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.

Abstract The supergiant Grasberg porphyry deposit in Papua, Indonesia (5.26 Gt @ 0.61% Cu and 0.57 g/t Au, with no cutoff applied) is hosted by the Grasberg Igneous Complex that fills an upward-flared diatreme ~1,800 m wide at the 4,250-m surface elevation. The Grasberg Igneous Complex is emplaced into folded and strike-slip faulted Tertiary and older sediments and comprises 3.6 to 3.3 Ma Dalam monzodiorite intrusions and subordinate volcanic rocks occupying much of the pipe, the central 3.2 Ma Main Grasberg intrusion, and the NW-SE-trending 3.2 to 3.0 Ma Kali dikes. The Grasberg Igneous Complex contains two porphyry systems: Gajah Tidur copper-(molybdenum) and Main Grasberg copper-gold. The Gajah Tidur intrusion belongs to the Dalam igneous group and is a 3.4 Ma porphyritic monzonite with its top at a 2,750-m elevation; it is overprinted by an extensive, domal, quartz stockwork, with a low-grade and intensely phyllic-altered core, surrounded by molybdenite-bearing veins, with a pre-Main Grasberg Re-Os age, as well as chalcopyrite and overprinting pyrite-covellite veins. The strongly potassic-altered, Main Grasberg monzodiorite porphyry extends from surface to the 2,700-m elevation and is overprinted by a cylindrical, ~1-km-diameter, intense quartz-magnetite stockwork cut by abundant chalcopyrite-bornite veins with rare molybdenite dated at 3.09 Ma. A 700-m-wide annulus of chalcopyrite overprinted by pyrite-covellite-mineralized phyllic alteration surrounds the stockwork. Altered and mineralized Main Grasberg and surrounding Dalam rocks were subsequently wedged apart by the largely unmineralized Kali dikes. Gold is predominantly associated with the Main Grasberg porphyry system where it occurs as 1- to 150- µ m (avg ~15 µ m) native gold inclusions within chalcopyrite and bornite. Melt and fluid inclusions from Main Grasberg stockwork quartz veins, which exhibit crack-seal textures, comprise K-feldspar-rich silicate melt, sulfide melt, virtually water-free salt melt, and coexisting hypersaline and vapor-rich fluids. Factors important in forming the Grasberg deposit include the following: (1) generation of highly oxidized fertile magma in a postsubduction tectonic setting; (2) efficient extraction of metals from the parental magma chamber; (3) prolonged maintenance of a fluid-accumulating cupola in a strike-slip structural setting that delivered multiple overlapping discharges of metal-rich fluid; (4) highly focused fluid flow into a narrow, permeable stockwork zone in which a steep temperature gradient enabled highly efficient copper and gold precipitation and led to high ore grades; (5) limited dilution by postmineral intrusions; (6) the youthfulness of the deposit minimized erosion and resulted in preservation of nearly all the high-grade Main Grasberg porphyry orebody; and (7) the proximity of the two porphyry centers enables them to be mined as a single, large deposit. The Gajah Tidur copper-(molybdenum) and Main Grasberg copper-gold porphyry centers overlap in space and formed within ~250,000 years of one another. However, their distinct metal endowment, depth of emplacement, and geometry indicate that they formed under different magmatic, hydrothermal, and structural conditions, which are the subject of ongoing research.