CHARACTERIZING THE STRUCTURE, MINERALOGY, AND PARAGENETIC SEQUENCE OF SHEETED QUARTZ VEINS IN THE ERTSBERG EAST SKARN SYSTEM, PAPUA, INDONESIA

The Ertsberg East Skarn System (EESS), a 3-Gt orebody at 0.59% Cu and 0.49 ppm Au, is located in the Ertsberg-Grasberg district in the highlands of Papua, Indonesia. Subduction of the Australian plate beneath the Pacific plate ca. 12 Ma uplifted and deformed a succession of Upper Cretaceous siliciclastic to Paleogene carbonate strata into which ca. 3 Ma magmatism and episodic hydrothermal fluid flow created stockwork- and skarn-hosted Cu-Au orebodies. The EESS is located on the northern margin of the Ertsberg pluton at its contact with steeply dipping strata forming the southern limb of the Yellow Valley Syncline. The mineralized zone extends from surface exposures at ~4300-m to below 2000-m, forming one of the world’s largest skarn Cu-Au systems.

A NW-SE-trending zone of sub-vertical sheeted quartz veins up to 30-m wide has been intercepted in at least 11 drill holes and can be traced for at least 150 m along strike though the Ertsberg diorite ~250 m inboard of the wallrock contact. The sheeted vein structure appears to be parallel to major district NW-SE faults and extends at least from the 2550- to 2700-m elevations, with all limits poorly constrained by current drilling information. Modeling of the drill intercepts of the sheeted veins indicates that the structure is planar. Composite images of photographed core from these drill holes were analyzed and suggest an average quartz vein width of ~1 cm, but many individual veins record multiple opening events. Cu sulfide mineralization is irregularly distributed through the sheeted vein zone and overprints quartz.

Thin sections of individual quartz veins were made for petrographic study and to analyze selvage alteration. Individual SEM-CL images were stitched into composite maps that reveal zonation textures that record changes in P-T-X conditions and crystal growth rates. The eight textures identified in more than one image were used to construct a paragenetic sequence of EESS quartz-sulfide vein architecture as it varies by elevation and host lithology. The CL textures observed are consistent with other Cu-Au porphyry systems, including the nearby Grasberg intrusive system. The sheeted vein zone may record a major early hydrothermal fluid pathway, but its significance for Cu-Au mineralization is under investigation.