Fluid inclusion constraints on the hydrothermal processes responsible for Cu-Au mineralization in the Ertsberg East Skarn System, Papua, Indonesia

The Ertsberg East Skarn System (EESS) is a 3-Gt orebody at 0.59% Cu and 0.49 ppm Au that extends from the surface at 4200m at least to 1800m elevation.  Skarn-hosted Cu-Au ores are localized in Upper Cretaceous siliciclastic to Lower Paleogene carbonate strata along the steeply dipping contact with the 3-Ma Ertsberg Diorite that locally hosts stockwork ores. Petrography, quartz SEM-Cathodoluminesence (SEM-CL), fluid inclusion microthermometry, and fluid inclusion LA-ICP-MS were used to characterize vein formation and fluid conditions in the ore-forming system.

Five vein-hosting lithologies and five stages of vein formation were defined petrographically, and 41 SEM-CL transects from 17 quartz vein samples were used to identify eight recurring quartz CL textures that vary by elevation and host lithology.  The CL texture paragenesis reveals quartz-sulfide vein architecture rife with brittle deformation features and evidence of repeated opening and mineralization of fractures.  Brittle deformation increased with depth in diorite-hosted veins and was most prevalent in skarn-hosted veins.

Fluid inclusion petrography of 16 samples, in-situ micro-thermometry of 107 fluid inclusions in eight samples, and LA-ICP-MS of 99 inclusions revealed four types of inclusions consistent with Cu-Au porphyry systems. Type 1 inclusions are 4-11% salinity, 40-70 vol% vapor bubble, homogenize between 332-396 C and can contain chalcopyrite daughter crystals. Type 2a inclusions are 27-51% salinity, 10-30 vol% vapor bubble, homogenize between 280-420 C and contain halite. Type 2a inclusions may also contain sylvite, anhydrite, chalcopyrite, magnetite, and hematite. Type 2b inclusions are 1-12% salinity, 65-90 vol% vapor bubble, homogenize between 340-392 C and typically do not contain daughter minerals. Type 3 inclusions at ambient conditions are 2-23% salinity, 15-50 vol% vapor bubble, homogenize between 283-403C and may contain opaque minerals.  LA-ICP-MS analysis indicate that EESS fluid inclusions have high Na, K, and Ca, and support mineralization by Cu, Fe, Mn, Pb and Zn-rich brine derived from phase separation of a primary magmatic fluid.  All fluid inclusion types were observed over the 1.7 km of elevation sampled – nearly 4x the vertical extent documented for other porphyry systems. The upper limit of the phase separation zone is present at ~2500 m, which corresponds with deepest known Cu-Au concentrations. These data support fault-controlled fluid migration and a lack of lithologic control on Cu-Au mineralization in veins.