Copper-Gold ± Molybdenum Deposits of the Ertsberg-Grasberg District, Papua, Indonesia
Clyde A. Leys, Mark Cloos, Brian T.E. New, George D. MacDonald, 2012. "Copper-Gold ± Molybdenum Deposits of the Ertsberg-Grasberg District, Papua, Indonesia", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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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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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.