STOCKWORK VEIN ZONE OF THE GRASBERG PORPHYRY CU-AU DEPOSIT: STRUCTURAL AND CHEMICAL HISTORY

The Grasberg Igneous Complex (GIC) is host to the largest (2+ billion tonnes with 1 wt. % Cu and 1 gm/tonne Au) ore deposit in the Ertsberg mining district of Papua, Indonesia. Most ore mineralization postdates the 3.2 Ma Main Grasberg Intrusion (MGI). The MGI is host to the stockwork zone, a 3-D network of abundant quartz and magnetite veining.

A sample suite of 75 polished slabs, mostly 10-15 cm long, was prepared from seven near-horizontal drill cores at the 3,700 meter elevation of the stockwork zone. Thin sections were also prepared for 38 samples. Samples with abundant veins (>50 vol%) come from a circular area ~300 m across in the center of the GIC. The 1,211 veins, about 16 per sample, range from veinlets (<1 mm) to about 15 mm thick. Quartz veins are commonly 5 to 10 mm wide. Numerous cross-cutting relationships indicate five stages of veining. Stage 1 consists of narrow (<1 mm) magnetite and biotite veinlets. Stage 2 consists of quartz veins with minor amounts of disseminated magnetite and/or chalcopyrite. Stage 3 veins are laminated with walls of magnetite and interior fills of quartz with disseminated magnetite and/or chalcopyrite. Stage 4 are quartz veins with narrow centerlines of chalcopyrite with varied amounts of magnetite or pyrite. Stage 5 consist of narrow (up to 2 mm) veins and thin veinlets of chalcopyrite with varied amounts of biotite and/or pyrite. Some Stage 4 and 5 veins contain microscopic traces of bornite. They are localized to an area about 150 meters across in the center of the GIC. This observation, combined with the limits of abundant veining mapped zones of pervasive hydrothermal alteration, indicate there was a center of upwelling ore-forming mineralization that was centered near the tip of the wedge of crosscutting Kali dikes. Pervasive alteration in the Kali is minor, but much lesser veining indicates a similar succession of stages.

The random orientation of veins indicates near-isotropic state of stress during extension fracturing. The stockwork zone is interpreted as a product of episodic collapse of the roof above a fluid-charged cupola at the top of a cooling stock of magma. Over time the cupola migrated downward and repeated fracturing and veining produced the stockwork zone of intense mineralization. Stages 4 and 5 copper mineralization is explainable as a result of deeper seated sidewall bubbling.