Paper Endapan Mineral - Copy

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

1 Naskah 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 Porfr

yang Khas di Kawasan Tambang Ertsberg Papua, Indonesia

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

PT. Freeport Indonesia, Tembagapura, Papua

ABSTRACT

The 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 permitstemporal correlation of porphyry and skarn styles of alteration and mineralization. Differences in style

of alteration and veining between porphyry and endoskarn reect degree of interaction of magmatic

uids 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 signicantly.

Keywords: stockwork zone, porphyry copper, mineralization style, endoskarn alteration, Ertsberg,Papua, Indonesia

ABSTRAK

Zona Stockwork Ertsberg (ESZ) merupakan suatu tipe cebakan Cu-Au yang khas di kawasan penam-

bangan Ertsberg. Zona Stockwork Ertsberg ini bukan cebakan tipe porri 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 porri 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 porri 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 porri dan Intrusi Ertsberg


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Ertsberg Stockwork Zone: A Unique Porphyry Copper Style Mineralization in the Ertsberg

Mining District, Papua, Indonesia (L. Soebari et al .)

3

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, GB T, Dom

Alluvium

Kembelangan

Group

Sirga Fm.

Kais Fm.

Faumai/Waripi Fm.

N G L G

EXPLANATION

J

- K

T e r t i a r y

Q

738000mE734000mE

9 5 5 0 0 0 0 m N

9 5 4 6 0 0 0 m N

9 5 5 0 0 0 0 m 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

G r a s b e

r g F a u l

t

C a r t e n s

z V a l l e y F a u l

t

Y V S

E2F

E 3 F

E 1 F

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


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4

Kembelangan Group

The Kembelangan Group of ~3400 m thick

is largely composed of siliciclastics divided

into four formations: the Middle to UpperJurassic 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 ofincreasing 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 rst dated by

McDowell et al . (1996) at 2.65 to 3.09 Mausing 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 ne- 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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6

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 ratherthan 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 eld, 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 dikesand the Main Ertsberg have been observed.

These shears, in all observed cases, occur

in endoskarn altered contacts, are 3-10 cm

wide, and are lled with nely ground wall

rock. Figure 3 presents a simplied 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 exoskarnof 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 difcult

to map in detail. Phyllic alteration (quartz-

sericite-pyrite) is not widespread in this sys-

tem but is usually conned 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 ofthe porphyry dikes. There are three main

aspects to the potassic alteration of this rock:

1) alteration of mac 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

signicant 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 EESSon 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 (ne-

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 (darkyellow) 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 ne 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 mac 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

nal stage of retrograde uid ow up the

contacts between the porphyry dikes and the

Main Ertsberg would explain the pattern of

this propylitic alteration being conned 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 ne-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 suldesin this mineralization event, but assays of

drill core indicate that this mineralization

event is richer in Au (up to 15 g/t) than

the rst 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 andsealed 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 veinsin clinopyroxene skarn are inlled 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 sulde 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 suldes 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 sulde deposition

was more spread out than in the single

skarn specimen, and extended from quartz

veining to after anhydrite deposition. Thegeneralised sequence of deposition is

shown on Figure 8.

DISCUSSION - NEW DEPOSIT

MODEL FOR ESZ

The ESZ system does not t 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 reect degree of interaction of magmatic uids 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 lled a “fault” orzone of weakness that cut the Main Erts-

berg Intrusion. The fault and the contacts

along the porphyry dikes that later lled 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 nal stages of cooling of

the ESZ hydrothermal system, uids 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 sulde 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)

ErtsbergStocwork Zon(ESZ)

Barren Core or Center Yes No No

Potassic Zone with elevated Cu-Au grades Yes No Yes

Phyllic Zone with decreased Cu-Au grades Yes No No

Phyllic Zone mostly barren of Cu-Au grades Yes No Yes

Argillic Zone Yes Yes No

Intrusive host rock for Cu-Au mineralization Yes No Yes

Stockwork veining Yes No Yes

Supergene enrichment Yes No No

Structural Control Yes Yes Yes

Sulde Zoning No? Yes No

Stratigraphic/Lithologic Control No? Yes Yes?

Sedimentary host for Cu-Au mineralization No Yes No

Anhydrous calc-silicate skarn minerals No Yes Yes

Hydrous calc-silicate skarn minerals Yes? Yes Yes

Retrograde alteration overprinting of progradealteration

Yes? Yes Yes

Mineralization occurs in the nal stages of thehydrothermal 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 signicantly.

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.

“ F

a u l t ”

Sediments

Quartz-AnhydoteChalcopyrite veins

Biotite-BorniteVeiniets mesh

Propyliticalteration

Unaltered Intrusion

Exoskarnalteration

CarbonateSediments

EndoSkarn

Dyke

0 500 m

LS 1012

Dyke

Potassic

alteration

ESZ system is the presence of endoskarnalteration 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 rst 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


14/14


14

postdates the emplacement of the por-

phyry dikes.

5. The highest grades in the ESZ system

are conned 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 gures.

REFERENCES

Allen, J.A, 1997. Porhyry and Endoskarn Au_Cu

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McMahon, T.P., 1994. Pliocene Intrusions in The

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