Pliocene Intrusions in the Gunung Bijih (Ertsberg) Mining District, Irian Jaya, Indonesia: Petrography and Mineral Chemistry

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Abstract
At least 16 Pliocene hypabyssal intrusions crop out within the Gunung Bijih (Ertsberg) Mining District in Irian Jaya (western New Guinea), Indonesia. Several Cu-Au ore deposits are associated spatially and genetically with these young hypabyssal intrusions. Approximately 188 mt of Cu-Au skarn ore has been delineated or produced from the district. Most of these skarns are associated with a quartz monzonite to quartz monzodiorite stock (the Ertsberg Intrusion). The Grasberg Complex, a series of nested intrusions, hosts a supergiant porphyry copper deposit containing ∼9.8 mt of Cu-Au ore. Textural and mineralogical differences among the intrusions are largely the result of crystallization at different crustal levels. The intrusions were emplaced at ≤ 2 km depth into deformed sedimentary rocks that originally were deposited on the northern margin of the Australian continent. Emplacement of at least one intrusion was controlled by cross-cutting NW- and NE-trending fault sets. Intrusions within the Gunung Bijih District can be divided into a “high-K” group of latites, trachydacites, and trachytes, and a “low-K” group of medium- to high-K andesites and dacites. The high-K group of intrusions is volumetrically more important (at current levels of exposure), containing the two largest igneous bodies in the district: the Ertsberg Intrusion and the Grasberg Complex. The Ertsberg Intrusion is a single intrusion with an estimated volume of 10 to 20 km3. The Grasberg Complex, which has an outcrop area comparable to that of the Ertsberg, consists of a series of small (≤4 km3) intrusions. All other intrusions in the study area appear to be ≤1 km3 in volume. The Ertsberg Intrusion is equigranular, with only a few phenocrysts and a generally coarser grain size than the other intrusions. In contrast to most district intrusions, the Ertsberg contains only trace amounts of amphibole. Despite its large (for the district) volume, the Ertsberg is relatively homogeneous in terms of mineral content and mineral compositions. These observations, along with the occurrence of near end-member magnetite in the Ertsberg, suggest that this intrusion was emplaced in the shallow (≤2 km) crust as a single pulse of crystal-poor magma, which then cooled slowly. At this depth, the dissolved H2O content of the magma remained too low to stabilize amphibole throughout crystallization. Coexisting titanite, diop-side, magnetite, and quartz, with no ilmenite, suggests oxygen fugacity in the Ertsberg was at least one log unit above the nickel-nickel oxide (NNO) buffer. Other intrusions in the district, including those in the Grasberg Complex, are porphyritic, containing 40% to 60% phenocrysts in fine-grained (usually ≤10 μm) groundmass. These intrusions commonly contain 5% to 20% amphibole or pseudomorphs of alteration minerals after amphibole. These intrusions crystallized phenocrysts at depth (10-15 km?) where increased confining pressure allowed dissolved H2O contents to build up enough to stabilize amphibole. Strongly zoned plagioclase phenocrysts in low-K intrusions indicate that low-K intrusions reached H2O saturation early in their crystallization history, and that low-K magmas crystallized with higher dissolved H2O contents than did porphyritic high-K magmas. None of the porphyritic intrusions contain mineral assemblages indicative of oxygen fugacity, but the lack of orthopyroxene in these intrusions may indicate oxygen fugacities greater than the minimum estimated for the Ertsberg Intrusion.