RECENT ADVANCES IN URANIUM MINERAL EVOLUTION (Invited Presentation)

The origins and near-surface distributions of the approximately 250 known uranium minerals elucidate principles of mineral evolution. This history can be divided into four phases. The first, from ~4.5 to 3.5 Ga, involved successive concentrations of uranium from its initial uniform trace distribution into magmatic-related fluids from which the first U⁴⁺ minerals, uraninite (UO₂) and coffinite (USiO₄), precipitated in the crust. The second period, from ~3.5 to 2.2 Ga, saw the formation of large low-grade concentrations of detrital uraninite (containing several weight percent Th) in the Witwatersrand-type quartz-pebble conglomerates deposited in a highly anoxic fluvial environment. Abiotic alteration of uraninite and coffinite, including radiolysis and auto-oxidation, may have resulted in the formation of a limited suite of uranyl oxide-hydroxides. Earth’s third phase of uranium mineral evolution, during which most known U minerals first precipitated from reactions of soluble uranyl (U⁶⁺O₂)²⁺ complexes, followed the Great Oxidation Event (GOE) at ~2.2 Ga and thus was mediated indirectly by biologic activity. Most uraninite deposited during this phase was low in Th and precipitated from saline and oxidizing hydrothermal solutions (100 to 300°C) transporting UO₂²⁺-chloride complexes. Examples include the unconformity- and vein-type U deposits (Australia and Canada) and the unique Oklo natural nuclear reactors in Gabon. During this phase, most uranyl minerals would have been able to form in the O₂-bearing near-surface environment for the first time through weathering processes. The fourth phase of uranium mineralization began approximately 400 million years ago, as the rise of land plants led to non-marine organic-rich sediments that promoted new sandstone-type ore deposits. The modes of accumulation and compositions of uraninite are a sensitive indicator of global redox conditions. The near-surface mineralogy of uranium provides a measure of a planet’s geotectonic and geobiological history. In the absence of extensive magmatic-related fluid reworking of the crust and upper mantle, uranium will not become sufficiently concentrated to form its own minerals or ore deposits. Furthermore, in the absence of surface oxidation, all but a handful of the known uranium minerals are unlikely to form.