Southwest Margin of the Grasberg Igneous Complex, Papua, Indonesia: Prolonged Interaction of Acidic Magmatic Fluid with Carbonate Wall Rock

The 3 Ma Grasberg Igneous Complex (GIC) of Papua Indonesia is host to a supergiant porphyry Cu-Au orebody. The upper GIC is emplaced into a folded and faulted sequence of Miocene limestones. Three units are mapped at the contact: The Heavy Sulfide Zone (HSZ) a pyrite-rich shell that grades outward into the Marginal Breccia (MB), a unit that consists mostly of rounded clasts of white limestone surrounded by a black shaley matrix. Near the surface, the MB is overlain by the Banded Clay, a unit composed of halloysite.

Igneous intrusion created several meters of calc-silicate skarn at the contact. The precipitation of magnetite in the core of the deposit, hydrolysis of SO2, and precipitation of pyrite in the periphery made escaping magmatic fluids highly acidic. In the outer part of the system, infiltrating fluids altered igneous minerals to sericite, quartz and pyrite. Acidic fluids dissolved carbonate wall rock forming the MB. Copious amounts of pyrite precipitated to form the HSZ. Layered infills, at least 50 cm thick, indicate dissolution at least locally generated very high porosity.

The MB and HSZ are the result of a prolonged interaction between cooling acidic magmatic fluids and carbonate wallrock. The outward-flaring geometry of the upper 1000 m of the GIC system resulted from the dissolution of hundreds of meters of wallrock which caused the outwards collapse of the GIC. High permeability along the contact maintained fluid pressure gradients between the cupola and the wall rock that were higher than they would have been if the GIC was emplaced into less reactive wallrock. The steady and prolonged flow of magmatic fluids from the fluid-charged cupola towards the enveloping high-permeability HSZ/MB shell prevented explosive fragmentation of the hydrothermal system. This is probably a significant factor contributing to the formation of the extraordinary Grasberg Cu-Au orebody.