially or entirely replace garnet and diopside. Fine-grained phlogopite, commonly intergrown with muscovite, occurs locally as a late prograde and/ or an early retrograde phase. Chalcopyrite occurs in veinlets with coarse phlogopite in at least one sample, and chlorite is observed replacing that phlogopite. Interstitial quartz and calcite are ubiquitous in the Dora, suggesting a late lower-temperature hydrothermal event, in contrast to the early, high-temperature quartz that occurs within the altered intrusions.

4. Discussion

Although the three major skarn orebodies of the Ertsberg district are associated with the Ertsberg intrusion, significant differences appear to be related to local conditions. The GB


orebody is surrounded by intrusive rock, whereas the GBT complex and the Dom skarn occur along the periphery of the intrusive complex and are fault-bounded adjacent to marble. The development of the GBT and Dom skarns may have been more influenced by local structural features. However, the Dom skarn more closely resembles the GB deposit in mineralogy and fossil replacement textures. Both appear to be hosted by the Ainod For- mation which occurs along strike to the northwest of the GB orebody and to the southeast of the Dom area (Fig. 2). The GBT complex skarns are hosted by the limestones of the Faumai Formation and the dolomitic carbonates of the Waripi Formation, based on stratigraphic and structural considerations observed in diamond-drill core and surface mapping.

Calc-silicate mineralogy is generally similar for the GB, GBT, and Dom orebodies, although ore mineralogy differs among the deposits. The GB and Dom orebodies are magnetite + calc-silicate skarns, whereas GBT skarn is dominated by the calc-silicate assemblage of forsterite, monticellite, or garnet; the IOZ and DOZ ores are hosted by a magnetite+ calc-silicate assemblage. In addition, the high-temperature phase melilite occurs in both the GB and Dom skarns but not in the GBT complex. Anhydrite is abundant within the GBT complex, particularly in the DOZ, but is rare within the Dom and GB deposits. Chalcopyrite is the dominant ore mineral in the GB, upper GBT complex, and Dom orebodies, whereas bornite generally is the most abundant Cu sulfide in the DOZ. Visible Au has been observed in the GB and the GBT complex orebodies, but is rare in the Dom deposit. A wide zone of specular hematite, adjacent to the Dom Fault, is unique to the Dom orebody, although a NE-trending hematite-rich zone was mapped on the south side of the GB orebody (Fig. 5A). Retrograde alteration is prevalent throughout the Dom. Local endoskarns are associated with all three of the deposits, although clearly subordinate to the ore-bearing exoskarn. Although the Ertsberg pluton commonly is referred to as "barren", in contrast to the Grasberg intrusive complex that hosts the porphyry Cu-Au orebody, this designation is one of degree because thin sulfide-filled fractures, typically Cu-bearing, are common in the Ertsberg pluton.

Sulfide deposition generally is prompted by decreasing temperatures and reflects higher oxidation and sulfidation states relative to earlier alteration (e.g. Einaudi et al., 1981 ). The skarn deposits of the Ertsberg district likewise follow this pattern, in that the ore minerals are relatively late stage and formed during decreased temperatures at the onset of retrograde alteration. The common large-scale intergrowths between magnetite and bornite or chalcopyrite may indicate a redox boundary where Fe is oxidized to form magnetite and sulfate is reduced to allow deposition of Cu-Fe-sulfides.

5. Conclusions

The major skarn orebodies of the Ertsberg district were formed by hydrothermal systems associated with the intrusion of the 2.9-Ma Ertsberg pluton into the Tertiary New Guinea Group. These locally dolomitic, fossiliferous strata were highly reactive protoliths that greatly influenced the evolution of the calc-silicate alteration zones. Although the pre- mineralization relations are obscured by later structural and alteration events, structural features probably also influenced the progressive development of the ore-forming hydrothermal systems.


The three major skarn orebodies are closely associated with the Ertsberg intrusion, occurring along the flanks of or within the pluton. This relationship suggests a genetic association of the skam-forming fluids with the Ertsberg magma. Many of the differences between the orebodies appear to be a function of the stratigraphic position of the skarn host. Large scale deformation resulted in regional and local faulting and brecciation, thus providing pathways for fluid migration through the margin of the pluton and into the surround- ing carbonates. Prograde skarn formation probably increased the bulk permeability of the sedimentary sequence, thus allowing infiltration of metasomatic and ore-forming fluids. Sulfides generally were precipitated after the bulk of prograde skam formation in the Ertsberg district, and probably were contemporaneous with the onset of retrograde alteration that accompanied a temperature decrease.

