Chapin Wash Formation. Two basal carbonaceous horizons contain an extensive low-grade mineralization (100-200 ppm U) peneconcordant with bedding. Diagenetic alteration to smectite clay minerals and heulandite-clinoptilolite zeolites was accompanied by penecontemporaneous silicification and calcification. Clays are significantly enriched in lithium (0.21-0.39%). Uranium is locally associated with high values of Mo, As, and V. Uranium was preconcentrated during early diagenesis by sorption onto colloidal humic substance, silica gel, and zeolites. Subsequent precipitation as coffinite was mainly controlled by bacteriogenic H 2 S gas. Low-temperature stratabound deposit formed by complex sorption and precipitation processes.--Modified journal abstract.