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Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 1-14

1Naskah diterima: 03 Desember 2012, revisi terakhir: 18 Maret 2013, disetujui: 20 Maret 2013

Ertsberg Stockwork Zone: A Unique Porphyry Copper Style Mineralization in the Ertsberg Mining District, Papua, Indonesia

Zona Stockwork Ertsberg: Mineralisasi Tipe Tembaga Porfir yang Khas di Kawasan Tambang Ertsberg Papua, Indonesia

Lasito Soebari, Iwan Sriyanto, Geoff de Jong, and Ahmad Muntadhim

PT. Freeport Indonesia, Tembagapura, Papua

ABSTRACTThe Ertsberg Stockwork Zone (ESZ) is a unique Cu-Au deposit type in the Ertsberg Mining District. The ESZ is neither a porphyry style deposit nor a skarn deposit, but exhibits characteristics of both deposit types. The ESZ mineralization in the Ertsberg monzodiorite occurs near giant East Ertsberg Skarn System, close to the northern margin of the intrusion. Mineralization is completely enclosed by the “barren” Ertsberg Intrusion and centred about 5 - 15 m porphyritic hornblende dikes that cut the Ertsberg Intrusion. A model was presented in which a hydrothermal system rose through the Ertsberg Intrusion along a “fault” or zone of weakness. The prograde event resulted in a potassic alteration in the centre of the system with a propylitic halo at the periphery. Porphyry dikes then intruded the “fault”. Endoskarn alteration along the margin of these dikes resulted from a continued high temperature hydrothermal alteration was focused along the contacts. Cu and Au were introduced into the system as quartz- anhydrite-pyrite-chalcopyrite veins cut across the dikes and the Main Ertsberg Intrusion. As the system cooled, the contact zones of the porphyry dikes and the Main Ertsberg Intrusion were propyliticaly altered. The change in mineralogy and paragenetic sequence across the transition permits temporal correlation of porphyry and skarn styles of alteration and mineralization. Differences in style of alteration and veining between porphyry and endoskarn reflect degree of interaction of magmatic fluids with Ca-Mg carbonate sediments. Compared with rocks nearby Grasberg deposit, the Ertsberg Stockwork Zone deposit has much weaker development of hydrolytic alteration styles, an absence of breccias in igneous rocks, suggesting the physico-chemical conditions of mineralization for the two deposits differed significantly.Keywords: stockwork zone, porphyry copper, mineralization style, endoskarn alteration, Ertsberg, Papua, Indonesia

ABSTRAKZona Stockwork Ertsberg (ESZ) merupakan suatu tipe cebakan Cu-Au yang khas di kawasan penam-bangan Ertsberg. Zona Stockwork Ertsberg ini bukan cebakan tipe porfiri dan bukan pula skarn, namun memperlihatkan karakteristik gabungan keduanya. Mineralisasi ESZ dalam monzodiorit Ertsberg hadir dekat Sistem Skarn Ertsberg Timur yang besar, dekat ke tepi utara intrusi. Mineralisasi ini seluruhnya ditutupi oleh Intrusi Ertsberg yang “kosong” dan terpusat sekitar retas horenblenda porfiri dengan tebal 5 - 15 m, yang memotong Intrusi Ertsberg. Sebuah model yang memperlihatkan pemunculan sistem hidrotermal melalui Intrusi Ertsberg sepanjang sesar atau zona lemah telah dibuat. Kegiatan “prograde” telah menghasilkan alterasi potasik di pusat sistem dengan halo propilitis pada batas luarnya. Retas porfiri kemudian mengintrusi sesar. Alterasi endoskarn yang hadir sepanjang tepi retas tersebut adalah akibat alterasi hidrotermal suhu tinggi yang terfokus sepanjang kontak. Cu dan Au hadir dalam sistem yang berupa urat-urat kuarsa-anhidrit-pirit-kalkopirit yang memotong retas dan Intrusi Ertsberg utama. Ketika sistem mendingin, zona kontak retas porfiri dan Intrusi Ertsberg


