DECIPHERING THE EVOLUTION OF ORE FLUIDS AT THE BATU HIJAU COPPER-GOLD PORPHYRY DEPOSIT, SUMBAWA, INDONESIA

The Batu Hijau Cu-Au porphyry deposit is a world-class island arc porphyry deposit, Sumbawa, Indonesia. The 12 by 6 km district holds an estimated 914 million tonnes of ore at an average grade of 0.53% Cu and 0.40 g/t Au. Hydrothermal alteration and mineralization crosscut a tonalite porphyry complex and surrounding volcanic and volcaniclastic host rocks and are spatially associated with the old and intermediate tonalites. Within the veins, four types of fluid inclusions have been trapped in quartz, B35, B60, B85, and B20H (the numerical value indicating the average volume of the vapor phase, and the ‘H’ indicating a halite phase is present). Fluid inclusions associated with vein mineralization have been evaluated using petrography, microthermometry, SXRF, and LA-ICP-MS.

Microthermometry provided salinities and homogenization temperatures of inclusions in fluid inclusion assemblages. Ice melting temperatures in B35 inclusions gave an average salinity of 2.3 wt % NaCl equiv. Homogenization temperatures are bimodal with a lower temperature population from 250 - 330 °C, and a higher temperature population from 370 - 390 °C.

B20H brine inclusions homogenized by both halite melting and by liquid-vapor homogenization. More than 55% of brine inclusions homogenized by halite melting from 310 - 440 °C. Brine inclusions homogenizing by liquid-vapor homogenization ranged from 300 - 490 °C, with most inclusions homogenizing from 300 - 400 °C; salinities for these inclusions ranged from 30 - 44 wt% NaCl equiv, with an average salinity of about 41%.

Although quantitative homogenization temperatures cannot be determined for vapor-rich inclusions we heated B60 and B85 inclusions and determined they would homogenize at >350 °C with most homogenizing at >420 °C.

SXRF analyses of large, individual B35, B60, B85, and B20H fluid inclusions have produced elemental maps illustrating relative elemental abundances. These data are being compared with quantitative elemental data from LA-ICP-MS analyses.

Results are consistent with input of a single-phase, low salinity fluid at the base of the hydrothermal system that likely transported Cu, Au, and other metals from depth. As this fluid moved upwards it separated into immiscible brine and vapor. Mineralization appears to be spatially associated with the presence of B20H inclusions.