HYDROTHERMAL ALTERATION AND MINERALIZATION OF THE DEEP GRASBERG PORPHYRY CU-AU DEPOSIT, PAPUA, INDONESIA

Hydrothermal fluids play an active and important role in the development of intrusion-hosted metal deposits. Thus, defining the evolution of these hydrothermal fluids is essential to understanding the mineralizing systems. The Grasberg porphyry Cu-Au deposit records changes in the composition of the hydrothermal fluids primarily in quartz crystals found in the extensive stockwork. The crystals record both spatial and temporal evolution of the hydrothermal fluids contained within each of the 3-Ma concentrically emplaced quartz monzodiorite to diorite intrusive units.

Microbeam scanned luminescence techniques reveal nearly ubiquitous distinct zonation patterns within quartz crystals in the deep Grasberg stockwork between 2450 and 3050 m elevation. The zonation pattern typically mimics the euhedral shape of the crystal, although minor defects in this pattern have been observed. Although zonation is found consistently within each of the intrusions, the zonation pattern is not identical, even among crystals in direct contact. Trace element mapping of these zoned quartz crystals reveals that Al and Ti vary with the zonation pattern, such that higher Al and Ti content correspond to the lighter luminescence zones; Fe shows no such correlation. The observed zonation pattern may be attributable to slight variations in fluid composition or temperature as the crystals were precipitated.

Fluid inclusion populations indicate that a range of fluid compositions were trapped during quartz precipitation and later mineralizing events. Four distinct types of fluid inclusions have been identified in quartz crystals within the stockwork: fluid + vapor; fluid + vapor + halite; fluid + vapor + sulfide; fluid + vapor + halite + sylvite + anhydrite + sulfide. The majority of inclusions in most crystals are limited to a simple composition of fluid + vapor +/- halite; however, many crystals contain each of the four types. Studies are in progress to use luminescent zonation as a guide to establishing the relationship among the fluid inclusion types.

High resolution X-ray computed tomography reveals the 3-D nature of the stockwork, including the relationship of quartz-filled fractures to the metallic mineral concentrations.