Petrology of the southwest margin of the Grasberg Igneous Complex, Papua, Indonesia




Lambert, Adam L.

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The 3 Ma Grasberg Igneous Complex (GIC) of Papua, Indonesia is host to a super-giant porphyry copper-gold deposit. The GIC is shallowly emplaced into folded and faulted limestones that are as young as late Miocene. The Heavy Sulfide Zone (HSZ) is a pyrite-rich shell that surrounds the GIC. The HSZ grades into the Marginal Breccia. Near the surface the Marginal Breccia is overlain by the Banded Clay, a halloysite-clay rich unit. This study has determined the relationship of these units and characterized the pervasive alteration from the petrology of 588 samples collected from five drill cores and 23 outcrop samples. The initial Dalam phase of intrusion generated ~5 m of skarn (epidote, garnet, and clinopyroxene) at the margin of the GIC below 3700 m elevation. The precipitation of magnetite in the core of the GIC, the hydrolysis of SO₂, and the precipitation of pyrite in the periphery of the deposit generated abundant HCl and H₂SO₄. In the outer GIC, primary igneous minerals were altered to sericite, quartz, and pyrite by the acidic hydrothermal fluids. These fluids dissolved carbonate host rock and precipitated abundant pyrite. Layered dissolution infill indicates porosity was locally very high. High permeability along the margin of the GIC would have channeled hydrothermal fluid upwards. As supporting carbonate rock was dissolved forming the Marginal Breccia, open fractures formed along the contact with the GIC. Pyrite precipitated into these fractures, forming layering in places. Unsupported pieces of igneous rock and skarn broke off and fell into openings which became filled with pyrite. This process caused most of the growth of the HSZ to progress inward, leaving rocks that were once at the margin of the GIC separated from it by lenses of massive pyrite. Minor sphalerite and galena precipitated at lower temperatures at the outer edge of the HSZ. Only trace amounts of copper sulfides are present within the pyrite-rich HSZ because nearly all the copper was precipitated in the center of the deposit. Any H₂S or SO₂ remaining in the upwelling fluids mixed with oxygenated meteoric waters forming concentrated H₂SO₄, which intensely altered volcanic sediments forming the Banded Clay. The dissolution-enhanced permeability along the contact zone facilitated the drainage of magmatic fluids. This acted to increase the fluid pressure gradient driving pervasive infiltration between the center and the margin of the GIC. This was a significant contributing factor to the creation of the super-giant orebody. The dissolution of carbonate wall-rock caused the GIC to collapse outwards. This caused extension fracturing within the GIC that diverted fluid flow upwards and back into igneous rock. As a result some of the outermost igneous rock in the upper part of the GIC is relatively fresh, with primary groundmass orthoclase and plagioclase phenocrysts. The outward collapse of the GIC generated the flaring-upward shape that is a distinct attribute of the Grasberg system.