Abstract

The Mount Read Volcanics of western Tasmania host a number of polymetallic volcanogenic massive sulfide deposits, a cluster of disseminated Cu-Au deposits at Mount Lyell, and a high-grade gold deposit (avg grade 27 ppm Au) at Henty. The Henty deposit occurs at the base of the Tyndall Group, a sequence of rhyolite lavas and crystal-rich volcaniclastics. Fossiliferous sediments and welded ignimbrites within this group are indicative of a local shallow-water environment. Mineralization occurs in a conformable package of altered rocks in the footwall of a major fault. The host rocks were strongly deformed during a later compressional event. The deposit has a distinct asymmetry in its alteration and metal distribution. An exhalite unit is zoned outward from massive pyrite with minor lenses of massive sulfide, to massive calcite with red jasper, to a chlorite-calcite rock. Gold occurs with pyrite, chalcopyrite, and galena in late fractures in an intensely silicified zone in the footwall of the massive pyrite. This zone is surrounded by silica-sericite-pyrite-altered rocks with disseminated base metals. During the deformation, the silicified rocks were repeatedly fractured whereas the sericite-rich rocks became high-strain zones. Pressure solution of sulfides enhanced gold transport into late brittle fractures. High gold values are confined to the footwall of proximal parts of the exhalite, indicating that gold was introduced during the volcanogenic massive sulfide event and that remobilization was a localized process.

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