Sources of mineralising fluids for the Olympic Dam deposit (South Australia) : SmNd isotopic constraints

Abstract

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 relationships between the different ore and breccia types. Pyrite-, chalcopyrite- and bornite-chalcocite-rich hematitic ores share an initial ENd signature of ∼ −2.5 which suggests that these ore types are cogenetic. These ores, together with sulphide-poor hematitic breccias, yield a 14-point Sm-Nd isochron age of 1572 ± 99 Ma. The host Roxby Downs Granite has an initial ϵ Nd of ∼ −5, and therefore cannot alone have been the source of the mineralising fluids for the hematitic ores. In contrast to hematitic rocks, volumetrically minor magnetite-rich assemblages have the same initial Nd signature as the host granite, suggesting that they are cogenetic. Assuming that both hematite and magnetite were precipitated from solution, their contrasting isotopic signatures require that ore deposition involved two distinct fluids. The isotopic data do not allow the distinction between a model involving concurrent activity of such fluids in a mixing regime, and a model involving temporally separate overprinting of magnetite by hematite +Cu-sulphides. The ore signature of ϵ Nd ≈ −2.5 cannot have been derived solely from pre-existing crustal rocks of the Gawler Craton; it must have involved a contribution from a mantle-derived source rock or magma. The most likely source of this Nd component are the mantle-derived rocks produced during the Gawler Range Volcanics event, e.g. altered alkaline mafic/ultramafic dykes within the deposit, which have initial isotopic signatures as high as ϵ Nd + 4. Assuming end-member compositions equivalent to these dykes and the Roxby Downs Granite, the ore signature implies that ∼ 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 the deposit are ⩾ 13% mafic/ultramafic rocks and ⩽ 87% Roxby Downs Granite or its volcanic equivalent. Massbalance calculations indicate that 13% rock fraction of mafic/ultramafics can account for ∼ 50% of the Cu contained in the deposit. The involvement of mafic/ultramafic rocks is therefore inferred to have been crucial to the genesis of the Cu-enriched ores at Olympic Dam.