Significance of high temperature fluids and melts in the Grasberg porphyry copper‑gold deposit

Abstract

A study of quartz-sulfide veins from the Grasberg Cu-Au deposit has shown that the quartz crystals initially grew from the walls of the veins into open fractures. Cathodoluminescence (CL) imaging shows concentric growth zones in the larger crystals and changes in the orientation of the growth zones with occasional truncation of the zoning. Towards the centre of veins the crystals become smaller and irregularly shaped but exhibit CL banding parallel to vein walls, which is normally associated with crack-seal processes. The vein quartz contains silicate and sulfide-rich melt inclusions, virtually water-free salt-melt inclusions, and coexisting hypersaline and vapour-rich inclusions. Late stage, secondary aqueous inclusions are also present in some veins. Most fluid inclusions homogenised at temperatures below 620 °C. However, hypersaline (B2) inclusions had a very complex behaviour and exhibited partial homogenisation to salt melt and clear immiscible fluid at temperatures ranging from 820 °C to 1300 °C. These inclusions contain the highest Cu concentrations (up to 6.3 wt%), suggesting they play a role in metal transport.The contemporaneous trapping of the K-feldspar-rich melt inclusions, hypersaline and vapour-rich inclusions suggests a common source for these fluids and a process involving heterogeneous entrapment of immiscible silicate melt, hypersaline fluid and vapour as previously proposed for other porphyry copper deposits. Furthermore, the Grasberg deposit is estimated to have formed at pressures below 400 bar, which may explain the presence of salt-melt inclusions as the vapour + halite field of the H2O-NaCl system is dominant below 800 °C at these pressures.The unrealistically high temperatures reported herein may have resulted from entrapment of either immiscible silicate melt, hypersaline fluid and vapour or cumulates of an evolving silicate melt saturated in K-spar, brine and vapour. Although these temperatures may not be directly useful, these inclusions give important clues to metal-enrichment processes. These melt and fluid inclusions record cycles of transitory, high-temperature (>700 °C) hydrofracturing, melt and fluid release, and vein formation in a cooler (500–600 °C) background host-rock thermal regime. High sulfur and Cu contents of sulfide-melt and hypersaline B2 inclusions are interpreted to be the fluid composition before the main sulfide precipitation event, and, therefore, may be used as tracers of the magmatic body from which they exsolved at depth.