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utama terpropilitkan. Perubahan dalam mineralogi dan runtunan paragenetis melalui transisi dapat dipakai sebagai korelasi temporal alterasi dan mineralisasi tipe porfiri dan skarn. Perbedaan jenis alterasi dan penguratan antara porfiri dan skarn memperlihatkan tingkatan interaksi cairan magma dengan sedimen karbonat C-Mg. Dibandingkan dengan batuan dekat cebakan Grasberg, tipe alterasi hidrolitik cebakan Zona Stockwork Ertsberg yang perkembangannya lebih lemah, dan tak adanya breksi dalam batuan beku, memberikan kesan adanya perbedaan kondisi fisika-kimiawi yang signifikan dalam proses mineralisasi kedua cebakan.Kata kunci: zona stockwork, tembaga porfir, jenis mineralisasi, alterasi endoskarn, Ertsberg, Papua, Indonesia

INTRODUCTION

The Ertsberg Stockwork Zone (ESZ) is lo-cated near the crest of the Central Range of Papua, Indonesia within the Ertsberg Mining District (P.T. Freeport Indonesia’s Contract of Work “A”) (Pennington, 1993). The ESZ deposit is hosted entirely within the Ertsberg Intrusion approximately 200 m south of the Ertsberg East Skarn System (EESS) (Coutts et al., 1999) which lies along the contact between the Ertsberg Intrusion and the host sediments. The EESS is also known in the literature by the names GBT, IOZ, and DOZ, which are the names of the various under-ground mines that exploit the metals hosted in this huge contact skarn system.

Intensive exploration activities on the ESZ commenced in early 2000 and by the end of 2000 Freeport announced an ESZ reserve of 101 million tonnes at 0.55% Cu and 0.80 g/t Au. The year 2001 exploration program resulted in additional reserves added to the ESZ. Currently ESZ deposit is being block cave mined as part of DOZ mine integration, which produce 80 k ton ore per day.

The ESZ ore body is oriented NW-SE and occurs over a strike length of 650 m. On average, the ESZ resource is 300 m in width with Cu-Au mineralization occurring between the 3150 and 3700 m levels (at its shallowest point, the ESZ is ~200 m beneath the surface). The ore body narrows to the southeast, where mineralization occurs be-

tween the 3200 and 3500 m levels. The ESZ Cu-Au mineralization remains open to the northwest and PTFI plans to drill test this area in the near future. Barren igneous rock of the Ertsberg Intrusion encloses the ESZ orebody on all sides and above.

The purpose of this paper is to describe the key geological characteristics of the ESZ and to put forth a new deposit model for this unique type of ore body in the Erts-berg Mining District.

REGIONAL GEOLOGIC SETTING

Papua lies on the northern edge of the Aus-tralian Plate. Presently, the Sorong-Yapen Fault Zone forms a transform plate boundary between the Australian Plate and the Pacific Plate. Prior to~4 Ma the Australian Plate was subducting beneath the Pacific Plate. This process culminated in the formation of a fold-and-thrust mountain belt termed the Central Range, which reaches heights approaching 5000 m above sea level. Pro-terozoic through Late Tertiary rocks form the Central Range stratigraphy. Within the Ertsberg District, Mesozoic through Late Tertiary age sedimentary rocks are exposed. These belong, respectively, to the mostly siliciclastic Kembelangan Group and the mostly carbonate New Guinea Limestone Group. Some glacial deposits locally overlie the bedrock at a high elevation.

Ertsberg Stockwork Zone: A Unique Porphyry Copper Style Mineralization in the Ertsberg Mining District, Papua, Indonesia (L. Soebari et al.)

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Structures

Two principal styles of deformation have accommodated the fold-and-thrust belt related shortening across the Erstberg Min-ing District. Km-scale folding is the most obvious mechanism of the two. Folds tend to strike 290 - 1100 across the District and the most impressive example of such fold-ing is the Yellow Valley Syncline. Parallel to the km-scale folds are NW-SE striking reverse faults, some of which have km-scale offsets (e.g., the Wanagon Fault and the Idenberg #2 Fault). Crossing these struc-tures are NE-SW striking strike-slip faults (e.g. the Grasberg Fault and the Carstensz Valley Fault) with left-lateral offsets up to a few hundred meters, but typically with

less than that (a few meters offset is more common). The largest intrusions in the Grasberg and Ertsberg Districts, have been emplaced where NW-SE reverse faults and NE-SW strike-slip faults intersect (Figure 1). Mineralization in the Ertsberg District is probably also controlled by these fault intersections.

