Ertsberg Stockwork Zone: A Unique Porphyry Copper Style

Transcription

Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 1-14
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
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 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
ABSTRAK
Zona Stockwork Ertsberg (ESZ) merupakan suatu tipe cebakan Cu-Au yang khas di kawasan penambangan 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
Naskah diterima: 03 Desember 2012, revisi terakhir: 18 Maret 2013, disetujui: 20 Maret 2013
1
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 located 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 underground 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 be2
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 Ertsberg Mining District.
REGIONAL GEOLOGIC SETTING
Papua lies on the northern edge of the Australian 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. Proterozoic 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.)
Structures
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.
Two principal styles of deformation have
accommodated the fold-and-thrust belt
related shortening across the Erstberg Mining 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 folding 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 structures 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
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 landslide deposits (colluvium).
EXPLANATION
738000mE
Q
NG
rg F
Ca
Ertsberg (E)
YVS
W
BG
GB
E
GBT
o
4 S
Faumai/
Waripi Fm.
E2F
E3
F
9546000mN
COWA
Arafura
Sea
Others
Sirga Fm.
F
Paciﬁc Ocean
o
Grasberg (GIC)
Kais Fm.
E1
DOM
8 S
Intrusion
J-K
9550000mN
GIC
t
aul
yF
le
Val
Tertiary
sz
rten
Alluvium
Skarn
BG, GB, GBT, Dom
9550000mN
sbe
Gra
t
aul
ESZ
o
136 S
738000mE
NGLG
734000mE
Kembelangan
Group
NG = North Grasberg Intrusion
E1F = Ertsberg No. 1 Fault
WGF = Wanagon fault
YVS = Yellow Valley Syncline
GG = Bigosan
GB = Gunung Bijih
GBT = Gunung Bijih Timur
GBTA = Gunung Bijih Timur Atas
Project Location
Figure 1. Project location and geological map of Ertsberg District (Source: PTFI internal report).
3
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 Jurassic to Lower Cretaceous Woniwogi Formation, 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 monzodiorites, 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 Grasberg and Ertsberg (2.6 - 3.5 Ma) (Mc
Mohan, 1994) are large stocks (kilometer
scale in exposure size) and occur further
4
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 equigranular medium-grained phase, and (2) a later
porphyritic fine- to medium-grained phase
of meter-scale dikes that are related to mineralization at the ESZ.
The “Main Ertsberg”
The equigranular part of the Ertsberg Intrusion 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 intrusion. The main mineralogy of this rock type
is plagioclase (42 - 52%), clinopyroxene
(30 - 35%), hornblende (5%), and potassium 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.
a
c
b
d
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, bornite) 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.
The “Porphyry Dikes”
The volumetrically minor (<5% of the
volume of the mapped Ertsberg Intrusion)
porphyritic fine- to medium-grained meterscale 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 phenocrysts (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
5
clear. The main mineralogy of these dikes
consists of plagioclase phenocrysts (50 55%), hornblende (12 - 15%), clinopyroxene (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 mapping 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 endoskarn and propylitic alteration. At several
locations on the surface, and also at a few locations 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).
6
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 crosssection 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-Anhydrite-Pyrite-Chalcopyrite Veining, and (4)
Propylitic Alteration (Figures 4 and 5). The
boundaries between these different alteration types are quite irregular and difficult
to map in detail. Phyllic alteration (quartzsericite-pyrite) is not widespread in this system 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 biotiteactinolite,
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).
7
Figure 5. Level Plan at 3500 meters showing the alteration patterns of the ESZ. The quartz-anhydrite- pyritechalcopyrite veins are not shown at this scale (Source: PTFI internal report).
3) quartz plus bornite veinlets with no anhydrite. Potassium feldspar alteration is not a
significant aspect of the potassic alteration
event at the ESZ. There is a roughly cylindrical 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 Ertsberg 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 Intru8
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
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.
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 chalcopyrite veins crosscut the potassic and endoskarn 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% anhydrite over a length of ~5 m. These veins are
widespread, but there is a marked increase
in intensity of quartz-anhydrite-pyritechalcopyrite 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 anhydrite.
An important aspect of the quartz-anhydritepyrite-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 interface 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 (finegrained 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
9
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 hydrothermal system was dying away.
In the Main Ertsberg rock type, the propylitic alteration overprints earlier potassic
alteration and has resulted in the conversion of the secondary biotite to chlorite
plus actinolite. This zone of propylitic
alteration forms an irregular ring around
the periphery of the ESZ hydrothermal system. 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 periphery of the system.
In the porphyry dikes, the propylitic alteration is texturally destructive and has
resulted in the conversion of the mafic phenocrysts to chlorite and the groundmass to
chlorite+epidote+calcite. Propylitic alteration 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.
by Allen (1997) shows that Au grains on
the scale of ~100 - 200 microns occurring
in welded chalcopyrite grains are in contact 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-pyritechalcopyrite
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
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
10
Figure 7. Photomicrograph of quartz veined. Bornitechalcopyrite intergrowth interstitial to quartz and
sealed with anhydrite.
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 magnetite and tremolite overgrown by bornitechalcopyrite intergrowths and sealed with
anhydrite (Figure 6a). The adjacent wallrock is pervasively retrogressed to tremolite; magnetite in this zone is locally overgrown 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 polymorph, 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 mineralization 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 conventional Cu-Au porphyry deposit model or a
typical Cu-Au skarn deposit model, but it
Skarn: paragenetic sequence of mineralisation:
Clinopyroxene endoskarn
Quartz veining
Magnetite
Tremolite, retrograde
Bomite-chalcopyrite-digenite
Gold
Anhydrite
Porphiry: paragenetic sequence of mineralisation:
Quartz
Magnetite
Biotite
Tellurides
Bomite-chalcopyrite-digenite
Gold
-I-
-II- -III-
Anhydrite
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 between porphyry and endoskarn reflect degree of interaction of magmatic fluids with Ca-Mg carbonate sediments.
11
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 hydrothermal system centered about late porphyry 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 distribution 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 Ertsberg 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 alteration of both the Main Ertsberg rock type
and the porphyry dikes. Quartz-anhydritepyrite-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
Porphyry
Cu-Au
System
(Grasberg)
Cu-Au
Skarn
System
(EESS)
Ertsberg
Stocwork
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
Sulfide 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 prograde
alteration
Yes?
Yes
Yes
Mineralization occurs in the final stages of the
hydrothermal event
Yes
Yes
No
Characteristics of Deposits
12
”
“Fault
Sediments
Exoskarn
alteration
Unaltered Intrusion
Biotite-Bornite
Veiniets mesh
Dyke
Dyke
Quartz-Anhydote
Chalcopyrite veins
Potassic
alteration
Propylitic
alteration
0
Carbonate
Sediments
Endo
Skarn
500 m
LS 1012
Figure 9. Summary cross-section view illustrating the main aspects of the ESZ deposit model.
Main Ertsberg rock type. Quartz-anhydritepyrite-chalcopyrite veins occur in all orientations 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 differed significantly.
CONCLUSIONS
1. The ESZ has similarities and differences 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
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 hydrothermal 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 associated with the potassic alteration zone
which probably predates the porphyry
dikes, and the later mineralization stage
is part of a quartz- anhydrite-pyritechalcopyrite veining event which clearly
13
postdates the emplacement of the porphyry 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-
14
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 Intrusion, 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. Freeport Indonesia Co., Pollard and Taylor, Geological
Services Pty. Ltd. C/- School of Earth Sciences,
James Cook University.

Ertsberg Stockwork Zone: A Unique Porphyry Copper Style

Transcription

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