Mid-crustal uranium and rare metal mineralisation in the Mount Isa Inlier: a genetic model for formation of orogenic uranium deposits

Uranium mineralisation near Mount Isa in northwest Queensland, Australia, is widespread yet poorly understood. Within this region in the Western Fold Belt, one hundred and ninety uranium-rare metal occurrences are known. This uranium mineralisation is similar to worldwide examples of albitite-hosted or sodium-metasomatic uranium deposits, which host albite-carbonate ore zones enriched in incompatible elements. Various metal sources and ore-forming processes have been suggested to explain the origin of this unusual zirconium- and heavy rare-earth element-enriched uranium mineralisation in the Mount Isa Inlier, including metamorphogenic, basinal fluid-related and intrusion-related genetic models.
   
   This thesis focuses on three key themes; (1) characterisation of the processes involved in albitite-hosted and refractory rare metal-enriched uranium mineralisation regionally, (2) the possibility that ore-forming elements were sourced from local granite intrusions, (3) understanding the nature of regional metamorphic fluids that were widespread at the time of uranium mineralisation. These themes are then incorporated into a new genetic model for this orogenic uranium mineralisation which also explores potential links to regional iron oxide-copper-gold mineralisation, which are mildly uranium-enriched (IOCG+U).
   
   This study combines field reconnaissance at Mount Isa with drill core logging, petrology, geochemistry, stable and radiogenic isotope analyses, geochronology, advanced microscopy and mineral trace element analytical techniques from uranium ore zones and potential source rocks. This large dataset enables the development of a detailed understanding of how uranium mineralisation was related to regional fluids and available source rocks during the deformation, metamorphism and magmatism of the Isan orogeny. The emplacement of the uranium-rich Sybella batholith represented the last major magmatic event locally at Mount Isa, when several granitoid phases were intruded into the local superbasin, approximately 90 million years before orogenesis and the uranium mineralisation event. During orogenesis, ultrasaline metamorphic fluids sourced from voluminous mid-crustal evaporite-bearing sequences, enabled enhanced dissolution of carbonate and phosphate, creating unusually alkaline and mildly-oxidised metamorphic fluids distinct from typical orogenic gold-forming fluids. When these unusual fluids infiltrated granite margins during or just after peak metamorphism associated with the Isan orogeny, they dissolved fluorite and mobilised uranium along with typically immobile elements like Zr and HREE. These elements were sourced from highly metamict zircons that are found throughout the most uranium-rich phases of the Sybella batholith.
   
   The results of this study suggest exploration models for orogenic uranium mineralisation should be focused towards uranium- and fluorine-rich anorogenic granites emplaced into favourable evaporite-bearing sedimentary sequences that were subjected to later orogenic processes. The association between Proterozoic IOCG+U and orogenic uranium systems is related to granite sources for U and associated elements, regardless of whether uranium is derived directly from magmatic-hydrothermal fluids or leached from crystalline granites by hydrothermal fluids during later metamorphism. However, IOCG+U-related fluids are comparatively oxidised and acidic, and in the case of the Mount Isa Inlier, were produced in a later stage of orogeny than the orogenic uranium-related fluids.