EVALUATION OF THE ROLE OF CAMBRIAN GRANITES IN THE GENESIS OF WORLD-CLASS VHMS DEPOSITS IN TASMANIA

An analysis of the distribution, composition and alteration zonation o f Cambrian granites which intrude the Mt Read Volcanics of western Tas mania provides evidence that there may have been a direct input of mag matic fluids containing Fe, Cu, Au and P to form the copper-gold volca nic-hosted massive sulphide (VHMS) mineralisation in the Mt Lyell dist rict, Interpretation of regional gravity and magnetic data indicates t hat a narrow discontinuous body of Cambrian granite (2-4 km wide) exte nds along the eastern margin of the Mt Read Volcanic belt for over 60 km. The Cambrian granites are altered magnetite series types which sho w enrichment in barium and potassium, and contrast markedly with the f ractionated ilmenite series Devonian granites related to tin mineralis ation elsewhere in the Dundas Trough. Copper mineralisation occurs in a linear zone above the apex of the buried Cambrian granite body at th e southern end of the belt, from Mt Darwin to the Mt Lyell district ov er a strike length of 25 km. Gold and zinc mineralisation are concentr ated higher in the volcanic stratigraphy more distant from the granite . Overlapping zones of alteration extend from the granite into the sur rounding volcanic rocks. An inner zone of K-feldspar alteration is ove rprinted by chlorite alteration, which passes outwards into sericite a lteration. Magnetite +/- pyrite +/- chalcopyrite +/- apatite mineralis ation is concentrated in the chlorite alteration zone as veins and low grade disseminations. The Mt Lyell copper-gold stringer and dissemina ted mineralisation is hosted in felsic volcanic rocks 1 to 2 km west o f the interpreted buried granite position. Magnetite-apatite +/- pyrit e veins in the Prince Lyell deposit at Mt Lyell an very similar to the veins in the halo of the granite, further south, and provide evidence for magmatic fluid input during the formation of the copper-gold VHMS deposits.A model involving deeply penetrating convective seawater, mi xing with a magmatic fluid released from the Cambrian granites, best e xplains the features of VHMS mineralisation in the Mt Lyell district.