Postgraduate Report
THE 2001 GEOLOGICAL SOCIETY OF AMERICA MEETING
Kylie Prendergast
JCU, Qld
2001 AIG Postgraduate Bursary Winner
Introduction
Firstly I would like to sincerely thank the AIG for funding the air travel to the 2001
Geological Society of America Meeting (GSA; Boston, 1-10th November) through
allocation of one of their student bursaries. The AIG Bursary enabled me to deliver my
presentation titled “Metal zonation and gold mineralisation at Big Gossan, Irian Jaya,
Indonesia” (see abstract Prendergast, Taylor, Clarke and Pollard (2001)). The Big
Gossan project is one of three sections of my PhD study “Distal porphyry-related
Au±Zn-Pb mineralisation and carbonate alteration in the Ertsberg District, Irian Jaya,
Indonesia”.
The majority of scientists who have worked in the Ertsberg District are from American
Universities and organisations. As it is the premier Geological Meeting in America, the
GSA meeting provided an excellent opportunity to meet and exchange ideas with other
scientists working in my research field.
The Ertsberg District and Big Gossan
The world class Ertsberg mining district lies in the mountainous Central Ranges of Irian
Jaya (Figure 1), an active collisional boundary between the southwest-migrating IndoPacific plate and the north-moving Australian plate (Hamilton, 1979). The oldest rocks
in the district are the Jurassic-Cretaceous Kembelangan Group (clastic and carbonate
sedimentary rocks) and the Tertiary New Guinea Limestone Group (carbonaterich
sedimentary rocks) (Dow et al. 1988). Sixteen hypabyssal Pliocene intrusions have also
been reported in the district (McMahon, 1994).
The Ertsberg district is well known for its multiple Cu-Au skarn and porphyry-related
deposits (Figure 2) that collectively constitute the largest Cu-Au resource in the world.
For both scientific and economic reasons, there is considerable interest in any advances
in understanding the origins and processes responsible for mineralisation in the district.
In addition to the multiple skarn and porphyry-related deposits, there are numerous latestage, carbonate-hosted, Au±Zn-Pb occurrences over an area of several kilometres and
at depths up to 1.5 km. Early work indicates that these occurrences are part of a late,
multistage system that overprints earlier Cu-Au skarn and porphyry-related deposits.
Although some evidence suggests the Au±Zn-Pb is related to a high-sulphidation system
(e.g. spatially associated acid leaching, high-sulphidation state sulphides and high-Te
content), there is little evidence of high-acid alteration (e.g. the presence of pyrophyllite,
alunite) that typically accompanies classic high-sulphidation systems.
The Big Gossan Cu-Au skarn is hosted by a fault at the contact between the Tertiary
New Guinea Limestone Group and the Jurassic-Cretaceous Kembelangan Group.
Meinert et al. (1997) recognised vertical and lateral metal zonation in the Big Gossan
skarn where Mo increases with depth and Cu, Au, Ag, Pb, Zn, As and Co increase
towards the top of the system and towards the western and eastern margins. The current
data indicate that most of this zonation is a product of overprinting sulphide stages and
the availability of structural channelways Prendergast et al. (2001).
Several generations of brecciation in the Big Gossan skarn are associated with infill and
alteration by pyroxene, garnet, magnetite, amphibole, and chalcopyrite-pyrite with a CuAu-Ag element association, whereas sphalerite, galena and minor pyrite postdate the
Cu-Au mineralisation Prendergast et al. (2001). Carbonate breccia occurs in a fault zone
above the skarn and in the hanging wall adjacent to the skarn. Areas of high gold (with
associated sphalerite, galena, pyrite, arsenopyrite and magnetite) and low copper have
been documented in the Big Gossan skarn and adjacent carbonate breccias (Allen, 1992;
Rae et al., 1994; Rubin and Kyle, 1997; Prendergast et al. 2001).
The major hanging wall breccia contains infill and alteration composed of amphibole,
calcite, magnetite and talc and is crosscut by fractures and faults containing gold
associated with massive pyrite, sphalerite, galena and localised Bi-Pb sulphides
(Prendergast et al. 2001; Rae et al., 1994). Gold in the minor fault breccias above the
skarn is associated with massive banded sphalerite, pyrite, galena and minor
arsenopyrite Prendergast et al. (2001).
One or more late stage galena-sphalerite-pyrite-gold events overprint the main Cu-AuAg orebody. These Au±Zn-Pb zones may be part of a system separate to the
development of the Big Gossan Cu-Au skarn mineralisation as indicated by different
gold fineness and sulphide association. The apparent vertical and lateral metal zonation
recognised by Meinert et al. (1997) is not the result of a single-stage evolving fluid.
