Uranium redox and deposition transitions embedded in deep-time geochemical models and mineral chemistry networks

Abstract

Uranium (U) is an important global energy resource and a redox sensitive trace element that reflects changing environmental conditions and geochemical cycling. The redox evolution of U mineral chemistry can be interrogated to understand the formation and distribution of U deposits and the redox processes involved in U geochemistry throughout Earth history. In this study, geochemical modeling using thermodynamic data, and mineral chemistry network analysis are used to investigate U geochemistry and deposition through time. The number of U 6+ mineral localities surpasses the number of U 4+ mineral localities in the Paleoproterozoic. Moreover, the number of sedimentary U 6+ mineral localities increases earlier in the Phanerozoic than the number of U 4+ sedimentary mineral localities, likely due to the necessity of sufficient sedimentary organic matter to reduce U 6+ –U 4+ . Indeed, modeling calculations indicate that increased oxidative weathering due to surface oxygenation limited U 4+ uraninite (UO 2 ) formation from weathered granite and basalt. Louvain network community detection shows that U 6+ forms minerals with many more shared elements and redox states than U 4+ . The range of weighted Mineral Element Electronegativity Coefficient of Variation (wMEE CV ) values of U 6+ minerals increases through time, particularly during the Phanerozoic. Conversely, the range of wMEE CV values of U 4+ minerals is consistent through time due to the relative abundance of uraninite, coffinite, and brannerite. The late oxidation and formation of U 6+ minerals compared to S 6+ minerals illustrates the importance of the development of land plants, organic matter deposition, and redox-controlled U deposition from ground water in continental sediments during this time-period.