PORPHYRY CU-AU MINERALIZATION IN THE ERTSBERG INTRUSION, WEST PAPUA, INDONESIA: STYLE, TIMING, AND SOURCES OF HYDROTHERMAL FLUIDS

The Ertsberg Intrusion (EI) is located approximately 1.5 km southeast of the Grasberg super-porphyry Cu-Au deposit (GIC), in the Gunung Bijih (Ertsberg) Mining District, West Papua, Indonesia. Intrusion- and carbonate-hosted mineralization is associated with the 3.28-2.97±0.54 Ma multi-phase Ertsberg Intrusion. The orientation of the mineralized zone is parallel to the direction of porphyry dike emplacement and regional structures.

Potassic, phyllic, propylitic and endoskarn alteration types are recognized in the EI, distributed over 7 vein types. Two vein types initiate pre-porphyry dike emplacement, and mineralization occurs pre and post-dike emplacement. Cu-Au mineralization is associated with pre-dike biotite-bornite-anhydrite veinlets, and post-dike quartz-anhydrite-bornite+chalcopyrite//green sericite veins. Cu mineralization is not associated with late quartz-pyrite//white sericite mineralization; however, Au (in electrum and tellurides) has been recognized in each alteration type except the endoskarn.

Sulfides associated with each alteration type in the EI have δ³⁴S values that range between -3.0 to 3.6‰. Sulfate δ³⁴S values are variable between different alteration types: potassic (9.6-11.1‰) and hydrolytic (10.2-16.6‰). The bulk isotopic values associated with the porphyry-style mineralization remained consistent throughout the life of the hydrothermal, suggesting a common, magmatic source (Porter, 2004). Sulfide-sulfate equilibrium temperatures for potassic alteration in the EI average 574°C, approximately 125°C cooler than sulfide-sulfate equilibrium temperatures in the GIC. Preliminary oxygen isotope data from potassic alteration in the EI give δ¹⁸O values between 9.2-10.1‰ for quartz; >1‰ higher than δ¹⁸O values for quartz in potassic alteration in the GIC.

Initial mass balance calculations using the EI volume estimate, and the known mineralization associated with the EI show that the EI has an insufficient volume of H₂O to account for the known volume of hydrothermal alteration and mineralization. Coupled with preliminary sulfur and oxygen isotope data, and Re-Os isotope source data, this suggests additional input of hydrothermal fluids from deeper magmatic and sedimentary sources.