Mineralogical and Microanalytical Characterisation of Uranium Mineralisation

摘要

A sound mineralogical understanding of an orebody or prospect is critical for planning and optimisation of ore processing and waste management, can contribute to improved genetic models and can potentially be used in vector approaches to exploration targeting. This can apply to any ore system but is particularly applicable to uranium-bearing mineralisation. The high mobility of uranium, the often fine-grained nature of U-minerals and the high tendency for absorption onto clays and other minerals can often make for complex ore textures and paragenetic relationships.Drawing on examples from different types of ore system (roll front uranium, iron oxide-copper-gold-(uranium), granite-hosted uranium), this presentation will show how we address these issues using a state-of-the-art range of microanalytical infrastructure at University of Adelaide.The mineralogical deportment of elements of interest (U, other key components such as Th, and also potential unwanted elements), can be evaluated using a combination of scanning electron microscope (SEM), mineral liberation analyser (MLA) and quantitative techniques, including electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The latter offers micron-scale resolution combined with sub-ppm-level sensitivity. Grain-scale chemical mapping permits a visualisation of variation in the concentrations of major, minor and trace elements in the context of prevailing textures.In many uranium ores, textural and compositional heterogeneity are present at a scale smaller than that of the microprobe beam or laser spot. Advances in submicron-scale characterisation and, critically, the ability to microsample in-situ are offered by dual-beam focused ion beam (FIB)-SEM platforms. The FIB-SEM allows for cross-section imaging, 3D 'slice and view' microscopy and information on grain orientation via electron backscatter diffraction. The scanning transmission electron microscopy (STEM) detector on the FIB platform allows for imaging of textures, phase identification by energy-dispersive X-ray spectroscopy (EDXS) and element mapping at a resolution well beIowr that of LA-ICP-MS. Importantly, FIB-SEM can be used to prepare and thin foils for nanoscale imaging and electron diffraction using transmission electron microscopy (TEM). In 2015, these nanoscale capabilities will be significantly expanded by the addition of a new FEI Titan Themis aberration-corrected TEM offering atomic-scale resolution, and the facility to obtain compositional data and chemical maps of 10 x 10 nm areas.When used in combination, this portfolio of quantitative and qualitative techniques bridge nanometre to millimetre scales of observation. They also have application to assessment and interpretation of other mineralogically-complex deposits, including rare earth elements deposits.