Stratigraphy

The sedimentary stratigraphy of the Ertsberg District is broadly divided into two groups: the Mesozoic Kembelangan Group and the Tertiary New Guinea Limestone Group. Quaternary deposits are limited to glacial till, alluvium, alpine peat, and some land-slide deposits (colluvium).

Others

Grasberg (GIC)

Ertsberg (E)

Intrusion

SkarnBG, GB, GBT, Dom

Alluvium

KembelanganGroup

Sirga Fm.

Kais Fm.

Faumai/Waripi Fm.

NG

LG

EXPLANATION

J-K

Ter

tiar

yQ

738000mE734000mE

9550

000m

N95

4600

0mN

9550

000m

N

738000mE

COWA

Pacific Ocean

ArafuraSea

4 S

8 S

o

o

136 So

ESZ

GB GBTE

DOM

GIC

BG

W

NG

E1F

E2F

E3F

WGF

YVS

GG

GB

GBT

GBTA

= North Grasberg Intrusion

= Ertsberg No. 1 Fault



= Wanagon fault

= Yellow Valley Syncline

= Bigosan

= Gunung Bijih

= Gunung Bijih Timur

= Gunung Bijih Timur Atas

Project Location

NG

Grasberg Fault

Cartensz Valley Fault

YVS

E2F

E3F

E1F

Figure 1. Project location and geological map of Ertsberg District (Source: PTFI internal report).


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Kembelangan Group

The Kembelangan Group of ~3400 m thick is largely composed of siliciclastics divided into four formations: the Middle to Upper Jurassic Kopai Formation, the Upper Juras-sic to Lower Cretaceous Woniwogi Forma-tion, the Lower to Middle Cretaceous Piniya Formation, and the Upper Cretaceous Ekmai Formation. The Ekmai Formation is divided into three members, from lower to upper are Sandstone Member, Limestone Member, and 3 - 4 m thick Shale Member.

New Guinea Limestone Group

The New Guinea Limestone Group having thickness of ~1700 m consists largely of carbonates. The group is divided into four formations, those are the Paleocene Waripi Formation, the Eocene Faumai Formation, the Oligocene Sirga Formation, and the Upper Oligocene to Middle Miocene Kais Formation. The Kais Formation is divided into four members informally referred to as “Tk1”, “Tk2”, “Tk3, and “Tk4”.

Intrusive Units

All intrusions described in the Ertsberg District are potassium rich, so they are commonly referred to as “alkalic”. These rocks tend to be described as monzodio-rites, quartz monzodiorites, monzonites, trachyandesites, etc. There appears to be a progression through space and time of increasing size of intrusive events in the Ertsberg District. Older intrusions (on the order of 4 - 5 Ma?) such as the South Wanagon Suite and the Utikinogon Suite are small (meters to hundreds of meters in surface exposure size) sills on the south side of the District and its surroundings, whereas the younger intrusions like Gras-berg and Ertsberg (2.6 - 3.5 Ma) (Mc Mohan, 1994) are large stocks (kilometer scale in exposure size) and occur further

to the north. Other intrusions (such as Kay, Idenberg, and Lembah Tembaga), probably of intermediate age (3 - 4 Ma?), are more plug-like in their shape and are hundreds of meters across in maximum size. This paper focuses on mineralization hosted entirely within the youngest, largest intrusion in the District, the Ertsberg Intrusion.

The Ertsberg Intrusion

The intrusion is situated on the south limb of the Yellow Valley Syncline. The age of the Ertsberg Intrusion was first dated by McDowell et al. (1996) at 2.65 to 3.09 Ma using conventional K-Ar techniques. Using the 40Ar-39Ar technique, Pollard and Taylor (2001) dated a sample of the equigranular Main Ertsberg Intrusion at 2.66 ± 0.03 Ma (Pollard and Taylor, 2001).

There are at least two main intrusive events of similar monzodioritic composition that occur in the Ertsberg Intrusion: (1) an early volumetrically dominant equigranu-lar medium-grained phase, and (2) a later porphyritic fine- to medium-grained phase of meter-scale dikes that are related to min-eralization at the ESZ.

