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Abstract

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°c and about 300°c 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° -150°C, 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
10-31·5 atm.), low fs2 (10-11_10-12·3 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
10-34·5 atm.), low fs2 (10-10·2-10-10·9 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 δ34S∑S values for Stages 1-3
and Stage 4 fluids average +7.3‰ and +7.5‰ respectively, and are
consistent with the Pine Hill Granite being the major or sole sulphur
source for the fluids.

Item Type:	Thesis - PhD
Authors/Creators:	Patterson, DJ
Keywords:	Mines and mineral resources, Geology
Copyright Information:	Copyright 1979 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1979. Bibliography: 1. 191-206
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