Characterisation of uranium-pyrite associations within organic-rich Eocene sediments using EM, XFM-µXANES and µXRD

Sediments that are rich in organic matter (OM) can trap and accumulate metals, including uranium (U), and provide favourable conditions for the formation of minerals such as pyrite. In OM-sediments, pyrite may play an important role in sorption and reduction of U and other metals that are sensitive to redox change. Mulga Rock, located in Western Australia, is an Eocene polymetallic deposit hosting economic uranium within its OM-rich sediments. Our previous work on the Mulga Rock deposits showed that OM may host U(VI) for long periods and suggested that reduction to U(IV) may occur through different pathways. Framboidal-like pyrite, embedded in lignite, displaying U(IV)-rich rims, > 100 µm, were investigated using synchrotron-X-ray fluorescence microscopy-micro-X-ray absorption near edge structure (XFM-µXANES), laboratory micro-X-ray diffraction (µXRD) and electron microscopy. Micro-XRD analyses indicated that the U(IV) phase was mostly coffinite [USiO4] mixed with uraninite [UO2]. Analyses of transects across U-pyrite rims using XFM and XFM-µXANES revealed higher concentrations of U(IV) near the pyrite, and proportionally more U(VI) away from the edge of the pyrite. We infer that within OM environments, pyrite has a local influence on the oxidation state of U, which may be influential when considering U mobility.