The “Main Ertsberg”

The equigranular part of the Ertsberg Intru-sion is informally referred to as the “Main Ertsberg” and this term will be used for the remainder of this report. It comprises >95% of the volume of the mapped Ertsberg intru-sion. The main mineralogy of this rock type is plagioclase (42 - 52%), clinopyroxene (30 - 35%), hornblende (5%), and potas-sium feldspar (3 - 5%). The largest grains in a typical sample of Main Ertsberg rock are 1 - 3 mm in diameter (Figure 2a). Primary biotite may locally comprise up to 5% of the total rock volume, but no clear pattern of distribution of primary biotite in this rock type has been described.


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The “Porphyry Dikes”

The volumetrically minor (<5% of the volume of the mapped Ertsberg Intrusion) porphyritic fine- to medium-grained meter-scale dikes are informally referred to as the “porphyry dikes”. The porphyry dikes tend to be significantly richer in horn-

blende compared to the Main Ertsberg rock type. The key characteristic of this rock type is elongate hornblende pheno-crysts (up to 3 mm long) set in an aphanitic groundmass of mostly plagioclase. Contact relationships between these late dikes and the Main Ertsberg are, in some underground drift exposures and drill cores, sharp and

Figure 2. Photographs of (a) Main Ertsberg equigranular monzodiorite, (b) “Porphyry dike” richer in hornblende compared to the Main Ertsberg rock type. (c) Pale grey dioritic porphyry enveloped by brown garnet-clinopyroxene endoskarn with ghosted porphyritic texture. (d) Copper sulfides (chalcopyrite, born-ite) occur in vein quartz and disseminated in adjacent porphyry. Different generations of quartz veining are apparent: early veins with sparse sulfides oriented parallel to the core axis, are cut at a high angle by a later black vein core.

a c

b

d


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clear. The main mineralogy of these dikes consists of plagioclase phenocrysts (50 - 55%), hornblende (12 - 15%), clinopyrox-ene (5 - 7%), and plagioclase groundmass (35%). Again, biotite is a local accessory mineral that comprises a maximum ~5% of the rock volume present. Sphene (titanite) is a common, but conspicuous, accessory mineral that comprises much less than 1% of the rock volume (Figure 2b). The occurrence of the porphyry dikes, as presently mapped, has a maximum strike length of 600 m. The dikes are generally in the range of 1 - 20 m wide, but north of the ESZ, where these dikes are hosted by skarned sediments rather than the Main Ertsberg Intrusion, they are up to 100 m wide. Drilling and surface map-ping have established that these dikes occur as far down as the 2590 m and as high up as 3800 m level (exposed at the surface). These dikes are generally aligned with the regional structural grain of the Central Range, but at

lower levels the strike of the dikes is 290 - 300º, whereas at higher levels the strike of the dikes is in the range 300 - 310º.

In the field, the contact between the Main Ertsberg rock type and porphyry dikes is characterized by a color change from dark gray to white or light gray (compare Figures 2a and b). Alteration typically overprints the contact so although this color change is locally sharp it may also be blurred by en-doskarn and propylitic alteration. At several locations on the surface, and also at a few lo-cations along underground workings, brittle sheared contacts between the porphyry dikes and the Main Ertsberg have been observed. These shears, in all observed cases, occur in endoskarn altered contacts, are 3-10 cm wide, and are filled with finely ground wall rock. Figure 3 presents a simplified level plan geologic map at 3126 m showing the spatial relationship of the Main Ertsberg (Te1

Figure 3. Level Plan at 3126 meters showing the geology of the ESZ. Note the spatial relationship the exoskarn of the East Ertsberg Skarn System (EESS) to the Erstberg Stockwork Zone (ESZ) (Source: PTFI internal report).


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Ertsberg Monzodiorite) and the porphyry dikes (Te3 Ertsberg porphyry) of the ESZ, the skarn of the EESS, and the marblelized host rocks outside the Ertsberg Intrusion. Figure 4 shows a typical geologic cross-section through the ESZ and surroundings.

ALTERATION OF THE ESZ

This section focuses on the alteration of the Ertsberg Stockwork Zone, as opposed to the EESS skarn alteration that mostly lies to the northeast of the ESZ along the contact of the Ertsberg Intrusion with the host sediments.

