GRASBERG PORPHYRY CU-AU OREBODY: FLUID-CHARGED CUPOLAS, PERVASIVE ALTERATION, AND STOCKWORK FORMATION

The Grasberg Igneous Complex (GIC) in the highlands of Papua Indonesia is host to a supergiant porphyry-type Cu-Au orebody. The GIC is about 2 km across at the surface near ~4000 m elevation and narrows to about 800 m at 3000 m. There were three distinct magmatic phases: Dalam, MGI, and Kali. The Dalam phase was prolonged and involved a period of volcanism in a maar-caldera. The MGI is a medial cone-shaped plug that widens upwards. The Kali is a composite of dikes forming a wedge with a tip near the GIC center. Plutonism was facilitated by concurrent strike-slip faulting.

The orebody is the product of two distinct pulses of mineralization. The first pre-dates emplacement of the MGI, has a high Cu/Au ratio with some Mo, and is dominated by infiltration of magmatic fluids that caused intense pervasive alteration. The second predates Kali-phase diking, produced the bulk of high-grade Cu+Au ore and involved voluminous infiltration and fracture fluid flow. Above 3500 m, a stockwork (defined as vein network recording a 3-D dilation) formed that is about 400 m across with an outer limit mapped where the vein volume exceeds 5% of the rock. Volumes of veins per cubic meter locally exceed 50%. The GIC stockwork, largely hosted in the MGI, involved two distinct stages, an early low-Cu episode that mostly produced qtz + mt and a later Cu+Au ore-stage. Upwards and outwards, the ore-related mineralization shows a progression from bor to ccp to pyr.

Concentric zones of intense pervasive alteration and stockwork veining are most simply explained as a manifestation of the existence of a cupola inside the stock (boundary of immobile and mobile magma) that is charged with magmatic fluid. When fluid volume is only enough to displace some of the melt in the porous mush at the top of the stock, infiltration alteration dominates. When fluid production rates are high enough to fill the porous zone and displace enough melt to form a pocket, episodic collapse of the roof of the cupola can occur that produces 3-D stockwork veining. The production of Cu-rich fluids happens when sidewall bubbling extends below depths of about 6 km. In the case of the GIC, ore-forming cupolas developed twice. The fluid-charged pre-MGI cupola was located SW of the center of the GIC at an elevation of about 2700 m. The fluid-charged pre-Kali cupola migrated down from above 3500 m to about 2700 m.