A Re–Os isotope study of sedimentary rocks and copper–gold ores from the Ertsberg District, West Papua, Indonesia

Abstract

The ores of the Ertsberg district consist of both porphyry stockwork and contact controlled mineralization related to the Grasberg and Ertsberg intrusion centers. High-grade copper mineralization with gold occurs in skarns on the periphery of each of the intrusion centers. The periphery mineralization includes the Kucing Liar skarn-related ores, the massive sulfide lenses on the borders of the Grasberg Intrusive Complex, and the giant skarn bodies of the Deep and Intermediate Ore Zones, together with the ores of the Dom complex on the margin of the Ertsberg diorite. The Ertsberg orebody, the basis of discovery of the district, is a copper–gold, roof-pendant of sedimentary strata in the diorite. In order to clarify genetic and geochronologic links between the peripheral sediment-hosted ore bodies and the intrusion-hosted ore bodies, Re–Os isotopes of the ores and subjacent Precambrian and Phanerozoic crust were analyzed. Re–Os geochronology of molybdenites was used to resolve the age relationships of the ore bodies. The Grasberg and Kucing Liar molybdenites have older molybdenite mineralization ages of 2.88 ± 0.02 Ma, and 3.01 ± 0.02 Ma, respectively, whereas the Ertsberg molybdenite has a younger age of 2.54 ± 0.02 Ma. These data agree with Ar chronologies of the alteration associated with mineralization ( Pollard and Taylor, 2000 [Pollard, P.J., Taylor, R., 2000. Geochronology of intrusive rocks and Cu–Au mineralization in the Ertsberg District. In: Proceedings of Freeport-Institut Teknologi Bandung (ITB) Geological Symposium, Bandung, Java, pp. 17–19.]), and also indicate that the Grasberg and Kucing Liar molybdenites are closely similar in age, and older than the Ertsberg Intrusive System. Re–Os data from pyrite, chalcopyrite, magnetite, gold, and the surrounding sedimentary rocks from the district define a mixing relationship between samples from the initial stockwork mineralization and peripheral ores of each intrusion center. Sulfide and oxide minerals from primary stockwork mineralization are isotopically homogeneous and define trends on isochron diagrams with distinct Os initial ratios, whereas sulfides and oxides taken from peripheral ores are isotopically heterogeneous and define trends on mixing diagrams. One possible end member for the mixing is sedimentary rock surrounding the porphyries. The measured Os ratios of the sedimentary rocks range from 0.52 to 2.21, and overlap the Os initial ratios of the sulfides and oxides sampled from both stockwork and contact mineralization. The data suggest a model in which the continental crust supplied some of the Os (and by inference other metals) during magma genesis in the lower crust and the remainder came from sedimentary rocks near the intrusion centers.