Combined Focused Ion Beam – Scanning Electron Microscope and Synchrotron X-ray Fluorescence analysis of multi-solid and melt inclusions from the super-giant Grasberg Cu[sbnd]Au deposit, Indonesia

Previous studies of the mineralised quartz veins from the Grasberg Cu[sbnd]Au deposit in Indonesia have shown that they contain multi-solid inclusions and silicate melt inclusions. The multi-solid inclusions exhibit salinities up to 75 wt% NaCl equiv. and ultrahigh homogenisation (Vapour + Liquid + Solid → Liquid) temperatures, up to 1281 °C, but the processes leading to their formation are poorly understood. To determine the composition of these inclusions they were analysed by Focused Ion Beam – Scanning Electron Microscope (FIB-SEM) and Synchrotron X-ray Fluorescence (SXRF) techniques. FIB-SEM analysis of two representative multi-solid inclusions (MSI) showed that they contain a large halite and a smaller sylvite crystal and mixed K[sbnd]Fe chloride crystals. K-feldspar crystals occurring inside and outside the multi-solid inclusions indicate that they were trapped from a melt phase. The ore-mineral chalcopyrite was also identified in one inclusion. Opening the multi-solid inclusions under vacuum resulted in a coating enriched in Na, K, Fe and Cl that precipitated on the crystals and walls of the inclusions. FIB-SEM analysis of a coexisting silicate melt inclusion showed that it consisted mainly of a devitrified glass with approximately K-feldspar composition that also contained small crystals of iron oxide. Further milling of the melt inclusion exposed a Cu[sbnd]Fe-rich silicate mineral. SXRF analysis of another multi-solid inclusion indicated that Br and Zn occur in the aqueous phase whereas K, Ca, Fe, Pb and Mn are hosted by solid phases within the inclusion. Modelling of mineralisation processes demonstrate that a supercritical, intermediate density fluid (with ~5 wt% NaCl) underwent an initial pressure drop causing phase separation and the formation of a vapour-rich fluid and a small fraction of brine, which led to the formation of salt hydrates, salt melts or both within the MSI. The coexistence of silicate melt inclusions and the occurrence of hematite and, K-feldspar in MSI suggest that a coeval silicate melt is also associated with mineralisation at the Grasberg deposit. The high salinity, multi-solid inclusions and melt inclusions form early in the vein paragenesis and record the origin and initial processes of ore deposition. Furthermore, the high metal-bearing capacity of salt melts suggests that the observation of coexisting silicate and salt melts may indicate the presence of a fertile mineral system.