Br/Cl geochemistry of hydrothermal brines associated with Proterozoic metasediment-hosted copper mineralization at Mount Isa, northern Australia

Abstract

Instrumental neutron activation analyses (INAA) of small bulk samples of vein quartz are used to estimate the Br/Cl ratio of saline fluid inclusions associated with late-metamorphic copper mineralization in quartz-dolomite breccias at Mount Isa. The results are compared with similar data from veins associated with district-scale metasomatism in potential copper source rocks near Mount Isa and with quartz samples from other locations in northern Australia which may have trapped fluids approaching the Br/Cl composition of Paleoproterozoic halite evaporites. Br/Cl ratios in some of the evaporite-related samples are similar to present-day seawater (0.0035) while others range up to 0.015. All samples from the Mount Isa copper ores (fluids with total salinities mostly between 10 and 25 wt% NaCl eq.) have consistently high Br/Cl ratios between 0.008 and 0.018, on average 3.5 times higher than modern seawater. Fluids associated with district-scale alteration of metabasalts up to 30 km away from the deposit have high Br/Cl ratios identical to those of the ore fluids. This indicates a common source of the salts and supports a tentative correlation between mine- and district-scale alteration events and may ultimately help to trace potential fluid-flow paths to or from the site of ore deposition. The observed fluid compositions are contrary to the low Br/Cl ratios that would be expected from a simple model of ascending fluids derived by prograde metamorphism of salt-bearing Paleoproterozoic metasediments. Mass-balance calculations based on mine- and district-scale alteration suggest that the high Br/Cl fluids may have originated above the present level of erosion, as basin brines which infiltrated the terrain during the waning stages of regional metamorphism. They may have formed in an inferred evaporitic cover sequence of Mesoproterozoic age, as residual bitterns or by diagenesis and partial dissolution of halite, similar to modern Br-enriched basin brines. Secular variation in oceanic Br/Cl, preferential chlorine removal from evaporite-derived metamorphic fluids by fluid-rock interaction, or an unusually Br-rich magma source beneath the deposit, could provide alternative explanations for the high Br/Cl ratios of the Mount Isa brines, but are considered to be less likely processes in light of the available geologic data.