MINERALOGICAL AND GEOCHEMICAL CHARACTERISATION OF CORES FROM AN IN SITU RECOVERY URANIUM MINE
This study investigates the mineralogy and geochemistry of natural aquifer materials associated with a uranium (U) ore deposit at the Smith Ranch-Highlands Mine, Wyoming, USA. The ore occurs in a roll front deposit and is mined via in situ recovery (ISR). This method of mining is used to recover U from low-grade deposits while producing no tailings or waste rock. This method of mining can, however, contaminate adjacent aquifers with residual U and other elements. The objective of this project is to characterize core taken down-gradient of a mined unit to provide information on the capacity of down-gradient aquifer solids to attenuate U. Core samples were obtained from hydraulically down-gradient of the mine unit B at two locations. Powder X-ray diffraction analyses showed the presence of quartz, clays, feldspar, goethite and pyrite. Electron microprobe analysis, scanning electron microscopy and physical examination confirmed that the core is a heterogeneous mixture of grain sizes with different shapes and composed of clays, sandy materials and coal lenses. Synchrotron X-ray fluorescence mapping revealed the close association of U with Fe, V, Ca and coal lenses in the sediments. Uranium X-ray absorption spectroscopy (XANES) showed the presence of both U(IV) and U(VI) in samples containing coal lenses. Carbon (C) XANES indicated the presence of several C functional groups representative of organic C; however, inorganic C was not identified in the C-K-edge XANES analysis. Iron and S XANES also confirmed the presence of reduced and oxidised Fe and S species. Clays, coal lenses, Fe- oxides and pyrite observed in the core samples may control U concentrations and mobility in groundwater. Geochemical modelling using PHREEQC software showed that primary and secondary U minerals may not control U mobility in these materials but the presence of carbonate, Fe oxides and clays may control U concentrations and mobility through sorption, reduction and precipitation processes. Information from this work can contribute to the quantification and estimation of the risk of downstream/down-gradient human exposure in fate-and-transport models.
uranium, in-situ recovery mining, reclamation, geochemistry, XANES, PXRD
Master of Science (M.Sc.)