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 sty le volcanic-hosted massive sulphide (VHMS) deposit in the Cambrian Mt Read Volcanic belt of western Tasmania. In the footwall directly under lying 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 entire ly of quartz, and occur throughout the alteration pipe. The synmineral ization 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 q uartz, carbonate and barite gangue and Stage 2C veins of coarsely crys talline barite with variable amounts of pyrite, sphalerite, galena and carbonate, Stages 3-6 veins are postmineralization veins and are rela ted to the Devonian Tabberabberan Orogeny. Textural, petrographic and microthermometric investigations of fluid inclusions in the Hellyer st ringer system indicate that Type I, primary, liquid-vapour inclusions occur along growth planes of crustiform quartz crystals or within colo ur banding of zoned sphalerite. These inclusions are 10-15 mu m in siz e, and yielded homogenisation temperatures of 170-220 degrees C in ear ly 2A veins, 165-322 degrees C in main-stage 2B veins and 190-256 degr ees C in late-stage 2C veins. These data suggest a waxing and waning t hermal history. However, the average salinity remained between 8-11 Na Cl 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 r ecorded by microthermometry. However, Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2 (< 1 mole%) in the Stage 2B vei ns, and no detectable CO2 in 2A and 2C vein stages. Semi-quantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate t hat the Hellyer ore fluid was enriched in potassium and calcium but de pleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz indicates that the Stage 2B ore fluids have a sig nificantly higher base metal concentration compared to the Stage 2A ve ins. The postmineralization Stage 4 veins have a variable but lower ba se metal content. In this study, there was no fluid inclusion evidence of boiling. Cation composition, higher salinities relative to seawate r and the presence of CO2, suggest that recycled seawater alone cannot be the sole source of the ore fluids. This interpretation is in agree ment with previous isotopic studies in the Hellyer stringer system. Al though direct input of bulk ore constituents from a magma chamber cann ot be demonstrated from the present fluid inclusion data, such a contr ibution of ore fluids from a magmatic source cannot be ruled out. The possible input from the magmatic source may have occurred during the b ase metal-rich Stage 2B vein formation characterised by the intensifyi ng temperature of deposition, higher base metals and CO2 contents.