Geology and mineralization at Renison Bell, western Tasmania
Creators: name: Patterson, DJ
Publisher: name: University Of Tasmania
Year: 2023

description: Cassiterite mineralization at Renison Bell, Western Tasmania, is spatially and temporally related to the Upper Devonian Pine Hill Granite, and occurs in dolomites and associated elastic sediments of ?Proterozoic to Cambrian age. The sedimentary succession consists of an older siliceous elastic and carbonate sequence (the Success Creek Group) overlain by, and in faulted contact with, a predominantly volcaniclastic sequence (the Crimson Creek Argillite). elastic sediments in the Success Creek Group are a normal assemblage of thinly-bedded quartzites, siltstones and shales of probable shallow marine origin. Sediments in the Crimson Creek Argillite are siltstones and volcaniclastic graywackes of basaltic composition, with polymict conglomerates, tuffs, cherts and carbonate rocks occurring in a thin but widespread basal sequence. Mafic intrusives in the Crimson Creek Argillite are chemically similar to the enclosing sediments. Carbonate beds in the Success Creek Group and Crimson Creek Argillite consist principally of highly ordered ferroan dolomite, and their textural, mineralogical and stable isotopic features suggest they are diagenetically dolomitised marine limestones. The Pine Hill Granite is a porphyritic biotite granite with chemical and stable isotope compositions intermediate between endmember S and I types, and has been locally altered to greisen and muscovitic assemblages carrying minor cassiterite. Adjacent rocks have been thermally metamorphosed to hornblende-hornfels facies assemblages. Cassiterite mineralization occurs in stratabound replacement bodies in dolomite beds in the Success Creek Group and Crimson Creek Argillite, in veins with variable wallrock replacement in major fault zones and to a minor extent in recrystallized sandstone. The generalized paragenetic sequence in Renison Mine is: Stage 1: Cassiterite + silicates tentatively inferred as an early stage of replacement ore formation. Stage 2: Cassiterite + pyrrhotite + arsenopyrite + silicates + minor sulphides and oxides etc. Main stage of mineralization, associated with sideritic alteration of dolomites, tourmalinisation of elastic sediments. Stage 3: cassiterite + pyrrhotite + arsenopyrite + silicates +minor sulphides, forming veins in major fault zones. Stage 4: Minor veining by sphalerite +galena+ silicates+ carbonates¬¨¬± fluorite. Stage 5: Minor veining by quartz + calcite ¬¨¬± chlorite. Stage 6: Vug-filling sequence of carbonates, quartz, fluorite and sulphides. Temperatures of about 350¬¨‚àûc and about 300¬¨‚àûc are inferred for Stages 1-3 and Stage 4 respectively, from fluid inclusion, mineralogical and stable-isotope data. Temperatures during Stage 5 mineralization were 0 0 probably in the range 200¬¨‚àû -150¬¨‚àûC, and were probably lower during Stage 6 mineralization. Bulk fluid-inclusion analyses indicate that fluids during Stages 1-4 were Na-K-Cl brines with a total salt concentration of about 2 molal and Na/K ratios of about 7. Variable Ca and Mg concentrations may be partly due to contamination during sample leaching. Fluids were gas-poor with average gas/water molal ratios about 2 x 10-2 , and C02/CH4 ratios near 1. Nitrogen and argon were detected only in some samples. Stage 5 fluids were more saline, with average salt concentrations of about 5 molal and Na/K ratios about 20. Formation of cassiterite-bearing stages took place at low fo2 (about 10-31¬¨‚àë5 atm.), low fs2 (10-11_10-12¬¨‚àë3 atm.) and low pH (3.9-5.4) in the reduced sulphur field. The major control on ore deposition was probably an increase in pH within the indicated range due to dolomite replacement. Calculated stabilities of stannous chloride complexes indicate that tens to hundreds of parts per million of tin may be transported in this form in low fo2 , low pH conditions, and may be precipitated as cassiterite from sulphur-poor solutions by mechanisms which wouid cause precipitation of the accompanying sulphides. Sphalerite-galena mineralization took place at low fo2 (about 10-34¬¨‚àë5 atm.), low fs2 (10-10¬¨‚àë2-10-10¬¨‚àë9 atm.) and low pH (3.9-5.5) in the reduced sulphur field. Oxygen and hydrogen isotope ratios of the mineralizing fluids, calculated from analyses of gangue minerals and water extracted from fluid inclusions, suggest the Stage 1-3 fluids were an exsolved magmatic aqueous phase. A different source, probably heated contemporary groundwater, is inferred for fluids in later stages. Some mixing of fluid types may have occurred during Stage 3. Calculated ˜í¬•34S‚Äöv†v´S values for Stages 1-3 and Stage 4 fluids average +7.3‚ÄövÑ‚àû and +7.5‚ÄövÑ‚àû respectively, and are consistent with the Pine Hill Granite being the major or sole sulphur source for the fluids.