Noble gas and halogen constraints on Mt Isa Cu ores

Abstract

Noble gas isotopes of He, Ar, Kr and Xe and the halogens Cl, Br, I present within fluid inclusions of hydrothermal quartz or sulphide, provide new insights on the origin of mineralising fluids. He and Ar isotopic ratios enable contributions of meteoric, crustal or mantle volatiles to be quantified. Together with Kr and Xe they can also provide evidence for phase separation or the presence of modern air. The halogens Cl, Br and I provide otherwise unavailable information on the acquisition of salinity. Contrasting styles of Cu-rich ore deposit in the Mt Isa terrane have been selected for study. The noble gas and halogen data indicate the composition of Proterozoic crustal fluids of diverse origin and from a deeper level of the crust than has hitherto been sampled. Magmatic and metamorphic fluid end-members provide insights into the composition of the Proterozoic mantle and deep crustal devolatilisation processes. Mineralisation resulted from mixing of surface- derived crustal fluids and deeply derived magmatic fluids in the east, and metamorphic devolatilisation fluids in the west.