Abstract

The Boco prospect is a large, fault dismembered, pipelike, hydrothermally altered zone in the Mount Read Volcanics of western Tasmania. It is a synvolcanic alteration zone hosted by felsic volcanic rocks formed in a subaqueous proximal intracaldera setting. Previous detailed geochemical and geophysical surveys and extensive drill testing have indicated it contains no economic metals.

The strong to intense, pervasively quartz + phyllosilicate + pyrite-altered northern segment of the prospect is semiconcentrically zoned. Short wavelength infrared (SWIR) spectral analysis has revealed that phyllosilicate assemblages grade from phengitic white mica in the least altered peripheries, through normal potassic white mica, to central zones containing kaolinite, slightly sodic white mica, and pyrophyllite. Mass balance calculations indicate average net mass losses in the altered facies were about 10 to 30 g/100 g, mainly owing to loss of SiO2, which implies very high hydrothermal water-rock ratios. Whole-rock oxygen isotope compositions of the enclosing least altered felsic rocks (δ18O values 8.2–11.7‰) are indistinguishable from those of altered facies (9.6–11.8‰). We attribute the former to low-temperature diagenetic isotopic exchange with 0 per mil δ18O seawater in the peripheral least-altered zones, and the latter to exchange with 3 to 6 per mil δ18O hydrothermal fluids at high water/rock ratios and temperatures generally greater than 220°C, and locally greater than 270°C, in the intensely altered facies. Pyrite sulfur isotope compositions in the Boco altered facies (δ34S values 1.2–7.2‰) are distinctly lower than most Tasmanian massive sulfide deposits (6–15‰), compatible with a dominantly magmatic source of sulfur.

The alteration mineral assemblages, estimated mass changes, and isotopic data show that the Boco alteration system was formed by a large volume of focused acidic hydrothermal fluid which had an oxygen isotope composition of 3 to 6 per mil δ18O at and temperature greater than 270°C. The slightly 18O-enriched fluid isotope composition suggests derivation from either mixed magmatic fluid and seawater or isotopically evolved seawater. Its advanced argillic altered facies place Boco among a newly recognized class of southeast Australian Cambrian volcanic-hosted prospects and deposits. These include Chester, Basin Lake, Western Tharsis, and North Lyell in Tasmania, and Rhyolite Creek, Hill 800, and Mike’s Bluff in eastern Victoria. SWIR spectral analyses with field-portable spectrometers allow early discrimination of this type of hydrothermally altered system, and can potentially assist subsequent exploration in mapping facies zonation.

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