Abstract

The Renison granite in northwestern Tasmania is genetically related to cassiterite sulfide mineralization (carbonate replacement) at Renison Bell, one of the largest Sn deposits in the world. The magmatic hydrothermal system responsible for the mineralization in the roof sediments is also responsible for the development of three zones of extensive hydrothermal alteration in the granite. The tourmaline zone (tourmaline + quartz + topaz + fluorite + sericite + cassiterite) is in part surrounded by a sericite zone (sericite + quartz + tourmaline + calcite + cassiterite). An albite zone is characterized by albite + quartz + chlorite + sericite along with primary plagioclase and K feldspar. The Renison granite is a reduced felsic granite with biotite as the only mafic mineral. It is characterized by high contents of SiO 2 (70.9-75.3%) and K 2 O (4.69-5.91%), but low Na 2 O (2.52-3.12%). High amounts of Rb (up to 918 ppm) and Sr (as low as 19 ppm), as well as other trace element abundances in the most felsic rocks, indicate that parts of the magma were highly fractionated, and as a result, enriched in volatile constituents, mostly aqueous. During hydrothermal alteration, B, Fe, and F were introduced into the tourmaline zone, whereas Na and K were removed. In the sericite and albite zones, chemical and petrographic effects of alteration are less conspicuous and are marked by changes in the concentrations of Na and Ca and small changes in K. Norms of unaltered granites project as a tight cluster on the haplogranite plane (quartz-albite-orthoclase) whereas altered rocks are widely scattered. Normal trace element concentrations defining-fractionation trends of igneous origin have surveyed the effects of alteration in albite and sericite zone rocks as well as in weakly to moderately altered, tourmaline zone rocks. However, depletion in Rb, St, and Ba occurs in strongly tourmalinized rocks in which the primary mineral fabric is completely replaced by secondary (hydrothermal) mineral fabric...

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