Geochemistry of hypersaline fluid inclusions from the Starra (Fe oxide)-Au-Cu deposit, Cloncurry district, Queensland

Studies of liquid-vapor-multisolid (L-V-nS) and liquid-vapor +/- halite flu id inclusions suggest that at least three fundamentally different brines we re involved in the genesis of the Proterozoic ironstone-hosted Starra Au-Cu deposit. Element concentrations determined from proton-induced X-ray emiss ion (PIXE) analysis of groups of L-V-nS inclusions from the Parry barren ma gnetite ironstone stage (400 degrees -550 degreesC) and the younger mineral ization stage (220 degrees -360 degreesC) covary over concentration ranges greater than the deduced analytical errors. The L-V-nS inclusions in the tw o stages are ver); different, with higher ratios: of Fe and K to Ca. Mn, Cu , Zn, Sr, Ba, and Pb ill the ironstone stage. Quite distinct, pseudoseconda ry NaCl-CaCl2-H2O inclusions are abundant in mineralization-stage quartz, c alcite, and anhydrite. High concentrations of Cu ica. >1,000 ppm) are prese nt in some L-V-nS inclusions of both stages, and the data suggest that larg e amounts of Cu, Mn, Zn, As, Ba, and Pb passed through the Starra system as it evolved. Cl/Br ratios in the inclusions are mostly less than 0.002 and clearly different from those of ore fluids of the giant sedimentary-hosted copper orebody at Mount Isa. This could reflect a major magmatic contributi on to the salinity). at Starra that is consistent with the preferred origin of the water in published stable isotope studies. Ba concentrations of at least several wt percent in some mineralization-stage L-V-nS inclusions are associated with oxidized sulfur-bearing mineral parageneses. Given the low solubility of barite this suggests that a(S2)-f(O2) relationships varied d uring mineralization.