SKARN CU-AU OREBODIES OF THE GUNUNG-BIJIH (ERTSBERG) DISTRICT, IRIAN-JAYA, INDONESIA

The major Cu-Au skarn deposits of the Gunung Bijih (Ertsberg) district in central Irian Jaya are products of hydrothermal systems that devel oped in association with Pliocene magma emplacement in an active conti nental margin. The Cu-Au skarn orebodies occur within a Cretaceous to Tertiary sedimentary sequence that was deformed as the northern Austra lian continental margin entered a north-dipping subduction zone at app roximately 12 Ma. The intermediate-composition intrusions consist of f ine-grained porphyritic stocks, dikes, and sills that have K-Ar ages r anging from 2.7 to 4.4 Ma. Most intrusions are slightly potassic, but these data could be affected by alteration. The skarn orebodies in the Ertsberg district are hosted in deformed lower Tertiary New Guinea Gr oup carbonate strata along the periphery of the Pliocene Ertsberg intr usion. Major skarn orebodies include the Ertsberg (GB), the Ertsberg E ast (GBT) complex, including the GBT, the Intermediate Ore Zone (IOZ) and the Deep Ore Zone (DOZ), and the Dom. Chalcopyrite is the dominant ore mineral in the GB and Dom orebodies, whereas bornite dominates in the GBT complex. Native Au occurs within bornite and chalcopyrite in GB and GBT ores. The district calc-silicate alteration assemblages are characterized by high-temperature skarn minerals, including forsterit e, monticellite, and minor melilite. Diopsidic clinopyroxene is common , particularly in GBT. Anhydrite and phlogopite are abundant in the GB T complex, and the anhydrite: calcite ratio increases with depth from GBT to DOZ where anhydrite is ubiquitous and calcite rare. At least th ree types of garnets have been identified at the Dom and show a progre ssive increase in ferric iron content. Garnet decreases with depth in the GBT complex. Talc, serpentine, tremolite-actinolite, and chlorite are common retrograde minerals. Copper sulfide mineralization is textu rally associated with early retrograde alteration. Differences among t he skarn orebodies are related in part to variable protolith compositi on that affected skarn development within different stratigraphic posi tions. Distinctive fossil replacement textures preserved within skarn indicate that the Oligocene-Miocene Ainod Formation is the most likely protolith for the GB and Dom orebodies. The GBT and upper IOZ orebodi es probably are hosted by the Eocene Faumai Formation. The DOZ and low er IOZ orebodies, dominated by magnesian skarn alteration, appear to b e developed in a dolomitic unit within the lower New Guinea Limestone Group, which probably is equivalent to the Paleocene Waripi Formation.