The Grasberg Igneous Complex (GIC) is host to one of the largest copper and gold porphyry-type ore deposits discovered on Earth. Much of the rock volume in the GIC has been pervasively altered by the infiltration of hot, magmatic fluids. In parts of the deposit, alteration destroyed all igneous phases. Petrography reveals that two zones characterise almost the entire complex at the level of the open pit mine. The 1 km-wide core of the deposit is dominated by biotite + magnetite with an inner ~500 m-wide sub-zone containing andalusite. The exterior annular zone, -500 m across, is dominated by sericite + anhydrite + pyrite with small amounts of kaolinite. Pockets of rock contain epidote with chlorite in the distal portions of the GIC.

The Grasberg Igneous Complex, which formed at ~3 Ma, is host to one of the largest copper and gold porphyry-type ore deposits discovered on Earth. This study focuses on the magmatic characteristics of the three main phases of intrusion at the level of the open pit mine: the Dalam, subdivided into the Dalam Andesite, Dalam Volcanic and Dalam Fragmental, the Main Grasberg Intrusion (MGI), and the Kali (Early and Late). A sample suite consisting of 225 polished slabs and thin sections shows all units contain plagioclase and biotite as the dominant phenocryst phases. The Dalam Andesite, MGI and Late Kali contain(ed) hornblende as well. The Late Kali and the MGI also contained minor amounts of clinopyroxene. Apatite is ubiquitous as a trace phase. Magmatic magnetite is identifiable in the Late Kali. The magmatic groundmass in the Kali, MGI, and Dalam Andesite was potassium feldspar, albitic plagioclase, quartz and biotite. A similar groundmass assemblage probably existed in the other Dalam phase rocks, but hydrothermal alteration caused complete replacement. The phenocryst assemblages record no profound changes in magma chemistry over time, but the parent chamber was probably recharged at least twice, and possibly many times.