ORIGIN OF HYDROTHERMAL CU-ZN-PB MINERALIZATION IN THE OLARY BLOCK, SOUTH AUSTRALIA - EVIDENCE FROM FLUID INCLUSIONS AND SULFUR ISOTOPES

Styles of sulphidic mineralisation recognised in the Olary Block, Sout h Australia, include disseminated to massive stratiform and strataboun d occurrences, as well as post-peak metamorphic vein-type mineralisati on, 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 Inli ers. A fluid inclusion and sulphur isotope study was undertaken to exa mine 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 e valuate possible sources of sulphur in each of these occurrences. Flui d inclusions in epigenetic assemblages can be classified in general te rms of CO2-rich, CH4-rich, NaCl-rich and H2O-rich types. Dense and mod erately saline CO2 and CH4-rich fluids prevailed during or shortly aft er peak metamorphism, possibly as a result of metamorphic decarbonatis ation 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 dee p-seated hydrothermal reservoirs during retrogression and uplift, thes e fluids became increasingly saline and thus suitable to transport sig nificant amounts of metals in solution. Near surface-derived fluids en tered fluid conduits during further uplift and cooling, resulting in c ompositional dilution of the brines and decreasing fluid temperatures. Sulphur isotopic signatures in stratabound and epigenetic mineralisat ion in the Olary Block suggest a continuous supply of hydrothermal sul phur-bearing fluids from a deep-seated reservoir. Large-scale leaching of volcaniclastics and other sedimentary rocks or convective homogeni sation of a mixed source (e.g., evaporitic or seawater sulphate and ig neous sulphides) resulted in relatively uniform delta(34)S values of 5 to +10 parts per thousand in the hydrothermal fluids. Mixing with se dimentary sulphur in stratabound occurrences is indicated by lighter s ulphur isotopic compositions (delta(34)S greater than or equal to -15. 6) in sulphides associated with carbonaceous pelites. The distinct sul phur isotopic signature of epigenetic vein mineralisation in the Olary Block (delta(34)S(pyrrhotite) between +5.8 and +7.6 parts per thousan d) precludes simple derivation from pre-existing deposits by remobilis ation processes. It rather implies extraction from the same source reg ion at a later stage, possibly induced by tectonic pumping and hydroth ermal venting of a deep-seated crustal reservoir.