Abstract

The Rosebery deposit in western Tasmania is a polymetallic massive sulfide deposit hosted in felsic volcanics of the Cambrian Mount Read Volcanic belt. The deposit underwent upper greenschist facies regional metamorphism and related deformation during the Devonian Tabberabberan orogeny, resulting in folding, shearing, and faulting (thrusting) of the ore lenses. The south end of the Rosebery deposit has undergone metasomatic replacement related to a postorogenic Devonian granitoid pluton, whose existence is inferred from geophysical (gravity) data. Metasomatic mineral assemblages related to this pluton have been recognized transgressing Cambrian volcanic-hosted massive sulfide minerals at the south end of the Rosebery orebody. Cambrian volcanic-hosted massive sulfide mineralization at Rosebery consists of three primary zones: a lowermost pyrite-chalcopyrite zone, overlain by a sphalerite-galena + or - pyrite zone, and further overlain by a massive barite + or - carbonate zone. Three major Devonian replacement zones are also present: (1) a magnetite-biotite + or - chalcopyrite zone, (2) a pyrrhotite-pyrite zone, and (3) a tourmaline-quartz + or - magnetite zone. Minerals such as fluorite, garnet, and helvite are also present in the replacement zones. Metal zonation studies indicate that zinc occurs dominantly in a blanketlike enrichment zone in the primary sulfide lenses of the Rosebery deposit. Gold is largely concentrated at the top of the ore lenses. In the transgressive pyrrhotite-pyrite replacement zone at the south end of the Rosebery deposit, zinc is conspicuously depleted but gold concentrations of more than 20 g/t are observed. In contrast, the biotite-, magnetite-, and tourmaline-bearing replacement zones contain low gold grades (generally less than 5.0 g/t Au). Copper concentrations as high as 2 percent are observed in the biotite-magnetite zone, as well as in the pyrrhotite-pyrite zone. Microthermometry and laser Raman spectroscopy studies of fluid inclusions from the Devonian replacement zone indicate that the early biotite-magnetite and pyrrhotite-pyrite assemblages formed from the interaction of moderate- to high-temperature (> or =330 degrees C), saline (10-25 wt % NaCl equiv), CO 2 -bearing fluid with the original stratiform lead-zinc mineralization. The later stage tourmaline veining and associated replacement assemblages resulted from lower temperature (< or =300 degrees C), less saline (< or =10 wt % NaCl equiv) fluid. The high-temperature, high-salinity, and CO 2 -bearing metasomatic fluid support the hypothesis of derivation of this fluid from Devonian granite below the Rosebery deposit. During the Devonian metasomatic event, gold and, to a lesser extent, copper were most likely remobilized as chloride complexes because of the high temperature and high salinity of the fluid. In comparison to copper and gold, zinc and lead were dissolved and removed, probably as chloride complexes.

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