The distribution, mineralogy and paragensis of the Hellyer Baritic and Siliceous Caps

Baritic and siliceous caps are a feature of many volcanic-hosted massive sulphide deposits. Their distribution, mineralogy and geochemis~ are important to the understanding of the genesis of these deposits. The baritic and siliceous caps overlying the Hellyer volcanic-hosted massive sulphide deposit, western Tasmania, show a spatial affinity to the centres of hydrothermal activity, identified previously from copper contents of the ore. Mineralogical and textural investigations identified barite as a precursor to formation of the siliceous cap. In both caps intricate sulphide textures, including pristine collofom pyrites, are present Mineralogical and spatial relationships suggest an interdigitation of the baritic and siliceous caps. Sulphur isotope studies of barite (with 034S values between +38 and +50 per mil) and pyrite (with 034-S values between +6 and+ 18 per mil) revealed a duality in the source of sulphur during formation of the caps; incompletely reduced seawater sulphur, and magmatic sulphur. The wide range of o34S values are thought to be the result of fluctuating contributions from these two sulphur sources. Metal zonation and mineral geochemical studies show that base and precious metal contents of both caps are enriched proximal to underlying massive sulphides. In such zones, textural evidence supports hydrothermal overprinting and porosity infill by paragenetically late sulphides. Formation of the barite cap at or above the seawater interface is interpreted to be the result of oxygenated seawater mixing with spent hydrothermal fluids enriched in barium, during periods of low hydrothermal flux, at temperatures between 230 and 250°C. By contrast, silica cap precipitation requires the local dominance ofH2S and a combination of conductive cooling and mixing. This is evidenced by the presence of arsenopyrite, and the absence of hematite. The interdigitating spatial affmity combined with mineralogical and textural evidence suggests that formation of the baritic and siliceous caps at Hellyer was an integral part of orebody formation. Thus these facies evolved with the growing sulphide mound, in a manner consistent with the rone refming model proposed by Eldridge et al. (1983), for the growth of seafloor sulphide deposits.