FERROUS IRON FROM DETRITAL BIOTITE AS REDUCTANT IN FORMATION OF A ROLL-FRONT URANIUM DEPOSIT

Classic roll-front type uranium deposits in Wyoming Tertiary basins occur at the contact between a zone of reduced rock containing one or more ‘reductant’ phases and an oxidized zone in which the ‘reductant’ phase has been depleted. Common reductant phases include buried organic matter and pyrite, other potential reductants include methane gas, liquid hydrocarbons, and sulfate-reducing bacteria. The deposit grows and advances down the hydrologic gradient as the reductant phase is depleted in the oxidized zone. Roll-front deposits lacking obvious reductant phases and typical spatial relations between mineralized zones and the occurrence of potential reductant phases also occur. Framboidal pyrite is ubiquitous in both the oxidized and mineralized portions of the deposit in this study but only rare, cubic, non-framboidal pyrite occurs in the reduced zone down-gradient from the deposit. Framboidal pyrite is biogenic in origin, suggesting bacterial activity as the reductant. The existence of this pyrite in both the oxidized and mineralized zones, and its absence in the reduced zones, suggests that processes responsible for its deposition post-dated the initial formation of the ore deposit, and pyrite is not a likely controlling reductant phase in this deposit. Calcite is also rare to absent in all zones. Organic matter is rare in this deposit, and no systematic variation in abundance of organic matter between the oxidized and reduced zones has been recognized. X-ray analysis of clay minerals associated with this deposit reveal that clays in the oxidized zone include abundant corrensite, while corrensite is absent in the reduced zone, but biotite is common. Chlorite in the reduced zone and coexisting with the ore deposit has higher Mg/(Fe+Mg) than the biotite, and poorly crystallized ferrihydroxide materials are abundant. We infer that in-situ alteration of biotite to chlorite and corrensite generated ferrous iron in solution. This ferrous iron acted as the reductant responsible for the formation of this ore body. Formation of this deposit required (a) rapid erosion and deposition of biotite-rich sandstones, (b) diagenesis without formation of significant carbonate, and (c) diachronous formation of corrensite from biotite and reduction of U(VI) to U(IV) to produce coffinite and rare uraninite.