PETROLOGICAL AND GEOCHEMICAL SIGNIFICANCE OF A DEVONIAN REPLACEMENT ZONE IN THE CAMBRIAN ROSEBERY MASSIVE SULFIDE DEPOSIT, WESTERN TASMANIA

The Rosebery orebody is a polymetallic massive sulfide deposit hosted in felsic volcanic rocks of the Cambrian Mt. Read Volcanic belt, weste rn Tasmania. The deposit underwent upper-greenschist-facies regional m etamorphism and related deformation during the Devonian Tabberabberan Orogeny, resulting in folding, shearing, and faulting (thrusting) of t he ore lenses. The south end of the Rosebery deposit has undergone met asomatic replacement related to the inferred intrusion of a post-oroge nic Devonian granite, as interpreted from detailed gravity data. Miner alization at Rosebery consists of three primary sulfide-sulfate zones and a massive carbonate zone of Cambrian age: a lowermost pyrite - cha lcopyrite zone (>4% Cu), an overlying sphalerite - galena +/- pyrite z one, and an uppermost massive barite and carbonate zone. Devonian meta somatic mineral assemblages that overprint the south end of the orebod y delineate three major zones: (1) magnetite - biotite +/- chalcopyrit e, (2) pyrrhotite - pyrite, and (3) tourmaline - quartz +/- magnetite. Other metasomatic minerals, such as fluorite, garnet, and helvite, al so are present in the Devonian assemblages. Field and textural relatio nships suggest that replacement of primary lead-zinc sulfide lenses oc curred after folding. Magnetite - biotite +/- chalcopyrite assemblages are confined to the lower levels of the mine, whereas pyrrhotite - py rite and tourmaline - quartz +/- magnetite assemblages occur toward th e upper part of the orebody. No cross-cutting relationships between th e pyrrhotite - pyrite zone and the magnetite - biotite +/- chalcopyrit e zone have been observed, suggesting that the minerals of these zones are products of a single evolving metasomatic event. The tourmaline - quartz +/- magnetite assemblage seems to have formed late in the repl acement process, as indicated by irregular and patchy quartz - tourmal ine veins cutting the host rock and other sulfide lenses. Detailed und erground examination also indicates that tourmaline - quartz veins dem onstrably cut cleavage in the tuffaceous host-rocks, suggesting that t he tourmaline veins formed after the development of the Devonian cleav age.