Radionuclide-bearing minerals in Olympic Dam copper concentrates

Abstract

Olympic Dam hosts >80 million tonnes of copper, as copper‑iron sulphides, within hematite-dominant gangue. Processing of the relatively fine-grained copper ore is complicated by the presence of by-product uranium and its radiogenic isotopes, in particular ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po, which partially recover to the final copper sulphide concentrates. Although the majority (~85%) of the U-bearing minerals (uraninite, coffinite, brannerite, thorianite, thorite) are recovered to flotation tailings, the remaining ~15% occurs in copper concentrates as sulphide-gangue composite particles and micron- to nanoscale grains occluded within sulphide minerals. Sulphuric acid leaching of copper concentrates significantly reduces the concentration of U but has only a minor impact on the daughter isotopes. Further reduction of these isotopes may be achieved through selective targeting and removal of specific minerals known to be radionuclide (RN) hosts, but fine-grained, micro- to nanoscale ore textures preclude easy direct identification of these RN host phases. Nanoscale secondary ion mass spectrometry (nanoSIMS) has proven to be an excellent platform for in situ mapping of ultra-trace RN distributions, with sub-micron spatial resolution, in individual mineral grains. Uranium and thorium minerals are, as expected, the major hosts and display significant concentrations of the entire ²³⁸U decay chain. Although the majority of these minerals are removed via acid leaching of copper concentrate, sufficient RNs remain in the concentrate as micron- to nanoscale grains within sulphides and gangue minerals or along particle microfractures. Results show that brannerite, potentially more problematic due to its lower solubility in sulphuric acid than uraninite, does not appear to contain appreciable amounts of daughter isotopes. The insolubility of ThPO₄ produces the potential for sequestration of ²³²Th and ²³⁰Th in rare earth phosphates such as xenotime, which then retain the respective decay chains. Likewise, the insolubility of certain sulphates (Sr, Ba, Pb, Ra) provides a mechanism for precipitation and entrapment of RNs. Baryte was shown to accumulate Ra and Pb naturally in the deposit, and it can upgrade its RN content substantially during sulphuric acid leaching through coupled dissolution-reprecipitation mechanisms involving Pb²⁺ and Ra²⁺ liberated from dissolved uranium minerals. Identification and prioritization of RN host minerals is crucial for the development of a more efficient processing flowsheet.