GEOCHEMICAL AND MINERALOGICAL EVOLUTION OF THE MCARTHUR RIVER ZONE 4 UNCONFORMITY-RELATED URANIUM ORE BODY AND APPLICATION OF IRON OXIDATION STATE IN CLAY ALTERATION AS INDICATOR OF URANIUM MINERALIZATION
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The sandstone-hosted McArthur River Zone 4 U ore body and alteration system, located in the Athabasca Basin, are the focus of a detailed mineralogical and geochemical study aimed at reconstructing its evolution. The oxidation state of Fe in clay alteration from Zone 4 is measured using 57Fe Mössbauer spectroscopy and compared with other mineralized and barren sandstone-hosted alteration systems in the Athabasca Basin. The aim is to ascertain the role of Fe in forming U deposits and determine whether Fe oxidation state in alteration minerals can indicate proximity to mineralization. At Zone 4, early diagenetic kaolin is overprinted by zones of dravite, illite, chlorite, and late kaolinite forming around the P2 fault. Uranium mineralization occurred at ca. 1600 Ma and was triggered by mixing between oxidizing U-bearing basinal fluids and reducing basement-modified basinal fluids, the latter forming when basinal fluids interacted with basement lithologies. Early pre-ore silicification in the lower 200 metres of the Manitou Falls Formation above the ore body created favourable conditions for mineralization by focusing basinal fluids into the reduction site and enhancing ore preservation. However, it obstructed the post-ore migration of radiogenic Pb and U pathfinder elements from the deposit and limited the extent of hydrothermal sudoite alteration in the overlying strata. Sandstone-hosted alteration systems in the Athabasca Basin are commonly surrounded by an outer illite and an inner chlorite zone. Illites have high Fe3+/ƩFe ratios characteristic of formation from oxidizing basinal fluids, whereas, chlorites have lower and more varied Fe3+/ƩFe ratios, reflecting their origin from reducing, Fe2+-bearing basement-derived fluids having undergone variable mixing with oxidizing basinal fluids. Chlorites in mineralized systems where fluid-mixing occurred, such as at McArthur River Zone 4 and Maurice Bay, record higher Fe3+/ƩFe ratios than barren systems where fluid-mixing did not, such as at Wheeler River Zone K and Spring Point. The scarcity of U-bearing basinal fluids available for mixing with Fe2+-bearing basement fluids is a critical geochemical factor precluding mineralization in barren sandstone-hosted systems. The Fe3+/ƩFe ratio of chlorites has potential applications for discriminating barren and mineralized systems and as spatial vectors to ore when coupled with Pb isotope ratios.