Four main stages of alteration characterizing the ESZ are: (1) Potassic Alteration, (2) Endoskarn Alteration, (3) Quartz-Anhy-drite-Pyrite-Chalcopyrite Veining, and (4) Propylitic Alteration (Figures 4 and 5). The

boundaries between these different altera-tion types are quite irregular and difficult to map in detail. Phyllic alteration (quartz-sericite-pyrite) is not widespread in this sys-tem but is usually confined to very narrow (cm-scale) zone along fractures. However, at one location (on the northwest side of the system) there is a 20 m wide occurrence of this phyllic alteration type.

Potassic Alteration

In the ESZ, the potassic alteration event only affected the Main Ertsberg rock type and probably predates the emplacement of the porphyry dikes. There are three main aspects to the potassic alteration of this rock: 1) alteration of mafic minerals to biotite-

actinolite,2) an irregular stockwork of hairline black

biotite-bornite±magnetite veinlets, and;

Figure 4. Typical cross-section through Ertsberg Stockwork Zone looking northwest (Sorce: PTFI internal report).


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3) quartz plus bornite veinlets with no anhy-drite. Potassium feldspar alteration is not a significant aspect of the potassic alteration event at the ESZ. There is a roughly cy-lindrical distribution of potassic alteration, but its shape in level plan in the range of 3000 - 3500 m is slightly ellipsoidal with a long axis of at least 500 m (unconstrained) and a short axis of ~250 m.

Endoskarn

Endoskarn alteration occurs at the contacts between Main Ertsberg and the porphyry dikes. This alteration occurs in both rock types. Endoskarn alteration of the Main Ertsberg is characterized by phlogopite, green diopside, tremolite, garnet, and some magnetite. This alteration of the Main Erts-berg is most intense in the lower parts of the ESZ system near the contact with the skarned sedimentary host rocks of the EESS on the north side of the Main Ertsberg Intru-

sion. Endoskarn alteration of the porphyry dikes is characterized by brown garnet, clinopyroxene, and epidote (Figure 2c). The endoskarn alteration generally destroys the texture of the Main Ertsberg rock type, but it may either enhance or destroy the texture of the porphyry dikes depending on the intensity of the alteration. Moderately intense endoskarn alteration enhances the porphyry dike rock texture by altering the groundmass to fine garnets and altering the hornblende phenocrysts to chlorite and epidote while retaining the euhedral shape of the hornblende. Very intense endoskarn alteration obliterates porphyritic texture of the dikes by altering the entire rock mass to garnet and clinopyroxene. Tremolite is the main retrograde alteration product of clinopyroxene endoskarn. Colour in thin section ranges from pale green to colourless (Figures 6a and 6b). Tremolite is developed along the margins of quartz veins and in

Figure 5. Level Plan at 3500 meters showing the alteration patterns of the ESZ. The quartz-anhydrite- pyrite-chalcopyrite veins are not shown at this scale (Source: PTFI internal report).


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crosscutting fractures and around cavities. It replaces pseudomorphs pyroxene, and also grows into interstitial open space.

Quartz-Anhydrite-Pyrite-Chalcopyrite Veining

Planar quartz, anhydrite, pyrite, plus chal-copyrite veins crosscut the potassic and en-doskarn alteration. These veins occur in the Main Ertsberg and the porphyry dike rock types. Locally these veins can be observed in underground drifts to be nearly 100% chalcopyrite grading to nearly 100% anhy-drite over a length of ~5 m. These veins are widespread, but there is a marked increase in intensity of quartz-anhydrite-pyrite-chalcopyrite veins within 50 m or so of the porphyry dikes. These quartz bearing veins are distinguishable from the quartz veins introduced during the potassic alteration event by (1) their greater widths (cm-scale rather than mm-scale), (2) the presence of sericite selvages that may extend millimeters to centimeters from the vein boundary, (3) substantially more pyrite, (4) the general lack of bornite, and (5) the presence of an-hydrite.

An important aspect of the quartz-anhydrite- pyrite-chalcopyrite veins is that above the 3500 m level where the anhydrite has been leached away by groundwater leaving bad ground conditions for mining. At the inter-face between the leached zone and the still massive intrusive rock, groundwater tends to pool, creating a hazard for mining beneath this interface. Dewatering drill programs performed in the last two years in support of the adjacent IOZ block cave mine have been very effective for solving this groundwater pooling problem.

