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 (similar to 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 U-238-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.