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

Previous studies of the mineralised quartz veins from the Grasberg CuAu 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 KFe 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 CuFe-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.