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Copper-Gold ± Molybdenum Deposits of the Ertsberg-Grasberg District, Papua, Indonesia

By
Clyde A. Leys
Clyde A. Leys
1
Freeport-McMoRan Copper and Gold, Inc., 333 N. Central Ave., Phoenix, Arizona 85004
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Mark Cloos
Mark Cloos
2
Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712
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Brian T.E. New
Brian T.E. New
3
Department of Earth Sciences, James Cook University, Queensland, Australia 4811
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George D. MacDonald
George D. MacDonald
1
Freeport-McMoRan Copper and Gold, Inc., 333 N. Central Ave., Phoenix, Arizona 85004
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Published:
January 01, 2012

Abstract

The Ertsberg-Grasberg district hosts two giant Cu-Au (± Mo)-rich porphyry and skarn-hosted Cu systems that formed between 3.3 and 2.5 Ma. These Cu-Au systems are associated with two separate K-rich dioritic intrusions, the Grasberg Igneous Complex and the Ertsberg Intrusive Complex, which were shallowly emplaced into a sedimentary sequence of Tertiary carbonate and Jurassic-Cretaceous siliciclastic rocks. The district is located near the crest of the Central Range of western New Guinea, in the easternmost Indonesian province of Papua (formerly Irian Jaya). Economic mineralization in each of these systems is vertically continuous over at least 1,500 m. Using a non-economic cutoff grade of 0.1% Cu, the Grasberg-related system contains 7.5 billion tonnes (grading 0.70% Cu and 0.64 ppm Au) in two deposits, the Grasberg porphyry system and the Kucing Liar skarn. At the same 0.1% Cu cutoff, the Ertsberg-related system contains 3.6 billion tonnes (grading 0.60% Cu and 0.44 ppm Au) in four deposits, the Ertsberg skarn, the Ertsberg East skarn system, the Dom skarn, and the Big Gossan skarn. A significant aspect of these orebodies is their ability to deliver a large tonnage of much higher than these average Cu and Au grades, which is required to offset the high costs of mining in the challenging environment in which they are exploited.

The Ertsberg, Dutch for “ore mountain,” was discovered in 1936. Freeport evaluated the prospect in the 1960s, and began mine development in 1969. Discovery of the Ertsberg East skarn and the Dom skarn ore-bodies quickly followed initial development in the district. The Grasberg deposit was discovered by exploration drilling in 1988, targeting Au mineralization in an intensely quartz-magnetite stockwork-veined outcrop that had been depleted of its Cu due to supergene leaching. This was followed by discovery of the high-grade Big Gossan skarn in 1992 and the massive Kucing Liar skarn in 1994.

Geologic studies have shown that the Central Range was produced by collisional tectonism that resulted when the northern edge of the Australian plate entered and jammed the subduction zone beneath the Melanesian oceanic arc. Magmas were generated during the breakoff of the oceanic end of the Australian plate as a result of decompression melting of asthenospheric and lithospheric mantle, which upwelled into the subterranean rift. This short episode (4.4–2.6 Ma) of intermediate-composition magmatism formed the district's porphyry Cu deposits. Magmatism and mineralization occurred in a structural corridor dominated by left-lateral strike-slip reactivation of the preexisting compressional regime faults, implying a tensional environment as a significant control to shallow emplacement. Pull-apart connections between strike-slip faults created pathways for magma ascent and the focused flow of magmatic fluids. Porphyry-type mineralization developed where the fluids ascended through igneous rocks, and skarns developed where they interacted with carbonate strata, especially impure dolostones.

Porphyry-style alteration follows typical patterns of a potassic core grading outward into phyllic alteration, and surrounded by a propylitic halo at the shallower levels of the system. Advanced argillic alteration is weakly represented in the igneous rocks. Stockwork vein systems form in the central high-temperature, potassic-altered zones and are the locus of the highest Cu and Au grades in the porphyry orebodies. High Au/Cu ratios (>1 g/t:%) are characteristic of ores formed within these central high grade zones; ratios diminish outward from the center of the systems more sharply than does the Cu grade. The overall Au/Cu ratio at Grasberg is 1 and is 1.3 in the porphyry-hosted ores at Ertsberg East skarn system. Chalcopyrite is the dominant Cu mineral throughout the potassic zones and bornite increases with depth. The majority of the Au is contained as free inclusions within these two Cu minerals, and within the potassic zone; covellite dominates in the phyllic zone with lesser chalcopyrite. Gold associations in the phyllic zone are complex. Where covellite dominates the Cumineral assemblage, Au is most commonly contained within pyrite or as free grains within the silicate rock matrix.

Skarn alteration mineralogy is strongly controlled by the host stratigraphy and is similar at both complexes. Prograde skarn mineralogy in the calcareous Kais and Faumai formations is dominated by monticellite and diopside, with lesser forsterite. Forsterite plus diopside dominate the dolomitic lower Waripi formation and the limestone member of the Ekmai formation. Massive magnetite mineralization is contemporaneous with pro-grade alteration, preferentially replacing dolostone beds and areas of apparent high fluid flow. Retrograde alteration is represented by chlorite-serpentine-talc in the upper limestone formations, and by actinolite-tremolite-phlogopite-talc-serpentine-chlorite plus calcite in the lower, more dolomitic formations. Chalcopyrite dominates over bornite in the magnetite-poor ores in the Ertsberg East skarn system, whereas bornite dominates in magnetite-rich ores. At Kucing Liar, chalcopyrite dominates over bornite, even within the magnetite-rich ores, but is replaced by covellite + pyrite on all orebody margins. Copper and Au are concentrated within magnetite replacement bodies, where present, in both of these skarns; Au/Cu ratios are ∼0.5 at Ertsberg East skarn system and ∼1 at Kucing Liar.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
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Michael Harris
Michael Harris
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Francisco Camus
Francisco Camus
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Society of Economic Geologists
Volume
16
ISBN electronic:
9781629490410
Publication date:
January 01, 2012

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