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Abstract: 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 f O2 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, f O2 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. [Copyright &y& Elsevier]

Mt Chalmers is a mound-style, volcanic-hosted massive sulfide (VHMS) deposit in central Queensland, Australia. The ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of Early Permian age. The two ore lenses (Main lode and West lode) consist of Cu–Zn–Pb massive sulfide underlain by Cu-rich stringer mineralization. Textural and petrographic investigations of fluid inclusions indicate that primary Type I inclusions up to 25 μm are found in quartz from the stringer mineralized zone, and microthermometric studies of these inclusions yielded homogenization temperatures of 160–285 °C and salinities of 5–10.5 NaCl equiv. wt.%. Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2 (0.1–1 mol%) in the Mt Chalmers VHMS systems. Semiquantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Mt Chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids. Cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids. High base metal content and the presence of CO2 in the fluid inclusions imply that magmatic input of ore metals, copper in particular, accompanying seawater leaching of the footwall volcanic pile is a distinct possibility.In terms of fluid composition, the K–Ca–Fe variation of the Mt Chalmers ore fluid is comparable with those of typical epithermal deposits (e.g., Thames, New Zealand) and porphyry copper deposits (Bingham, UT). The Cu–Zn–Fe/10 plot of the Mt Chalmers ore fluids, indicates that there is significant copper in the system, comparable to copper enrichment in a porphyry copper system. The Mt Chalmers ore fluids also show similar copper content with the Cu-rich end-member ore fluid composition of the mineralized Stage 2B veins of the Hellyer VHMS deposit, whereas the Stage 2A veins of Hellyer are more enriched in lead and zinc. Overall, the ore fluids have a variable chemistry with a continuum of compositional data from VHMS to epithermal-porphyry style ore fluids.Shallow-water emplacement (<300 m) for the VHMS mineralization has been postulated for the Mt Chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies. However, fluid inclusion studies do not rule out a moderate to deeper submarine environment, as there is no fluid inclusion evidence of boiling. At Mt Chalmers, boiling of ore fluids would probably have occurred (as in most epithermal systems) if the ore fluid exhaled onto the seafloor under a shallow environment. [Copyright &y& Elsevier]