Acknowledgments

This article represents a progress report on a continuing investigation of the tectonics and mineral resources of Irian Jaya by the University of Texas at Austin and the Institute of Technology in Bandung, Indonesia, sponsored by Freeport Indonesia. We acknowledge the other members of the University of Texas research team, specifically M. Cloos, D. Gonzalez, T. McMahon, A. Quarles, B. Sapiie, and R. Weitand, for their contributions to the under- standing of central Irian Jaya geology. Also, we are grateful for the opportunity to interact with numerous members of Freeport's present and past geological staff who generously shared their hard-won collective geologic knowledge of the Ertsberg district. As acknowl- edged for several illustrations, Freeport's geologists have generated much of the geologic background that formed the base for this research project. Reviews of earlier drafts of the manuscript by M. Cloos, D. Flint, J. Hedenquist, K. Hefton, L. James, G. Katchan, L. Meinert, J. Pennington, J. Price, H. Shimazaki, and S. Van Nort greatly improved the final version. R. Trevino prepared most of the illustrations. This is Ertsberg Project Contribution No. 2.

References

Anon., 1992. Freeport reports mineralization at Big Gossan. Min. Eng., 44: 12-13. Dozy, J.J., 1939. Geological results of the Carstensz expedition 1936. Leidse Geol. Meded., 11: 68-131. Dow, D.B., Robinson, G.P., Hartono, U. and Ratman, N., 1988. Geology of Irian Jaya. Irian Jaya Geological

Mapping Project, Geological Research and Development Centre, Indonesia, in Cooperation with the Bureau of Mineral Resources, Australia, on Behalf of the Department of Mines and Energy, Indonesia, and the Australian Development Assistance Bureau, 298 pp.

Einaudi, M.T., Meinert. L.D. and Newberry, R.J., 1981. Skarn deposits. In: B.J. Skinner (Editor), Seventy-Fifth Ann. Vol., Econ. Geol.: 317-391.

Gonzalez, D., 1994. Geology and mineralization of the Big Gossan area, Ertsberg (Gunung Bijih) district, Irian Jaya, Indonesia. MA thesis, Univ. Texas, Austin (in preparation).

Hamilton, W., 1979. Tectonics of the Indonesian region. U. S. Geol. Surv., Prof. Paper 1078, 345 pp. Hefton, K., 1990. Geologic map of the Big Gossan. Unpubl. 1:1,000 scale map, Freeport Indonesia. Hefton, K. and Pennington, J., 1993. Stratigraphic columnar section, Contract of Work Block "A", P. T. Freeport

Indonesia, Ertsberg district, Irian Jaya. Unpubl. 1:2,000 scale compilation, Freeport Indonesia.

H.J. Merti g et al. / Journal of Geochemical Exploration 50 (1994) 179-202 201

Katchan, G., 1982. Mineralogy and geochemistry of the Ertsberg (Gunung Bijih) and Ertsberg East (Gunung Bijih Timur ) skarns, Irian Jaya, Indonesia and the OK Tedi Skarns, Papua New Guinea. Ph.D. thesis, University of Sydney, 498 pp.

Le Maitre, R.W. (Editor), 1988. A classification of igneous rocks and glossary of terms. Blackwell Scientific Publ., Boston, MA, 193 pp.

MacDonald, G.D. and Arnold, L.C., 1993. Intrusive and mineralization history of the Grasberg deposit, lrian Jaya, Indonesia. Soc. Min. Eng., Preprint 93-92, 10 pp.

MacDonald, G.D. and Arnold, L.C., 1994. Geological and geochemical zoning of the Grasberg Igneous Complex, Irian Jaya, Indonesia. In: T.M. van Leeuwen, J.W. Hedenquist, L.P. James and J.A.S. Dow (Editors), Indo- nesian Mineral Deposits - - Discoveries of the Past 25 Years. J. Geochem. Explor., 50: 143-178.

McDowell, F.W., McMahon, T.P., Warren, P.Q. and Cloos, M., 1994. Pliocene Cu-Au-bearing igneous intrusions of the Gunung Bijih (Ertsberg) district, Irian Jaya, Indonesia: K-Ar geochronology (in preparation ).

McMahon, T.P., 1994. Petrology and geochemistry of the Pliocene intrusions in the Ertsberg (Gunung Bijih) district, Irian Jaya, Indonesia. Ph.D. Dissertation, Univ. Texas, Austin (in preparation).

Martodjojo, S., Sudradjat, D., Subandrio, E. and Lukman, A., 1975. The geology and stratigraphy along the road cut, Tembagapura, Irian Jaya, Volume 1. Unpubl. report, Tech. Inst. Bandung, Indonesia, 51 pp.

