The Magmatic and Hydrothermal Evolution of the Ertsberg Intrusion in the Gunung Bijih (Ertsberg) Mining District, West Papua, Indonesia

Abstract

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 intrusive complex. The orientation of the intrusion-hosted mineralized zone is parallel to the direction of porphyry dike emplacement in the intrusive complex and to regional structures. Potassic, phyllic, propylitic and endoskarn alteration types are recognized in the EI, distributed over 7 vein types. Three vein stages 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 (Stage III), and post-dike quartz-anhydrite-bornite+chalcopyrite//green sericite veins (Stage V), and quartz-anhydrite-chalcopyrite-pyrite//white sericite veins (Stage VI). Sulfides associated with each alteration type in the EI have d³⁴S values that range between -3.0 to 3.6‰. Sulfate d³⁴S between alteration types are variable: potassic (9.6- 11.1‰) and hydrolytic (10.2-16.6 ‰). The bulk isotopic sulfur (d³⁴S(SS)) composition for fluid in equilibrium Stage III veins is 7.5‰, which is higher than would expected for an oxidized calc-alkaline fluid, thus I invoke the addition of heavy sulfur from the sedimentary anhydrite nodules in adjacent carbonate host rocks. There is an overall decrease in bulk isotopic sulfur (d³⁴S(SS)) composition for hydrothermal fluid throughout the span of hydrothermal activity. A degassing mafic magma chamber at depth, and/or the leaching of previously deposited sulfides are likely responsible for this decrease. 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. Calculated oxygen isotopic compositions for water in equilibrium with anhydrite from early potassic veins in both the Ertsberg Stockwork Zone and GIC suggest this component was derived from a non-magmatic source; the sedimentary anhydrite nodules are a probable source. The calculated oxygen and hydrogen isotopic compositions for water in equilibrium sericite from intermediate veins in the ESZ and GIC show the fluid was derived from a magmatic water and/or magmatic vapor; however, the water responsible for late hydrolytic alteration in both intrusive centers provides evidence for mixing of magmatic water (vapor) with meteoric water. 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 sulfur, oxygen and hydrogen isotope data, and Re-Os isotope source data, this suggests additional input of hydrothermal fluids from deeper magmatic and sedimentary sources, with moderate addition of meteoric water into the hydrothermal system during Stage VI vein formation.