Biogenic non-crystalline U(IV) revealed as major component in uranium ore deposits

Historically, it is believed that crystalline uraninite, produced via the abiotic reduction of hexavalent uranium (U^(VI)) is the dominant reduced U species formed in low-temperature uranium roll-front ore deposits. Here we show that non-crystalline U^(IV) generated through biologically mediated U^(VI) reduction is the predominant U^(IV) species in an undisturbed U roll-front ore deposit in Wyoming, USA. Characterization of U species revealed that the majority (∼58-89%) of U is bound as U^(IV) to C-containing organic functional groups or inorganic carbonate, while uraninite and U^(VI) represent only minor components. The uranium deposit exhibited mostly ²³⁸U-enriched isotope signatures, consistent with largely biotic reduction of U^(VI) to U^(IV). This finding implies that biogenic processes are more important to uranium ore genesis than previously understood. The predominance of a relatively labile form of U^(IV) also provides an opportunity for a more economical and environmentally benign mining process, as well as the design of more effective post-mining restoration strategies and human health-risk assessment.