Origin of hydrothermal CuZnPb mineralisation in the Olary Block, South Australia: evidence from fluid inclusions and sulphur isotopes

Abstract

Styles of sulphidic mineralisation recognised in the Olary Block, South Australia, include disseminated to massive stratiform and stratabound occurrences, as well as post-peak metamorphic vein-type mineralisation, corresponding to the range of deposit types in the adjacent Broken Hill Block. However, it is still unclear as to whether the epigenetic veins are genetically related to stratabound mineralisation in either of these two portions of the Meso- to Palaeoproterozoic Willyama Inliers. A fluid inclusion and sulphur isotope study was undertaken to examine the nature of fluids associated with epigenetic mineralisation in the Olary Block, to determine possible genetic relationships between sulphides in epigenetic veins and stratabound mineralisation, and to evaluate possible sources of sulphur in each of these occurrences. Fluid inclusions in epigenetic assemblages can be classified in general terms of CO 2-rich, CH 4-rich, NaCl-rich and H 2O-rich types. Dense and moderately saline CO 2 and CH 4-rich fluids prevailed during or shortly after peak metamorphism, possibly as a result of metamorphic decarbonatisation and dehydration reactions at deeper levels as well as oxidation of graphite during retrograde metamorphism in shallower levels. Due to rehydration reactions and infiltration of metamorphic fluids from deep-seated hydrothermal reservoirs during retrogression and uplift, these fluids became increasingly saline and thus suitable to transport significant amounts of metals in solution. Near surface-derived fluids entered fluid conduits during further uplift and cooling, resulting in compositional dilution of the brines and decreasing fluid temperatures. Sulphur isotopic signatures in stratabound and epigenetic mineralisation in the Olary Block suggest a continuous supply of hydrothermal sulphur-bearing fluids from a deep-seated reservoir. Large-scale leaching of volcaniclastics and other sedimentary rocks or convective homogenisation of a mixed source (e.g., evaporitic or seawater sulphate and igneous sulphides) resulted in relatively uniform δ 34S values of +5 to 10‰ in the hydrothermal fluids. Mixing with sedimentary sulphur in stratabound occurrences is indicated by lighter sulphur isotopic compositions ( δ 34S ⩾ − 15.6) in sulphides associated with carbonaceous pelites. The distinct sulphur isotopic signature of epigenetic vein mineralisation in the Olary Block ( δ 34S inpyrrhotite between + 5.8 and + 7.6‰) precludes simple derivation from pre-existing deposits by remobilisation processes. It rather implies extraction from the same source region at a later stage, possibly induced by tectonic pumping and hydrothermal venting of a deep-seated crustal reservoir.