A petrological and fluid inclusion study of magnetite–scheelite skarn mineralization at Kara, Northwestern Tasmania: implications for ore genesis

Abstract

The Kara magnetite–scheelite deposit is located about 40 km south of Burnie in northwestern Tasmania. The deposit consists of a number of orebodies and the total mineable reserves are 1.7 Mt of magnetite at >30% Fe and 0.3 Mt of scheelite at 0.52% WO 3. The major orebodies at Kara are hosted by the Ordovician Gordon Limestone at the southern end of the Devonian Housetop Granite, adjacent to the granite or separated from it by the Ordovician Moina Sandstone. At least four paragenetic stages of skarn formation and ore deposition have been recognized: Stage I Clinopyroxene±garnet±vesuvianite±wollastonite±quartz±scheelite, Stage II Garnet–vesuvianite–magnetite±scheelite±apatite±quartz, Stage III Magnetite–amphibole–epidote–fluorite–quartz±chlorite±garnet±vesuvianite±scheelite±carbonate±pyrite±clinopyroxene, and Stage IV Hematite±fluorite±calcite±quartz. Fluid inclusions in clinopyroxene from Stage I homogenize at 460–620°C (mode 520°C). Stage II gave homogenization temperatures of 349–578°C (mode 500°C) from scheelite and 360–570°C (mode 500°C) from vesuvianite. The Stage I clinopyroxene gave a salinity range of 10.0–12.0 equiv. wt.% NaCl with only one low value of 2.0 equiv. wt.% NaCl. The Stage II mineral assemblages also showed moderate salinities of 12.0 and 17.8 equiv. wt.% NaCl from scheelite and vesuvianite. Fluid inclusions in quartz, scheelite, calcite and fluorite from the main scheelite mineralization phase, Stage III, homogenize at 230–360°C with a mode at 300°C and salinity varies from 0.2 to 19.8 equiv. wt.% NaCl. The mineral paragenetic and fluid inclusion studies indicate that magnetite–scheelite mineralization at Kara was formed as a proximal skarn assemblage in carbonate host. The skarn formation and ore deposition occurred in stages starting with prograde anhydrous clinopyroxene–garnet metasomatism at relatively high temperatures, >500°C. This early assemblage was replaced by the subsequent retrograde deposition of hydrous mineral phases (e.g. amphibole, epidote and chlorite) as temperatures decreased to 300°C or less. Deposition of scheelite predominantly occurred in association with hydrous minerals (e.g. amphibole in Stage III). The high-temperature and moderate-to-high-salinity ore fluids in early anhydrous skarn formation (Stages I and II) are consistent with fluids of magmatic origin. In comparison, the lower temperature and variable salinity from high (19.8 equiv. wt.% NaCl) to as low as 0.2 equiv. wt.% NaCl in Stage III suggests a possible involvement of low-salinity meteoric waters mixing with high-salinity fluids originating from the nearby granite in the later hydrous skarn and ore formation.