Meinert, L.D., 1989. Gold skarn deposits--geology and exploration criteria. In: R.R. Keays, W.R.H. Ramsay and D.I. Groves (Editors), The Geology of Gold Deposits: The Perspective in 1988, Econ. Geol. Monograph 6. Econ. Geol. Publ. Co., pp. 537-552.

Mertig, H.J., 1994. Geology and ore formation of the Dom Cu-Au skarn deposit, Gunung B ijih (Ertsberg) district, lrian Jaya, Indonesia. M.A. Thesis: Univ. Texas, Austin (in preparation).

Nash, C.R., Artmont, G., Gillan, M.L., Lennie, D., O'Connor, G. and Parris, K.B., 1993. Structure of the lrian Jaya Mobile Belt, Irian Jaya, Indonesia. Tectonics, 12:519-535.

Peacock, M.A., 1931. Classification of igneous rocks. J. Geol., 39: 54-67. Pieters, P.E., Pigram, C.J., Trail, D.S., Dow, D.B., Ratman, N. and Sukamto, R., 1984. The stratigraphy of western

Irian Jaya. In: Indonesian Petrol. Assoc., Proceedings, Vol. 1, 10th Annu. Conv., Jakarta, pp. 229-261. Pigram, C.J. and Panggabean, H., 1983. Geological Data Record, Waghete (Yapekpra) - - 1:250,000 Preliminary

Geological Map, Irian Jaya. Indonesian-Australian Geological Mapping Project, Geological Research and Development Centre, Indonesia, 126 pp.

Quarles, A.I., 1994. Structural geology and stratigraphy of the Central Range Fold and Thrust Belt in the vicinity of the Gunung Bijih (Ertsberg) mining district, Irian Jaya, Indonesia. Ph.D. Disssertation, Univ. Texas, Austin ( in preparation).

Quarles, A.I. and Cloos, M., 1994. Initiation of the Central Ranges Orogeny of New Guinea by arc~:ontinent collision at ~ 12 Ma: Constraints from regional sedimentation (in preparation).

Rubin, J.N., 1994. Skarn formation and ore deposition, Gunung Bijih Timur complex, Ertsberg district, lrian Jaya, Indonesia. Ph.D. Dissertation, Univ. Texas, Austin (in preparation).

Sapiie, B., 1994. Deformational history and origin of breccias in the Gunung Bijih (Ertsberg) mining district, Irian Jaya, Indonesia. Ph.D. Dissertation, Univ. Texas, Austin (in preparation).

Soeparman, S. and Budijono, 1989. Cu-skarn deposits in Ertsberg mine area, Irian Jaya, Indonesia. Geol. Indonesia, 12: 359-374.

Theodore, T.G., Orris, G.J., Hammarstrom, J.M. and Bliss, J.D., 1990. Gold-bearing skarns. U. S. Geol. Surv., Bull. 1930, 61 pp.

Van Leeuwen, T., 1994. 25 Years of mineral exploration in Indonesia. In: T.M. van Leeuwen, J.W. Hedenquist, L.P. James and J.A.S. Dow (Editors), Indonesian Mineral Deposits - - Discoveries of the Past 25 Years. J. Geochem. Explor., 50:13-90.

Van Nort, S.D., Atwood, G.W., Collinson, T.B., Flint, D.C. and Potter, D.R., 1991. Geology and mineralization of the Grasberg porphyry copper-gold deposit, Irian Jaya, Indonesia. Min. Engn., 43: 300-303.

Visser, W.A. and Hermes, J.J., 1962. Geological results of the exploration for oil in Netherlands New Guinea. Koninklijk Nederlands Geologisch Mijnbouwkundig Genootschap Verhandelingen, Geologische Serie, Vol. 20, 256 pp.

Weiland, R.J., Jr., 1993. Plio-Pleistocene unroofing of the Irian Fold-and-Thrust Belt south of the Gunung Bijih (Ertsberg) mining district, Irian Jaya, Indonesia: Apatite fission track thermochronology. M.A. Thesis, Univ. Texas, Austin, 84 pp.


Weiland, R.J., Jr. and Cloos, M., 1994. Plio-Pleistocene unroofing of the Irian Fold-and-Thrust Belt south of the Gunung Bijih (Ertsberg) mining district, Irian Jaya, Indonesia: Apatite fission track thermochronology (in preparation ).

Wilson, F., 1981. The Conquest of Copper Mountain. Atheneum, New York, 243 pp.

Skarn Cu-Au Orebodies of the Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia - Mertig 1993

Documents

Transcript of Skarn Cu-Au Orebodies of the Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia - Mertig 1993