The issues
The GSA meeting provided the opportunity for me to learn more about related
hydrothermal systems and explore issues in the Ertsberg District, including:
1. Deposits in other districts.
2. Characterisation of Au±Zn-Pb and porphyry-related hydrothermal systems in
carbonate rocks
3. Developments in the Ertsberg District
Many of the scientists I sought out at the GSA had recently published in areas directly
pertaining to my research. All expressed interest in my research and many provided
solutions to problems, new angles or support of my current research approach. I was
also able to network with members of the Society of Economic Geology (SEG), Society
for Geology Applied to Mineral Deposits (SGA) and the chief editors of the Society
journals (Economic Geology and Mineralium Deposita respectively).
Deposits in other districts
An interesting analogy to the Ertsberg District appeared in the Society of Economic
Geologists Fe-oxide Cu-Au Special Session immediately preceding the GSA. So called
“Fe-oxide-Cu-Au” deposits in “Fe-oxide Cu-Au” districts in Argentina (Arizaro
prospect, Salta province; Dow and Hitzman, 2001) and Chile (Productora, Chilean Iron
Belt; Fox and Hitzman, 2001) have an alteration and mineralisation paragenesis directly
comparable to the Grasberg porphyry deposit (Pollard et al. 2000).
It appears that the porphyry classification has lost some of its “members” to more
popular “Fe-oxide Cu-Au end-member” status. Consideration of the “porphyry versus
Fe-oxide Cu-Au” relationship (i.e. Pollard, 2000) was not addressed at the Special
Session and so the significance of districts containing both Grasberg-like porphyry
systems and Fe-oxide Cu-Au deposits was not examined.
Characterisation of Au±Zn-Pb and porphyry-related hydrothermal systems in
carbonate rocks
Background
The distinction between carbonate alteration caused by the different porphyry-related
hydrothermal systems in sedimentary-carbonate hosted districts is poorly understood. A
recent example was documented from carbonate-replacement, high sulphidation Cu-Au
mineralisation in the Superior district, Arizona, where adjacent feldspathic rocks are
altered to kaolinite- or sericite- bearing assemblages (Friehauf, 1996). The highsulphidation Cu-Au orebodies lie within a narrow (1m) hydrothermal carbonate
alteration halo characterised by overgrowths of hydrothermal Mn-Fe-rich dolomite,
hydrothermal carbonate veins and coincident sharply contrasting isotopic values
(Friehauf and Pareja, 1998).
There is also a scarcity of recently published material on porphyry-related, carbonatehosted Au±Zn-Pb systems. Lluis Fontboté (University of Geneve) reported on the San
Gregorio Zn-Pb deposit in Central Peru, the first carbonate-hosted, epithermal, highsulphidation Zn-Pb deposit recognised as such (Fontboté and Bendezu, 1999). His
documentation of porphyry-related mineralisation at San Gregorio provides the most
recent analogies to the Zn-Pb-Au systems in the Ertsberg district.
Outcomes of the GSA
A research approach to constrain different occurrences of carbonate alteration in the
Ertsberg District with C and O isotopes was enhanced by discussions with Kurt Friehauf
(University of Arizona) and Jeffery Hedenquist (Consultant) at the Boston GSA. Lluis
Fontboté recommended searching for parallels for Ertsberg Au±Zn-Pb in the carbonate
hosted Zn-Pb-Au systems in Mexico where carbonate alteration has been documented
but the magmatic-association is less clear.
Developments in the Ertsberg District
Consultants who have worked in the District and attended the Boston GSA include
Lawrence Meinert who has previously published on the Big Gossan deposit (Meinert et
al., 1997). Additionally, representatives from the University of Texas (Austin) and the
University of Arizona were present. Kurt Friehauf (University of Arizona) has recently
been working on the different intrusion phases within the Ertsberg Intrusion adjacent to
the Big Gossan deposit. Richard Kyle who published on precious metal mineralogy in
the Ertsberg district (Rubin and Kyle, 1997) and Structural Geologist Mark Cloos (both
University of Texas, Austin) were also attendees.
Mark Cloos has recently been involved with a number of structural geology projects in
the Ertsberg district. Although he insisted that the Big Gossan carbonate fault breccias
were really solution breccias, after further discussion I decided to disagree with him (the
fault breccias contain shear fabric, brittle-crackle features, and no evidence of collapse)
but recognise that “solution-processes” were contributory. Mark presented a talk on
bubbling magma chambers (Cloos, 2001) coining the phrase “super-giant porphyry
copper deposits”, they just keep getting bigger and better!
The most fruitful outcome from the GSA of direct advantage to my PhD study came
from discussions with University of Texas (Austin) and University of Arizona
representatives. Both Mark Cloos and Kurt Friehauf enthusiastically support a coordinated research approach in the Ertsberg district, which encourages exchange of ideas
and results.
References
Allen, J.M. 1992. Skarn mineralogy, alteration, veining and mineralisation in drillcore
samples from holes BG 1-5, 1-6 and 1-7A, Big Gossan, Irian Jaya. Unpublished internal
report for PT Freeport Indonesia (September 1992), 76p.
