A chemical model for the Devonian remobilization process in the Cambrian volcanic-hosted massive sulfide Rosebery deposit, western Tasmania

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 e nd 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 thi s pluton have been recognized transgressing Cambrian volcanic-hosted massive sulfide minerals at the south end of the R osebery orebody. Cambrian volcanic-hosted massive sulfide mineralization at Rosebery consists of three primary zones: a lowermost pyrite-chalcopyrite zone, overlain by a sphalerite-galena +/- pyrite zone, and further overlain by a mas sive barite +/- carbonate zone. Three major Devonian replacement zones are also present: (1) a magnetite-biotite +/- chalcopyrite zone, (2) a pyrrhotite-pyrite zone, and (3) a tourmaline-quartz +/- magnetite zone. M inerals such as fluorite, garnet, and helvite are also present in the repla cement zones. Metal zonation studies indicate that zinc occurs dominantly in a blanketlik e enrichment zone in the primary sulfide lenses of the Rosebery deposit. Co ld is largely concentrated at the top of the ore lenses. In the transgressi ve pyrrhotite-pyrite replacement zone at the south end of the Rosebery depo sit, zinc is conspicuously depleted but gold concentrations of more than 20 g/t are observed. In contrast, the biotite-, magnetite-, and tourmaline-be aring 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 bioti te-magnetite zone, as well as in the pyrrhotite-pyrite zone. Microthermometry and laser Raman spectroscopy studies of fluid inclusions f rom the Devonian replacement zone indicate that the early biotite-magnetite and pyrrhotite-pyrite assemblages formed from the interaction of moderate- to high-temperature (greater than or equal to 330 degrees C), saline (10-2 5 wt % NaCl equiv), CO2-bearing fluid with the original stratiform lead-zin c mineralization. The later stage tourmaline veining and associated replace ment assemblages resulted from lower temperature (less than or equal to 300 degrees C), less saline (less than or equal to 10 wt % NaCl equiv) fluid. The high temperature, high-salinity, and CO2-bearing metasomatic fluid supp ort 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 b ecause of the high temperature and high salinity of the fluid. In compariso n to copper and gold, zinc and lead were dissolved and removed, probably as chloride complexes.