CHARACTERIZATION OF U(-V) DEPOSITS IN THE LA SAL DISTRICT, UT AND CO AND THEIR RELATIONSHIP TO PARADOX BASIN FLUID FLOW

The tabular sandstone-hosted uranium (U) and vanadium (V) deposits of the La Sal district form a 1-3 km wide, 30-km long belt spanning the Utah-Colorado border. They exemplify one of the principal types of U(-V) mineralization of the Paradox Basin and encompassing Colorado Plateau. Building on previous work, new field, petrographic and geochemical data provide the framework for an updated district synthesis and its place in the broader geological context. Ore is hosted in the uppermost fluvial sandstones of the Salt Wash Member of the Jurassic Morrison Formation and forms elongate ore bodies up to 180 m long, 90 m wide, and 1 m thick. Sparse organic material, mainly in the form of coalified plant fragments, is present in the sandstones which are interbedded with finer grained silty, muddy and calcareous units. Principal changes in the rocks include bleaching by removal or reaction of early diagenetic hematite, a process that is coextensive with quartz overgrowths. Later formed were compositionally distinct carbonate cements, and reaction of feldspars to kaolinitic clays. Bleaching was prior to or contemporaneous with mineralization, which is restricted to bleached rocks. Petrography shows that ore minerals (uraninite, coffinite, and montroseite) at least in part predate growth of authigenic quartz cements, in turn followed by the formation of V-rich sheet silicates (which may reflect reaction of montroseite with quartz and other minerals) and carbonates and clay cements. Depositional features control where mineral growth occurred and include sedimentary structures (e.g., crossbedding), primary porosity, and the distribution of lithofacies in trends like the La Sal channel system. The thin section to district-scale observations suggest that mineralization reflects mixing of two fluids, as has been suggested in other Plateau deposits, or, alternatively as a reaction of oxidized fluids with an exogenous reduced component during bleaching. Increasing evidence in this area and elsewhere in the Paradox Basin suggests a complex fluid history including a major role for hydrocarbon-bearing fluids in bleaching and localization of metals beginning at least as early as the Late Triassic.