Comparison of granite-related uranium deposits in the Beaverlodge district (Canada) and South China - A common control of mineralization by coupled shallow and deep-seated geologic processes in an extensional setting

Many uranium deposits are related to granitic rocks, but the mineralization ages are much younger, thus excluding a direct magmatic-hydrothermal link between the mineralization and the granites. Two such examples are the Proterozoic "vein-type" uranium deposits in the Beaverlodge district in Canada and the Mesozoic granite-related uranium deposits in South China. Both areas have been extensively studied, but the critical factors that control the mineralization remain unclear. The uranium mineralization in the Beaverlodge district occurs in quartz - carbonate +/- albite veins and breccias developed within and near major deformation zones, and are mainly hosted by ca. 2.33 - 1.90 Ga granitic rocks and ca. 2.33 Ga Murmac Bay Group amphibolite. These rocks are unconformably overlain by the Martin Lake Basin, which was formed during a period of regional extension in the later stage of the Trans-Hudson orogeny and is filled with red beds. A ca. 1820 Ma mafic magmatic event is manifested as volcanic rocks occurring within the Martin Lake Basin and as dikes crosscutting the basement rocks and lower Martin Group strata. Uraninite U-Pb and Pb-Pb ages range from ca. 2290 Ma to < 300 Ma, with a peak overlapping the mafic magmatism. The granite-related uranium mineralization in South China occurs mainly as quartz - carbonate +/- fluorite veins and as disseminations in the host rocks adjacent to fracture zones. The deposits are hosted by, or occur adjacent to, granitic rocks, and are spatially close to Cretaceous - Tertiary red bed basins that were formed in a Basin-and-Range like tectonic setting related to roll back of the Pacific plate. These red bed basins contain intervals of bimodal volcanic rocks and are crosscut by coeval mafic dikes. Uraninite U-Pb ages dominantly range from 100 to 50 Ma, which is much younger than the host granites (> 145 Ma) and broadly contemporaneous with development of the red bed basins and mafic magmatism. Comparison of the two study areas reveals striking similarities in geologic attributes related to uranium mineralization, specifically, the development of large volumes of granitic rocks that are relatively enriched in uranium, and the development of red bed basins with accompanying coeval mafic magmatism in extensional tectonic settings. It is proposed that oxidizing basinal fluids from the red bed basins circulated into the underlying fertile granitic rocks along high-permeability structural zones, acquired uranium in the path through fluid-rock interaction, and precipitated the uranium where they encountered reducing agents. Elevated geo-thermal gradients associated with the mantle-derived magmatism greatly enhanced the mineralization by facilitating the uranium extraction and transport processes. Thus, it is the coupling of shallow (red bed basin and oxidizing basinal fluid development) and deep-seated (mantle-derived magmatism and related thermal activity) processes, together with the pre-enrichment of uranium in basement rocks (particularly granitic rocks) and preexisting deformation zones, that controlled the formation of the granite-related uranium deposits in both Beaverlodge and South China, and perhaps elsewhere in the world with similar geologic setting.