PETROLOGY AND GEOCHEMISTRY OF SPHALERITE FROM THE CAMBRIAN VHMS DEPOSITS IN THE ROSEBERY-HERCULES DISTRICT, WESTERN TASMANIA - IMPLICATIONSFOR GOLD MINERALIZATION AND DEVONIAN METAMORPHIC-METASOMATIC PROCESSES

Sphalerite is the major ore mineral in the Zn-rich, volcanic-hosted ma ssive sulphide deposits of western Tasmania. These deposits have been affected by regional metamorphism to upper greenschist facies, and ass ociated tectonic deformation related to the Devonian Tabberabberan Oro geny. The southern end of the Rosebery deposit has undergone metasomat ic replacement related to a post-orogenic Devonian granite intrusion. Sphalerite from VHMS deposits in the Rosebery district varies widely i n colour, grain size and texture. Compositional variation of the sphal erites was studied for three purposes (1) to investigate effects of th e Devonian overprinting, (2) to provide pressure (depth) estimates at the time of Fe-S-O replacement during the Devonian, and (3) to deduce the effect of a(FeS)(a(S2)) on gold deposition and subsequent remobili sation. Sphalerite from the Rosebery deposit shows an FeS range from 2 .0 to 20.0 mole%, with a bimodal distribution; a mode of 16.0 mole% Fe S was noted for the F(J) lens where Devonian metasomatism prevailed, w hereas a mode of 2.4-4.0 mol% FeS was found for the other lenses. Spha lerite from the Hercules deposit has a range of 2.0-10.0 mole % FeS, w hereas sphalerite from the South Hercules deposit ranges from 4.0-12.0 mole% FeS. VHMS sphalerites also contain minor copper, manganese and cadmium. The bimodal distribution of FeS content in Rosebery sphalerit e suggests that the primary VHMS mineralisation underwent at least two periods of post-depositional re-equilibration, The FeS content in sph alerite in equilibrium with hexagonal pyrrhotiteand pyrite indicates t hat the Devonian replacement occurred at a pressure of 3.0 +/- 0.5 kb, corresponding 8.0 +/- 0.1 km depth. The relationship between FeS cont ent in sphalerite and gold grades at Rosebery, Hercules and South Herc ules displays complex patterns that reflect either variations in the i nitial depositional conditions on the seafloor (pH, temperature and a( S2)), or later Devonian metamorphic and metasomatic recrystallisation.