BR CL GEOCHEMISTRY OF HYDROTHERMAL BRINES ASSOCIATED WITH PROTEROZOICMETASEDIMENT-HOSTED COPPER MINERALIZATION AT MOUNT ISA, NORTHERN AUSTRALIA/

Instrumental neutron activation analyses (INAA) of small bulk samples of vein quartz are used to estimate the Br/Cl ratio of saline fluid in clusions associated with late-metamorphic copper mineralization in qua rtz-dolomite breccias at Mount Isa. The results are compared with simi lar data from veins associated with district-scale metasomatism in pot ential copper source rocks near Mount Isa, and with quartz samples fro m other locations in northern Australia which may have trapped fluids approaching the Br/Cl composition of Paleoproterozoic halite evaporite s. Br/Cl ratios in some of the evaporite-related samples are similar t o present-day seawater (0.0035) while others range up to 0.0 1 5. All samples from the Mount Isa copper ores (fluids with total salinities m ostly between 10 and 25 wt% NaCl eq.) have consistently high Br/Cl rat ios between 0.008 and 0.018, on average 3.5 times higher than modem se awater. Fluids associated with district-scale alteration of metabasalt s up to 30 km away from the deposit have high Br/Cl ratios identical t o 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-f low paths to or from the site of ore deposition. The observed fluid co mpositions are contrary to the low Br/Cl ratios that would be expected from a simple model of ascending fluids derived by prograde metamorph ism of salt-bearing Paleoproterozoic metasediments. Mass-balance calcu lations based on mine- and district-scale alteration suggest that the high Br/Cl fluids may have originated above the present level of erosi on, as basin brines which infiltrated the terrain during the waning st ages of regional metamorphism. They may have formed in an inferred eva poritic cover sequence of Mesoproterozoic age, as residual bitterns or by diagenesis and partial dissolution of halite, similar to modem Br- enriched basin brines. Secular variation in oceanic Br/Cl, preferentia l chlorine removal from evaporite-derived metamorphic fluids by fluid- rock interaction, or an unusually Br-rich magma source beneath the dep osit, could provide alternative explanations for the high Br/Cl ratios of the Mount Isa brines, but are considered to be less likely process es in light of the available geologic data.