SOURCES OF MINERALIZING FLUIDS FOR THE OLYMPIC DAM DEPOSIT (SOUTH-AUSTRALIA) - SM-ND ISOTOPIC CONSTRAINTS

The Olympic Dam Cu-U-Au-Ag deposit, South Australia, is hosted by the Olympic Dam Breccia Complex, a high-level, hematite-rich hydrothermal breccia system. The deposit is characterised by extreme enrichments of REE which are a product of the ore-forming process, therefore making the Sm-Nd isotopic system an ideal tool for clarifying the complex rel ationships between the different ore and breccia types. Pyrite-, chalc opyrite- and bornite-chalcocite-rich hematitic ores share an initial e psilon(Nd) signature of similar to-2.5 which suggests that these ore t ypes are cogenetic. These ores, together with sulphide-poor hematitic breccias, yield a 14-point Sm-Nd isochron age of 1572 +/- 99 Ma. The h ost Roxby Downs Granite has an initial epsilon(Nd) of similar to-5, an d therefore cannot alone have been the source of the mineralising flui ds for the hematitic ores. In contrast to hematitic rocks, volumetrica lly minor magnetite-rich assemblages have the same initial Nd signatur e as the host granite, suggesting that they are cogenetic. Assuming th at both hematite and magnetite were precipitated from solution, their contrasting isotopic signatures require that ore deposition involved t wo distinct fluids. The isotopic data do not allow the distinction bet ween a model involving concurrent activity of such fluids in a mixing regime, and a model involving temporally separate overprinting of magn etite by hematite + Cu-sulphides. The ore signature of epsilon(Nd)appr oximate to-2.5 cannot have been derived solely from pre-existing crust al rocks of the Gawler Craton; it must have involved a contribution fr om a mantle-derived source rock or magma. The most likely source of th is Nd component are the mantle-derived rocks produced during the Gawle r Range Volcanics event, e.g. altered alkaline mafic/ultramafic dykes within the deposit, which have initial isotopic signatures as high as epsilon(Nd) + 4. Assuming end-member compositions equivalent to these dykes and the Roxby Downs Granite, the ore signature implies that simi lar to 30% of the Nd in the ores was derived from the mafic/ultramafic source. If Nd was leached from source rocks rather than derived from magmatic volatiles, the inferred rock fractions contributing REE to th e deposit are greater than or equal to 13% mafic/ultramafic rocks and less than or equal to 87% Roxby Downs Granite or its volcanic equivale nt. Mass-balance calculations indicate that 13% rock fraction of mafic /ultramafics can account for similar to 50% of the Cu contained in the deposit. The involvement of mafic/ultramafic rocks is therefore infer red to have been crucial to the genesis of the Cu-enriched ores at Oly mpic Dam.