THE RENISON GRANITE, NORTHWESTERN TASMANIA - A PETROLOGICAL, GEOCHEMICAL AND FLUID INCLUSION STUDY OF HYDROTHERMAL ALTERATION

The Renison granite in northwestern Tasmania is genetically related to cassiterite sulfide mineralization (carbonate replacement) at Renison Bell, one of the largest Sn deposits in the world. The magmatic hydro thermal system responsible for the mineralization in the roof sediment s is also responsible for the development of three zones of extensive hydrothermal alteration in the granite. The tourmaline zone (tourmalin e + quartz + topaz + fluorite + sericite + cassiterite) is in part sur rounded by a sericite zone (sericite + quartz + tourmaline + calcite cassiterite). An albite zone is characterized by albite + quartz + ch lorite + sericite along with primary plagioclase and K feldspar. The R enison granite is a reduced felsic granite with biotite as the only ma fic mineral. It is characterized by high contents of SiO2 (70.9-75.3%) and K2O (4.69-5.91%), but low Na2O (2.52-3.12%). High amounts of Rb ( up to 918 ppm) and Sr (as low as 19 ppm), as well as other trace eleme nt abundances in the most felsic rocks, indicate that parts of the mag ma were highly fractionated, and as a result, enriched in volatile con stituents, mostly aqueous. During hydrothermal alteration, B, Fe, and F were introduced into the tourmaline zone, whereas Na and K were remo ved. In the sericite and albite zones, chemical and petrographic effec ts of alteration are less conspicuous and are marked by changes in the concentrations of Na and Ca and small changes in K. Norms of unaltere d granites project as a tight cluster on the haplogranite plane (quart z-albite-orthoclase) whereas altered rocks are widely scattered. Norma l trace element concentrations defining fractionation trends of igneou s origin have survived the effects of alteration in albite and sericit e zone rocks as well as in weakly to moderately altered, tourmaline zo ne rocks. However, depletion in Rb, Sr, and Ba occurs in strongly tour malinized rocks in which the primary mineral Fabric is completely repl aced by secondary (hydrothermal) mineral fabric. There are at least th ree sets of fluid inclusions in the Renison granite. The earliest (typ e I inclusions) is characterized by high salinities ranging from 30.0 to 38.4 art percent NaCl equiv, a second (type II inclusions) has lowe r salinities (8.8-22.5 wt % NaCl equiv), and a third generation (type III inclusions) is marked by low salinities (4.5-12.3 wt % NaCl equiv) and the presence of COB. Hydrothermal activity represented by type II inclusions appears to be widespread, whereas type I and III inclusion s are mainly restricted to the tourmaline zone. Trapping temperatures recorded from type I and II inclusions are essentially in the same ran ge (343 degrees-455 degrees C) in the tourmaline zone. A substantial o verlap in the temperature and salinity of fluids recorded from all the alteration zones suggests a continuous flow of fluids. Decreases in t emperatures and salinities are thought to be responsible for changing fluid compositions which led to the formation of the three zones of al teration in the Renison granite.