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 ofAmerica Meeting (GSA; Boston, 1-10th November) through allocation of one of their student bursaries. TheAIG Bursary enabled me to deliver my presentation titled “Metal zonation and gold mineralisation at BigGossan, Irian Jaya, Indonesia” (see abstract Prendergast, Taylor, Clarke and Pollard (2001)). The BigGossan project is one of three sections of my PhD study “Distal porphyry-related Au±Zn-Pb mineralisationand carbonate alteration in the Ertsberg District, Irian Jaya, Indonesia”.
The majority of scientists who have worked in the Ertsberg District are from AmericanUniversities andorganisations. As it is the premier Geological Meeting in America, the GSA meeting provided an excellentopportunity 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), anactive collisional boundary between the southwest-migrating Indo-Pacific plate and the north-movingAustralian plate (Hamilton, 1979). The oldest rocks in the district are the Jurassic-Cretaceous KembelanganGroup (clastic and carbonate sedimentary rocks) and the Tertiary New Guinea Limestone Group (carbonaterichsedimentary rocks) (Dow et al. 1988). Sixteen hypabyssal Pliocene intrusions have also been reported inthe district (McMahon, 1994).
The Ertsberg district is well known for its multiple Cu-Au skarn and porphyry-related deposits (Figure 2) thatcollectively 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 formineralisation in the district. In addition to the multiple skarn and porphyry-related deposits, there arenumerous late-stage, carbonate-hosted, Au±Zn-Pb occurrences over an area of several kilometres and atdepths up to 1.5 km. Early work indicates that these occurrences are part of a late, multistage system thatoverprints 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 statesulphides and high-Te content), there is little evidence of high-acid alteration (e.g. the presence ofpyrophyllite, 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 LimestoneGroup and the Jurassic-Cretaceous Kembelangan Group. Meinert et al. (1997) recognised vertical andlateral metal zonation in the Big Gossan skarn where Mo increases with depth and Cu, Au, Ag, Pb, Zn, Asand Co increase towards the top of the system and towards the western and eastern margins. The currentdata indicate that most of this zonation is a product of overprinting sulphide stages and the availability ofstructural channelways Prendergast et al. (2001).
Several generations of brecciation in the Big Gossan skarn are associated with infill and alteration bypyroxene, garnet, magnetite, amphibole, and chalcopyrite-pyrite with a Cu-Au-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 copperhave 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 andtalc and is crosscut by fractures and faults containing gold associated with massive pyrite, sphalerite, galenaand localised Bi-Pb sulphides (Prendergast et al. 2001; Rae et al., 1994). Gold in the minor fault brecciasabove the skarn is associated with massive banded sphalerite, pyrite, galena and minor arsenopyritePrendergast et al. (2001).
One or more late stage galena-sphalerite-pyrite-gold events overprint the main Cu-Au-Ag orebody. TheseAu±Zn-Pb zones may be part of a system separate to the development of the Big Gossan Cu-Au skarnmineralisation as indicated by different gold fineness and sulphide association. The apparent vertical andlateral 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 andexplore 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 myresearch. All expressed interest in my research and many provided solutions to problems, new angles orsupport of my current research approach. I was also able to network with members of the Society ofEconomic Geology (SEG), Society for Geology Applied to Mineral Deposits (SGA) and the chief editors ofthe 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 directlycomparable 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-Auend-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-likeporphyry systems and Fe-oxide Cu-Au deposits was not examined.
Characterisation of Au±Zn-Pb and porphyry-related hydrothermal systems incarbonate rocks
Background
The distinction between carbonate alteration caused by the different porphyry-related hydrothermal systemsin sedimentary-carbonate hosted districts is poorly understood. A recent example was documented fromcarbonate-replacement, high sulphidation Cu-Au mineralisation in the Superior district, Arizona, whereadjacent feldspathic rocks are altered to kaolinite- or sericite- bearing assemblages (Friehauf, 1996). Thehigh-sulphidation Cu-Au orebodies lie within a narrow (1m) hydrothermal carbonate alteration halocharacterised by overgrowths of hydrothermal Mn-Fe-rich dolomite, hydrothermal carbonate veins andcoincident sharply contrasting isotopic values (Friehauf and Pareja, 1998).
There is also a scarcity of recently published material on porphyry-related, carbonate-hosted Au±Zn-Pbsystems. Lluis Fontboté (University of Geneve) reported on the San Gregorio Zn-Pb deposit in Central Peru,the first carbonate-hosted, epithermal, high-sulphidation Zn-Pb deposit recognised as such (Fontboté andBendezu, 1999). His documentation of porphyry-related mineralisation at San Gregorio provides the mostrecent 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 Cand O isotopes was enhanced by discussions with Kurt Friehauf (University of Arizona) and JefferyHedenquist (Consultant) at the Boston GSA. Lluis Fontboté recommended searching for parallels forErtsberg Au±Zn-Pb in the carbonate hosted Zn-Pb-Au systems in Mexico where carbonate alteration hasbeen 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 whohas previously published on the Big Gossan deposit (Meinert et al., 1997). Additionally, representatives fromthe University of Texas (Austin) and the University of Arizona were present. Kurt Friehauf (University ofArizona) has recently been working on the different intrusion phases within the Ertsberg Intrusion adjacent tothe 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 alsoattendees.
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 furtherdiscussion I decided to disagree with him (the fault breccias contain shear fabric, brittle-crackle features, andno evidence of collapse) but recognise that “solution-processes” were contributory. Mark presented a talk onbubbling magma chambers (Cloos, 2001) coining the phrase “super-giant porphyry copper deposits”, theyjust keep getting bigger and better!
The most fruitful outcome from the GSA of direct advantage to my PhD study came from discussions withUniversity of Texas (Austin) and University of Arizona representatives. Both Mark Cloos and Kurt Friehaufenthusiastically support a co-ordinated research approach in the Ertsberg district, which encouragesexchange 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, BigGossan, 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 ofthe 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, NorthwestArgentina. 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 ahigh sulphidation epithermal system. In Stanley, C.J. et al (eds), “Mineral deposits: processes to processing”. Proceedings of the fifthbiennial 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 Superiorporphyry-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), HydrothermalFe-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, WestPapua, 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 withthe 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 prospectGunung 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.