Evolution and source of ore fluids in the stringer system, Hellyer VHMS deposit, Tasmania, Australia: evidence from fluid inclusion microthermometry and geochemistry

The Hellyer deposit is a classic, large tonnage, high-grade, mound style volcanic-hosted massive sulphide (VHMS) deposit in the Cambrian Mt Read Volcanic belt of western Tasmania. In the footwall directly underlying the deposit, there is an extensively altered pipe which contains a well developed and preserved stringer zone. The vein paragenesis at Hellyer indicates that premineralization Stage 1 veins consist entirely of quartz, and occur throughout the alteration pipe. The synmineralization Stage 2 veins are the most abundant veins in the stringer zone and consist of three sub-stages: Stage 2A veins of crustiform quartz, pyrite, and carbonate with minor amounts of chalcopyrite, sphalerite and galena. Stage 2B veins with abundant base metal sulphides, minor quartz, carbonate and barite gangue and Stage 2C veins of coarsely crystalline barite with variable amounts of pyrite, sphalerite, galena and carbonate. Stages 3-6 veins are postmineralization veins and are related to the Devonian Tabberabberan Orogeny. Textural, petrographic and microthermometric investigations of fluid inclusions in the Hellyer stringer system indicate that Type I, primary, liquid-vapour inclusions occur along growth planes of crustiform quartz crystals or within colour banding of zoned sphalerite. These inclusions are 10-15 μm in size, and yielded homogenisation temperatures of 170-220°C in early 2A veins. 165-322°C in main-stage 2B veins and 190-256°C in late-stage 2C veins. These data suggest a waxing and waning thermal history. However, the average salinity remained between 8-11 NaCl equiv. wt% in all Stage 2 veins. Chalcopyrite-bearing primary fluid inclusions have been recognised in the base metal-rich Stage 2B veins. No evidence for presence of CO2 (e.g. formation of clathrates) was recorded by microthermometry. However, Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2 (< 1 mole%) in the Stage 2B veins, and no detectable CO2 in 2A and 2C vein stages. Semi-quantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Hellyer ore fluid was enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz indicates that the Stage 2B ore fluids have a significantly higher base metal concentration compared to the Stage 2A veins. The postmineralization Stage 4 veins have a variable but lower base metal content. In this study, there was no fluid inclusion evidence of boiling. Cation composition, higher salinities relative to seawater and the presence of CO2, suggest that recycled seawater alone cannot be the sole source of the ore fluids. This interpretation is in agreement with previous isotopic studies in the Hellyer stringer system. Although direct input of bulk ore constituents from a magma chamber cannot be demonstrated from the present fluid inclusion data, such a contribution of ore fluids from a magmatic source cannot be ruled out. The possible input from the magmatic source may have occurred during the base metal-rich Stage 2B vein formation characterised by the intensifying temperature of deposition, higher base metals and CO2 contents.