Cloos, M. 2001. Bubbling magma chambers, cupolas and porphyry copper deposits.
International Geology Review, vol 43, p. 285-311.
Dow, D.B., Robinson, G.P., Hartono, U. and Tarman, N. 1988. Geology of Irian Jaya:
Irian Jaya geological mapping project.
Geological Research and Development center, Indonesia, in cooperation with the
Bureau of Mineral Resources, Australia, on behalf of the Department of Mines and
Energy, Indonesia, and the Australian Development Assistance Bureau, 298p.
Dow, R.J. and Hitzman, M.W. 2001Tertiary aged Fe-oxide Cu-Au mineralisation at the
Arizaro prospect, Salta province, Northwest Argentina. Abstracts with programs: GSA
annual meeting 2001, vol. 33, no. 6, p. 129.
Fontbote, L. and Bendezu, R. 1999. The carbonate-hosted Zn-Pb San Gregorio deposit,
Colquijirca District, central Peru, as part of a high sulphidation epithermal system. In
Stanley, C.J. et al (eds), “Mineral deposits: processes to processing”. Proceedings of the
fifth biennial SGA meeting and the tenth quadrennial IAGOD symposium, London, UK,
22-25 August, 1999. Balkema, Rotterdam. p. 515-518.
Fox, K.A. and Hitzman, M.W. 2001. Superimposed magnetite and Fe-oxide Cu-Au
mineralisation at Productora, Chilean Iron Belt.
Abstracts with programs: GSA annual meeting 2001, vol. 33, no. 6, p. 129
Friehauf, K.C. 1996. Reaction paths of high-sulphidation state copper-gold ore fluids in
carbonate rocks – a case study at the Superior porphyry-related deposit, Arizona: Geol.
Soc. Amer. Abstracts with programs, vol. 28, no. 7, p. A403.
Friehauf, K.C. and Pareja, G.A., 1998, Can oxygen isotope halos be produced around
high temperature dolo-hosted ore deposits? – Evidence from the Superior district,
Arizona. Economic Geology, V. 93, No. 5, p. 639-650.
Hamilton, W. 1979. Tectonics of the Indonesian region: U.S. Geological Survey
Professional Paper 1078, 345p.
McMahon, T.P. 1994. Pliocene intrusions in the Ertsberg (Gunung Bijih) mining
district, Irian Jaya, Indonesia: petrography,
geochemistry and tectonic setting. Unpublished PhD dissertation, University of Texas,
Austin, 299p.
Meinert, L.D. 1997. Geology, zonation, and fluid evolution of the Big Gossan Cu-Au
skarn deposit, Ertsberg district, Irian Jaya, Econ. Geol. v92, p. 509-533.
Pollard, P.J. 2000. Evidence of a magmatic fluid and metal source for Fe-oxide Cu-Au
mineralisation; in Porter, T.M. (Ed), Hydrothermal Fe-oxide Copper-gold and related
deposits: a global perspective. Australian Mineral Foundation, Adelaide, pp 27-41.
Pollard, P.J., Taylor, R.G. and Kusnanto, B. 2000. Paragenesis of alteration and
mineralisation of the Grasberg Cu-Au deposit, West Papua, Indonesia. GSA abstracts
with programs, vol. 32, no. 7. pA-51.
Prendergast, K. Taylor, R.G., Clarke, G.W. and Pollard, P.J. 2001. Metal zonation and
the timing of gold mineralisation associated with the Big Gossan Cu-Au deposit,
Ertsberg District, Irian Jaya, Indonesia. Abstracts with programs: GSA annual meeting
2001, vol. 33, no. 6, p. 358
Rae, A.J., Allen, J.M. and Powell, N.G., 1994. Paragenesis, textural relationships and
chemistry of gold at the Big Gossan prospect Gunung Bijih (Ertsberg) district, Irian,
Jaya, Indonesia. Unpublished internal report for PT Freeport Indonesia.
Rubin, J.N. and Kyle, J.R. 1997. Precious metal mineralogy in Porphyry-, Skarn-, and
Replacement-Type Ore Deposits of the Ertsberg (Gunung Bijih) District, Irian Jaya,
Indonesia. Economic Geology. V92: p 535-550.
Philippines
Pacific Ocean
Ertsberg
District
Ind
on
es
Papua
New Guinea
ia
Australia
1000km
0
Figure 1. Location of the Ertsberg District, Irian Jaya, Indonesia
Grasberg
West Grasberg
Kucing Liar
Ertsberg
Wanagon
IOZ/DOZ
Big Gossan
CO W A G eo log y
DOM
La k e
Q - Al lu via l d e po si ts
T - Ba n de d cla y
T - Int r usi on s/ fra gm e n tal
T - Se d im en ta ry ro ck
K - Se d im en tary ro ck
J- K - Sedi m en tar y ro ck
2
0
2
4 Kilometers
Figure 2. Regional geology and location of the major Cu/Au skarn and porphyry
occurrences in the Ertsberg District.