Geologic setting and genesis of cassiterite-sulfide mineralization at Renison Bell, western Tasmania

Abstract

Cassiterite mineralization at Renison Bell is spatially and temporally related to the Upper Devonian Pine Hill Granite and occurs in dolomites and associated clastic sediments of probable Cambrian age. The sedimentary succession consists of an older siliceous clastic and carbonate sequence (the Success Creek Group) overlain by, and in faulted contact with, a predominantly volcaniclastic sequence (the Crimson Creek Formation). The Success Creek Group includes thinly bedded quartzites, siltstones, shales, and dolostones of probable shallow marine origin, and the Crimson Creek Formation consists of siltstones and volcaniclastic graywackes of basaltic composition. The Success Creek and Crimson Creek rocks are separated by the Red Rock sequence, consisting of polymict conglomerates, tuffs, cherts, and carbonate rocks. Mafic intrusives in the Crimson Creek Formation are chemically similar to the enclosing sediments. Carbonate beds in the Success Creek Group and the Red Rock sequence consist principally of highly ordered ferroan dolomite, and their textural, mineralogical, and stable isotopic features suggest they are diagenetically dolomitized marine limestones.The Pine Hill Granite is a porphyritic biotite granite which 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 Red Rock sequence, in veins with variable wall-rock replacement in major fault zones and to a minor extent in recrystallized sandstone. The generalized paragenetic sequence in the Renison mine is:stage 1: cassiterite + silicates--tentatively inferred as an early stage of replacement ore formation;stage 2: cassiterite + pyrrhotite + arsenopyrite + silicates + minor sulfides and iron oxides, etc.--the main stage of mineralization, associated with sideritic alteration of dolomites and tourmalinization of clastic sediments;stage 3: cassiterite + pyrrhotite + arsenopyrite + silicates + minor sulfides--forming veins in major fault zones;stage 4: minor veining by sphalerite + galena + silicates + carbonates + or - fluorite; and stage 5: vug-filling sequence of carbonates, quartz, fluorite, and sulfides.Temperatures of about 350 degrees and 300 degrees C are inferred for stages 1 to 3 and stage 4, respectively, from fluid inclusion, mineralogical, and stable isotope data. Temperatures during stage 5 mineralization were probably in the range 200 degrees to 150 degrees C and were probably lower during stage 6 mineralization.Bulk fluid inclusion analyses indicate that fluids during stages 1 to 4 were Na-K-Cl brines with a total salt concentration of about 2 molal and an Na/K ratio 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 of about 2 X 10 (super -2) and CO 2 /CH 4 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 of about 20. Formation of cassiterite-bearing stages took place at about 350 degrees C at low f (sub O 2 ) (about 10 (super -31 5) atm), low f (sub S 2 ) (10 (super -11) -10 (super -12 3) atm), and low pH (3.9-5.4) in the reduced sulfur 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 f (sub O 2 ) -low pH conditions, and may be precipitated as cassiterite from sulfur-poor solutions by mechanisms which would cause precipitation of the accompanying sulfides.Sphalerite-galena mineralization took place at low f (sub O 2 ) (about 10 (super -34 5) atm), low f (sub S 2 ) (10 (super -10 2) -10 (super -10 9) atm), and low pH (3.9-5.5) in the reduced sulfur field.Oxygen and hydrogen isotope ratios of the mineralizing fluids, calculated from analyses of gangue minerals and water extracted from fluid inclusions, suggest that the stage 1 to 3 fluids were an exsolved magmatic aqueous phase. Water from a different source, probably heated contemporary ground water, is inferred for fluids in later stages. Some mixing of fluid types may have occurred during stage 3. Calculated delta 34 S (sub Sigma S) values for stages 1 to 3 and stage 4 fluids average 6.0 per mil and 5.3%, respectively, and are consistent with the Pine Hill Granite being the major or sole source of sulfur for the fluids.