Mineral paragenesis, geochemistry and fluid characteristics of the Kara scheelite-magnetite skarn deposit, Northwestern Tasmania

The Kara scheelite-magnetite deposit is located in northwestem Tasmania 40km south of Burnie at latitude 41°18' S and longitude 145° 48'E. The deposit consists of several skam bodies with total remaining mineable ore reserves estimated at 1. 7 Mt magnetite @ >30% Fe, and 0.3 Mt scheelite @ 0.52 % W03 by 1992. The major orebodies at Kara are hosted by limestone or calcareous sandstones of the Ordovician Gordon Limestone Subgroup which are in contact with the Devonian Housetop Granite or separated from it by the Ordovician Moina Sandstone or Owen Conglomerate. The predominant skarn minerals display a zonation that ranges from epidote-quartz to garnet, magnetite-amphibole, vesuvianite and clinopyroxene zones away from the granite contact. Mineral paragenetic studies reveal at least four stages of skarn formation and ore deposition: 1) Stage I --clinopyroxene-gamet-vesuvianite±wollastonite±quartz±scheelite, 2) Stage II -gamet-vesuvianite-magnetite±scheelite±apatite±quartz, 3) Stage III -magnetite-amphibole-epidote-fluorite±quartz±gamet±vesuvianite ±chlorite±scheelite±sphene±hematite±pyrite±clinopyroxene, and 4) Stage IV -hematite±fluorite±calcite±quartz. Stages I and II mineral assemblages represent early skarn formation and are dominated by anhydrous minerals of clinopyroxene and garnet. Stages ill and IV minerals represent late skam-forming phases and pervasively replace early mineral assemblages. Scheelite occurs in Stages I-III and generally shows a close spatial association with hydrous minerals (vesuvianite in Stage I/II and amphibole in Stage III). Abundant scheelite is found in Stage ill where it forms very coarse grains up to >5cm. Magnetite occurs in Stages II and m but is also more abundant in Stage III. Microprobe analyses indicate that the majority of the skam minerals are calcic, and have high Fe3+fFe2+. Clinopyroxene is diopside-rich generally having a mole composition of >80% diopside and <25% hedenbergite, while garnet composition from Stages I to III are andradite-rich, displaying mole proportions of> 70% andradite and <30% grossular. Garnet appears to have the highest levels of tin (up to 0.37 wt. % Sn02) and may be the major carrier of tin in the skarn. Amphiboles are largely of hastingsite and magnesian hastingsite composition. Scheelite contains moderate amounts of molybdenum ( <3 wt. % Mo03) which give powellite mole proportions of up to 6% and is unaccompanied by molybdenite. Fluid inclusion microthermometric measurements of primary fluid inclusions for skam minerals reveal systematic declining temperatures from early mineral assemblages to later stages, but salinity values are variable. Stage I clinopyroxene yielded a homogenisation temperature range of 460° to 620°C (modes at 520°C and 600°C). The Stage II minerals gave a homogenisation temperature range of 350° to 570°C (mode at 500°C). The Stage ill mineral assemblage yielded homogenisation temperatures ranging from 240° to 360°C (mode at 300°C). Stage I clinopyroxene gave salinity values from 2.0 to 10.0 equiv. wt. % Naa. The Stage II mineral assemblages showed salinities of 12.0 to 16.0 equiv. wt. % NaCL The Stage ill mineral assemblage yielded salinity values of0.2 to 19.8 equiv. wt. % Naa (modes at 5 and 15 equiv. wt . % Naa). Isotopic measurements of the protolith Gordon Limestone reveal o13c composition from -1.6 to -4.4 %0 (PDB) and o18o composition of 10 to 23%0 (SMOW) which are both depleted with respect to the unmetamorphosed Gordon Limestone values (o13c = -1.5 to 1.8%0, PDB; 0180 = 22 to 27 %0, SMOW). The isotopic compositions of the Stages ill and IV skarn calcite range from -1.7 to -7.3%0 (PDB) (mean -4.5%0) for carbon and 3.4 to 14.0%0 (SMOW) (mean 11 %0) for oxygen. Calculated isotopic compositions yield S13Cco2 values of 0.0 to -5.0%0 (PDB) (mean -3%o) and B180H2o values of 0.0 to 10.0%0 (SMOW) (mean 7%o). Depletion in isotopic values for_the protolith marble are interpreted to be dominantly due to interaction with magmatic fluids from the nearby granite which are also responsible for the formation of skam calcite. The values below the magmatic range in the skam calcite are interpreted to be due to minor contribution from meteoric fluids. Oxygen isotope values of Stages II to ill magnetite vary from 2.7 to 4.7%o (SMOW) and yield o18oH20 values of 12.3 to 13.9%0, whereas oxygen isotopic compositions of Stages II to ill scheelite vary from 5.2 to 6.4%0 (SMOW) and yield o180H20 values of 6.0 to 9.0%0. These ol80H20 values are generally in agreement with the isotope values from skam calcite and confirm a magmatic source for the fluids. The geological, mineral paragenesis, mineral chemistry, fluid inclusion and stable isotope studies indicate that the Kara deposit formed in stages as a proximal skam assemblage in carbonate host rocks following the emplacement of the Devonian Housetop Granite and was characterised by early, high temperature (up to >600°C) mineral assemblages dominated by anhydrous minerals, and late low temperature (300°C) assemblages with abundant mineralisation (scheelite, magnetite) and hydrous minerals. Fluid inclusion and stable isotope data indicate that early skam formation was probably solely due to magmatic fluids, while later skam formation and ore deposition was developed from magmatic fluids mixed with minor meteoric fluids. Chemistries of clinopyroxene, garnet and scheelite (unaccompanied by molybdenite) suggest that the Kara skam deposit was formed under highly oxidised conditions. The deposit differs significantly from other scheelite deposits such as CanTung in the abundance of magnetite (up to >90 vol. % ) and lack of sulphides ( eg. pyrrhotite ).