Gibbins, S. L., Titley, S., Mathur, R., Eastoe, C., University of
Arizona, Tucson, AZ 85721
Friehauf, K.C., Kutztown University, Kutztown, PA 19530
Geological Society of America Abstracts with Programs, Reno, NV,
November 14-16, 2000, v. 32, no. x, p. Axx.
The Grasberg Cu-Au porphyry deposit in Irian Jaya, Indonesia, is one of the largest Cu-Au deposits in the world, and manifests a range of alteration types that extend from potassic to advanced argillic. Three intrusive stages form the Grasberg Intrusive Complex (GIC): the early Dalam Fragmental and Diorite, the Main Grasberg Intrusion and the late Kali Intrusion. These intrude intensely deformed Mesozoic-Cenozoic sedimentary units.
On the southwestern margin of the GIC two stages of anomalous advanced argillic alteration are localized within the Dalam that appear to be associated with gold concentrations exceeding 9 g/t. These stages of alteration consist of: (I) a pyrophyllite-kaolinite-(sericite-amorphous clay)-quartz mineral assemblage with associated sulfide phases of pyrite (marcasite)-covellite-enargite, followed by (II) a dickite-(illite)-quartz mineral assemblage with associated sulfide phases of massive pyrite and trace chalcopyrite-bornite. Stage I is ubiquitous in the area studied, whilst stage II alteration occurs at the contact zone between the GIC and the sedimentary wall rock. A Re/Os isotope study of the mineralization associated with stages I and II indicate they contain metals from different sources, allowing the inference that fluids responsible each stage could also be different. The initial Os ratios for stage I mineralization corresponds with values obtained from the 'typical' porphyry style mineralization in the GIC. This allows interpretation that metal for the typical porphyry style mineralization and stage I may have similar metal sources. The range of initial Os ratios from stage II mineralization allows interpretation that the fluids responsible for this event scavenged metals from a non-magmatic source, possibly the subjacent Proterozoic and Phanerozoic section.
Preliminary results from a sulfur isotope study are reported here. All of the pyrite-covellite samples from stage I and II show (34S values to range between 2.2 and 3.4 , suggestive of a magmatic source for sulfur, with the exception of one covellite-marcasite sample from stage I with (34S between 9.3 to -20.3 . This sample allows interpretation of a sedimentary origin for the sulfur.
November 14-16, 2000 Kurt C. Friehauf