Formation of magnetite-scheelite skarn mineralization at Kara, northwestern Tasmania: Evidence from mineral chemistry and stable isotopes

The Kara magnetite-scheelite deposit is located 40 km south of Burnie in no rthwestern Tasmania. The major orebodies at Kara are hosted by the Ordovici an Gordon Limestone at the southern end of the Devonian Housetop Granite, a djacent to the granite or separated from it by the Ordovician Moina Sandsto ne. At least four paragenetic stages of skarn formation and ore deposition have been recognized: stage I, clinopyoxene +/- garnet +/- vesuvianite +/- wollastonite +/- quartz +/- scheelite; stage II, garnet-vesuvianite-magneti te +/- scheelite +/- apatite +/- quartz; stage III, magnetite-amphibole-epi dote-fluorite-quartz +/- chlorite +/- garnet +/- vesuvianite +/- scheelite +/- carbonate +/- pyrite +/- clinopyroxene; and stage IV, hematite +/- fluo rite +/- calcite +/- quartz. Stages I and II mineral assemblages represent early skarn formation and are dominated by the anhydrous minerals clinopyroxene and garnet. Stages III a nd IV minerals represent late skarn-forming phases and pervasively replace early mineral assemblages. Scheelite occurs in stages I through III and gen erally shows a close spatial association with hydrous minerals (vesuvianite in stages I and/or II and amphibole in stage III). Abundant scheelite is f ound in stage III with amphibole where it forms very coarse grains in exces s of 5 cm. Microprobe analyses indicate that the majority of the skarn minerals are ca lcic and have high Fe3+/Fe2+. Clinopyoxene is diopside rich, generally havi ng a mole composition of >80 percent diopside and <25 percent hedenbergite, whereas garnet composition from stages I to III are andradite rich, displa ying mole proportions of >70 percent andradite and <30 percent grossular. G arnet appears to have the highest levels of till (up to 0.27 wt % SnO2) and may be the major carrier of tin in the skarn. Amphiboles are largely of ha stingsite and magnesian hastingsite composition. Scheelite contains moderat e amounts of molybdenum (<3 wt % MoO3), which give powellite mole proportio ns of up to 6 percent, and is largely unaccompanied by molybdenite. Isotopic measurements of the protolith Gordon Limestone reveal delta(13)C v alues from -1.6 to -4.4 per mil (PDB) and a delta(18)O composition of 10 to 23 per mil (SMOW), which are both lower than those of the unmetamorphosed Gordon Limestone (delta(13)C = -1.5 to +1.8 parts per thousand, PDB; delta( 18)O = 22-27 parts per thousand, SMOW). The isotopic compositions of the sk arn calcites range from -1.7 to -7.3 per mil (PDB; mean -4.5 parts per thou sand) for carbon and 3.4 to 14.0 per mil (SMOW; mean 11 parts per thousand) for oxygen. The isotopic compositions of the skarn calcites are also lower than that of the protolith marble. The depletion of O-18 and C-13 from the host Gordon Limestone to die protolith marble and the skarn calcites is in terpreted to be due to involvement of multiple fluid sources with dominant magmatic-hydrothermal fluids during infiltration metasomatic process. The Kara deposit formed as a proximal skarn assemblage in carbonate host ro cks following the emplacement of the Devonian Housetop Granite and was char acterized by early. high-temperature mineral assemblages dominated by anhyd rous minerals and late low-temperature assemblages with abundant mineraliza tion (scheelite, magnetite) and hydrous minerals. Chemistries of clinopyoxe ne, garnet, and scheelite suggest that the Kara skarn deposit was formed un der relatively oxidized conditions. The deposit differs significantly from other scheelite deposits, such as CanTung, which formed under relatively re duced conditions in the abundance of magnetite (up to >90 vol %), lower qua ntities of sulfides, and absence of pyrrhotite.