A fluid inclusion study of uranium and copper mineral systems in the Murphy Inlier, Northern Australia

The Palaeoproterozoic Murphy Inlier is situated at the southern end of the McArthur Basin in Northern Australia. The inlier contains over 50 uranium, copper, tin and base metal occurrences. Fluid inclusion studies were carried out on samples of quartz veining from the uranium and copper deposits as well as from the basement rocks to determine the composition of the fluids and to investigate how uranium and copper were transported in these fluids. Four types of fluid inclusions were observed in this study; Type A vapour-rich inclusions with ≥30 vol.% vapour, Type B two phase aqueous inclusion with ≤20 vol.% vapour, Type C multiphase inclusions with one or more solid phases and, Type D liquid-only inclusions.At least three different fluids were identified in the Murphy Inlier. There is a CaCl2 ± LiCl-rich brine, a NaCl-rich brine and a low salinity fluid. The fluids can also be grouped into a high temperature (typically homogenising above 210 °C) population and a low temperature (typically homogenising below 240 °C) population. Depending on location, the high temperature fluid may be enriched in CO2, N2, or CH4. In the uranium deposits, gas-rich inclusions are dominated by CO2 indicating that these fluids are relatively oxidized, while in the copper deposits both CO2 and CH4 are present indicating that these fluids are more reduced. The low temperature population of Type B inclusions have a mode of homogenisation at 190 °C in the uranium deposits and a mode at 120 °C in the copper deposits. Similarly, Type C inclusions have a mode of homogenisation at 235 °C in the uranium deposits and 170 °C in the copper deposits.Variations in the composition of the inclusions suggest that at least two stages of fluid mixing occurred. Firstly, there was mixing between a CaCl2 ± LiCl-rich brine and a NaCl-rich brine to produce a fluid of intermediate composition. This fluid then mixed with a low salinity fluid.Geochemical modeling has shown that both uranium and copper can be transported in the same fluid at high fO2 and moderate to high chloride concentrations. In the proposed model for mineralisation, uranium and copper were leached from the volcanics or sediments in the McArthur Basin and were simultaneously transported in the oxidised, Na-Ca-Li-bearing saline fluids which had undergone mixing within the basin. Uranium precipitated when this fluid was reduced, either by reaction with Fe-rich mafic volcanics, carbonaceous rocks, or by mixing with a CH4-bearing, low salinity fluid derived from the basement. Copper remained in the fluid until further changes in salinity, fO2 or pH occurred, most probably as a result of mixing with lower salinity meteoric fluids. The fluid may have continued to cool to near surface temperatures as evidenced by the trapping of liquid-only fluid inclusions in some veins.