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Abstract

Precambrian basins in Australia and Canada host massive unconformity-type uranium ore deposits, which constitute roughly 25% of the world’s known uranium resources and are the focus of international study as ancient analogs for nuclear waste repositories. Simulation of reactive mass transport and variable-density groundwater flow within these basins indicates that free convection of a uranium-bearing chloride brine at rates of about 1 ni/yr precipitated ore-grade quantities of uraninite (U02) near the unconformity between basin sandstones and graphiterich basement rocks within 100 to 1000 ky. Hydrothermal/diagenetic alteration surrounding the ore deposits resulted from mass transport through temperature gradients and across compositional boundaries. This alteration, consisting primarily of chlorite and muscovite precipitation, is strongly dependent on the pattern of groundwater flow and heat transport within the basins. The primary condition required for mineralization is the coincidence in space of graphite-rich basement rocks and upwelling limbs of free convection cells.

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