Epidote-Chlorite-Carbonate (Propylitic) Alteration

Propylitic alteration in the ESZ consists of epidote, chlorite, and carbonate (fine-grained calcite). There are two main spatial occurrences of the propylitic alteration: (1) within the Main Ertsberg rock type at the periphery of the ESZ system outside the outer edge of the potassic alteration zone and (2) in the centre of the ESZ system at the outer edges of the porphyry dikes. These two occurrences were probably formed at different times: the propylitic alteration at

a b

Figure 6. Photomicrographs of (a) Quartz veined clinopyroxene endoskarn. A vein of granular quartz (clear) cuts tremolite-altered clinopyroxene endoskarn (at right). Overgrowing quartz vein at left are magnetite (dark yellow) and pale green tremolite, sealed with late anhydrite (clear, with cleavage). (b) Clinopyroxene endoskarn partially altered to tremolite, surrounding a plagioclase grain (grey-white). Plagioclase is strongly altered to very fine sericite.


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the periphery of the ESZ being coeval with the potassic alteration event (i.e. part of the prograde alteration) and the propylitic alteration of the dikes resulting from the retrograde cooling as the ESZ hydrother-mal system was dying away.

In the Main Ertsberg rock type, the propy-litic alteration overprints earlier potassic alteration and has resulted in the conver-sion of the secondary biotite to chlorite plus actinolite. This zone of propylitic alteration forms an irregular ring around the periphery of the ESZ hydrothermal sys-tem. Inward migration of thermal-chemical boundaries as the prograde hydrothermal alteration event contracting would explain this relationship of propylitic alteration overprinting potassic alteration at the pe-riphery of the system.

In the porphyry dikes, the propylitic al-teration is texturally destructive and has resulted in the conversion of the mafic phe-nocrysts to chlorite and the groundmass to chlorite+epidote+calcite. Propylitic altera-tion of the porphyry dikes tends not to be present at the core of the widest dikes. The final stage of retrograde fluid flow up the contacts between the porphyry dikes and the Main Ertsberg would explain the pattern of this propylitic alteration being confined to the centre of the ESZ hydrothermal system along these contacts.

MINERALIZATION

There are basically two modes of occurrence of Cu-Au mineralization in the ESZ. The earliest phase of mineralization is hosted by the potassically altered Main Ertsberg rock type. This phase of mineralization brought Cu into the system in the form of a stockwork of black biotite-bornite veinlets with sporadic fine-grained chalcopyrite and quartz-bornite veinlets. Petrography done

by Allen (1997) shows that Au grains on the scale of ~100 - 200 microns occurring in welded chalcopyrite grains are in con-tact with bornite grains inside the bornite veinlets (Figure 7). The second phase of mineralization is the most obvious one to the casual observer of exposures in underground workings. This phase of mineralization is the quartz-anhydrite-pyrite- chalcopyrite veining event discussed above. Cu and Au were brought into the ESZ system in this event by depositing chalcopyrite, pyrite, and rare bornite into veins with quartz and anhydrite. It is unclear whether the Au is hosted as inclusions in quartz or in sulfides in this mineralization event, but assays of drill core indicate that this mineralization event is richer in Au (up to 15 g/t) than the first mineralization event (1 - 2 g/t is a typical high Au assay in the potassic altered Main Ertsberg rock type).

Cu-Au mineralization dies away slowly from the center of the ESZ system, where the porphyry dikes are located, to the outer edge of the potassic alteration zone. A typical high grade zone in the center of the system near the porphyry dikes assays at about 0.8% Cu and 0.7 g/t Au. A typical high grade zone in the outer parts of the potassic

Figure 7. Photomicrograph of quartz veined. Bornite-chalcopyrite intergrowth interstitial to quartz and sealed with anhydrite.


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zone assays at about 0.2% Cu and 0.3 g/t Au. The highest Cu-Au grades in the ESZ system typically occur over 1 - 5 m zones at the contacts between the porphyry dikes and the Main Ertsberg. Above 3600 m, all the way to the surface there is no mineralization above the upper periphery of the ESZ; the alteration is propylitic, rather than potassic at these levels in the system.

