FLUID EVOLUTION AND STRUCTURAL CONTROL ON URANIUM DEPOSITS IN SUCCESSOR BASINS IN NORTHERN CANADA AND NORTHERN AUSTRALIA
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Uranium deposits associated with Paleoproterozoic successor basins were investigated using structural, petrographic, geochronological and geochemical relationships to understand the character and timing of ore-forming fluids and the structural control on uranium mineralization. The work focused on two successor basins that share similar geological characteristics: the Martin Lake Basin in the Beaverlodge area in Canada, and the El Sherana Basin in the South Alligator River area in Australia. The Beaverlodge area records six temporally distinct stages of U mineralization spatially associated with the Martin Lake successor basin. Early minor stages are hosted in cataclasite and veins at ca. 2.29 Ga and in albitized granite in the Gunnar deposit between ca. 2.3 Ga and 1.9 Ga, which predates the main stage of U mineralization of hydrothermal breccias that formed at ca. 1.85 Ga. Later stages of mineralization are related to minor veins at ca. 1.82 Ga linked to alkaline mafic dikes associated with the Martin Lake Basin and to minor veins at ca. 1.62 Ga corresponding to the timing of unconformity-type U mineralization in the overlying Athabasca Basin. The main breccia-type U mineralizing event that affected all deposits in the Beaverlodge area formed at ca. 1.85 Ma from metamorphic fluids at ca. 330oC linked to metasomatism during regional metamorphism of the Trans-Hudson Orogen. The ore-forming fluids were likely derived from metamorphic remobilization of pre-existing U-rich basement rocks, and ascended upward along deep fracture systems that resulted from brittle reactivation of early ductile shear zones. The main event of U mineralization in the South Alligator River area formed at ca. 1.82 Ma, subsequent to deposition of the El Sherana Group at 1.84-1.83 Ma. The formation of these deposits is related to fluids derived from diagenetic processes in sandstone of the El Sherana Group. Mineralization formed when a 250oC, low latitude, oxidizing, U-bearing basinal brine from diagenetic aquifers in the Coronation sandstone descended downward into the unconformity along fracture systems created by brittle reactivation of the El Sherana-Palette fault system. Uranium deposits associated with successor basins in the Beaverlodge and South Alligator River area are older than those in the U-rich Athabasca and Kombolgie basins. Rocks that host these deposits have been folded, and then exhumed during subsequent tectonic events. These older U deposits can be considered as a potential source for detrital uraninite that fed sediments of the Athabasca and Kombolgie basins and therefore contributed to the inventory of uranium that formed unconformity-related U mineralization in the younger basins. Therefore, the occurrence of older U mineralization associated with successor basins can be considered as positive criterion for exploration of unconformity-related U mineralization in younger Paleoproterozoic basins.