Lithogeochemical and stable isotopic insights into submarine genesis of pyrophyllite-altered facies at the Boco Prospect, western Tasmania

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 (delta O-18 values 8.2-11.7 parts per thousand) are indistinguishable from those of altered facies (9.6-11.8 parts per thousand). We attribute the former to low-temperature diagenetic isotopic exchange with 0 per mil delta O-18 seawater in the peripheral least-altered zones, and the latter to exchange with 3 to 6 per mil delta O-18 hydrothermal fluids at high water/rock ratios and temperatures generally greater than 220 degrees C, and locally greater than 270 degrees C, in the intensely altered facies. Pyrite Sulfur isotope compositions in the Boco altered facies (delta S-34 Values 1.2-7.2 parts per thousand) are distinctly lower than most Tasmanian massive sulfide deposits (6-15 parts per thousand), 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 delta O-18 at and temperature greater than 270 degrees C. The slightly O-18-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