Mineralization Paragenesis

The veining and mineralization sequence in Ertsberg Stockwork Zone is studied by Allen (1997). The sequence; quartz veins in clinopyroxene skarn are infilled by mag-netite and tremolite overgrown by bornite-chalcopyrite intergrowths and sealed with anhydrite (Figure 6a). The adjacent wall-rock is pervasively retrogressed to tremo-lite; magnetite in this zone is locally over-grown by bornite-chalcopyrite- digenite intergrowths with rare inclusions of gold. Gold occurs only in bornite, suggesting it

may have been an original component of a high temperature copper sulfide poly-morph, and was partitioned into bornite on breakdown to bornite+chalcopyrite (Figure 7). It is notable that in this skarn sample, gold mineralisation occurs only within copper sulfides that overlap with retrograde amphibole; it postdates quartz veining and predates anhydrite. There is evidence in that quartz veining and miner-alization form a repetitive sequence. There is further evidence that sulfide deposition was more spread out than in the single skarn specimen, and extended from quartz veining to after anhydrite deposition. The generalised sequence of deposition is shown on Figure 8.

DISCUSSION - NEW DEPOSIT MODEL FOR ESZ

The ESZ system does not fit the conven-tional Cu-Au porphyry deposit model or a typical Cu-Au skarn deposit model, but it

Figure 8. The change in mineralogy and paragenetic sequence across the transition permits temporal correlation of porphyry and skarn styles of alteration and mineralization. Differences in style of alteration and veining be-tween porphyry and endoskarn reflect degree of interaction of magmatic fluids with Ca-Mg carbonate sediments.

-I- -II- -III-

Skarn: paragenetic sequence of mineralisation:

Porphiry: paragenetic sequence of mineralisation:

Clinopyroxene endoskarn

Quartz

Quartz veining

Magnetite

Magnetite

Biotite

Tremolite, retrograde

Tellurides

Bomite-chalcopyrite-digenite

Bomite-chalcopyrite-digenite

Gold

Gold

Anhydrite

Anhydrite


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contains elements of both deposit types. A comparison of the ESZ with Grasberg and the EESS systems is summarized in Table 1.

A unique Model for ESZ

The ESZ is a discrete Cu-Au bearing hy-drothermal system centered about late por-phyry dikes inside a large stock (the Main Ertsberg Intrusion) that is mostly unaltered and unmineralized laterally and vertically away from and above the ESZ. The distri-bution and alignment of the porphyry dikes along with shearing observed at their edges suggests that the dikes filled a “fault” or zone of weakness that cut the Main Erts-berg Intrusion. The fault and the contacts along the porphyry dikes that later filled the fault acted as conduits for the hydrothermal system of the ESZ. The potassic alteration and its associated peripheral propylitic halo

predated the intrusion of the late porphyry dikes (they are not potassically altered) and brought Cu and Au into the system. After the intrusion of the dikes, continued hydrothermal activity caused endoskarn al-teration of both the Main Ertsberg rock type and the porphyry dikes. Quartz-anhydrite-pyrite-chalcopyrite veins then cut across the entire system, again introducing Cu and Au. During the final stages of cooling of the ESZ hydrothermal system, fluids were focused along the contacts of the porphyry dikes causing propylitic alteration of the Main Ertsberg and porphyry dike rock types only within several meters of the contacts (Figure 9).

Two sulfide bearing vein events confer a “stockwork” aspect to this deposit. Black biotite- bornite veinlets form a 20-30 cm-scale mesh within the potassic altered

Table 1. Characteristic Comparison of ESZ with Porphyry (Grasberg) and Skarn Systems (EESS) in the Ertsberg District

Characteristics of DepositsPorphyry

Cu-AuSystem

(Grasberg)

Cu-AuSkarnSystem(EESS)

ErtsbergStocworkZon(ESZ)

Barren Core or Center Yes No NoPotassic Zone with elevated Cu-Au grades Yes No YesPhyllic Zone with decreased Cu-Au grades Yes No NoPhyllic Zone mostly barren of Cu-Au grades Yes No YesArgillic Zone Yes Yes NoIntrusive host rock for Cu-Au mineralization Yes No YesStockwork veining Yes No YesSupergene enrichment Yes No NoStructural Control Yes Yes YesSulfide Zoning No? Yes NoStratigraphic/Lithologic Control No? Yes Yes?Sedimentary host for Cu-Au mineralization No Yes NoAnhydrous calc-silicate skarn minerals No Yes YesHydrous calc-silicate skarn minerals Yes? Yes YesRetrograde alteration overprinting of prograde alteration Yes? Yes Yes

