Microthermometry and chemical composition of fluid inclusions from the Mt Chalmers volcanic-hosted massive sulfide deposits, central Queensland, Australia: implications for ore genesis

Abstract

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 CO 2 (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 CO 2 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.