Mineralization occurs in the final stages of the hydrothermal event Yes Yes No


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Main Ertsberg rock type. Quartz-anhydrite-pyrite-chalcopyrite veins occur in all orien-tations but tend spaced at the 1 - 5 m scale and crosscut both the Main Ertsberg and porphyry dike rock types. Compared with rocks nearby Grasberg deposit, the Ertsberg Stockwork Zone deposit has much weaker development of hydrolytic alteration styles, an absence of breccias in igneous rocks, suggesting the physiochemical conditions of mineralization for the two deposits dif-fered significantly.

CONCLUSIONS

1. The ESZ has similarities and differ-ences to both Grasberg and EESS, but the ESZ is a discretely different Cu-Au deposit type in the Ertsberg District, so a unique deposit model is presented here to describe it. A unique aspect to the

Figure 9. Summary cross-section view illustrating the main aspects of the ESZ deposit model.

“Fault”Sediments

Quartz-AnhydoteChalcopyrite veins

Biotite-BorniteVeiniets mesh

Propyliticalteration

Unaltered Intrusion

Exoskarnalteration

CarbonateSediments

EndoSkarn

Dyke

0 500 m

LS 1012

Dyke

Potassicalteration

ESZ system is the presence of endoskarn alteration in the center of the system. The endoskarn alteration in the Main Ertsberg rock type (and in the porphyry dikes) is spatially associated with the porphyry dikes.

2. Mineralization and associated hydro-thermal alteration in the ESZ is hosted and enclosed by a large stock (the Main Ertsberg Intrusion) that is barren on all sides and above the ESZ.

3. Late porphyry dikes that cut through the Main Ertsberg Intrusion are spatially associated with the center of the ESZ hydrothermal system.

4. Mineralization in the ESZ occurs in two stages: the first stage is associ-ated with the potassic alteration zone which probably predates the porphyry dikes, and the later mineralization stage is part of a quartz- anhydrite-pyrite-chalcopyrite veining event which clearly


14

postdates the emplacement of the por-phyry dikes.

5. The highest grades in the ESZ system are confined to within a few meters of the porphyry dikes.

ACKNOWLEDGMENTS

The authors would like to acknowledge the support and backing of the management of PT. Freeport Indonesia Company who permitted this paper to be published and presented to MGEI, Banda and East Sunda seminar 2012. Special mention is given to PTFI management who granted permission to write this paper. Additional thanks are given to Hans Manuhutu for drafting the figures.

REFERENCES

Allen, J.A, 1997. Porhyry and Endoskarn Au_Cu Mineralization in the DOZ, Ertsberg Diorite, Irian Jaya.

Coutts, B.P., Susanto, H., Belluz, N., Flint, D., and Edwards, A., 1999. Geology Deep Ore Zone, Erts-

berg East Skarn System, Irian Jaya. PT. Freeport Indonesia, Tembagapura, Irian Jaya. The 28th IAGI Annual Convention, Jakarta.

McDowell., F.W., McMahon, T.P., Warren, P.Q., and Cloos, M., 1996. Pliocene Cu-Au-Bearing Igneous Intrusions of The Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia: K-Ar Geochronology. Journal of Geology, 104, p. 327-340

McMahon, T.P., 1994. Pliocene Intrusions in The Ertsberg (Gunung Bijih) Mining District, Irian Jaya, Indonesia: Petrography, Geochemistry, and Tectonic Setting. Ph.D. dissertation, University of Texas at Austin.

Pennington, J.B., 1993. COW “A” Exploration Tembagapura, Exploration of The Ertsberg Intru-sion, Ertsberg Mineral District, Irian Jaya-Indonesia. Report for PT. Freeport Indonesia Company.

Pollard, P..J. and Taylor, R.G, 2001. 40Ar-39Ar dating of Intrusive and Hydrothermal Event in The Ertsberg District, Irian Jaya, Indonesia. Report for PT. Free-port Indonesia Co., Pollard and Taylor, Geological Services Pty. Ltd. C/- School of Earth Sciences, James Cook University.

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