CARBONATES AS A WINDOW INTO FLUID PROCESSES IN THE PARADOX BASIN, UT AND CO

Distinctive aspects of the geologic framework, paragenesis, and chemistry of carbonate minerals provide insight into the diverse conditions (from EMPA and SEM compositions) and timing (from carbonate U-Pb) of fluid events in the Paradox Basin. Included in the basin’s fluid history are the events that generated sandstone-hosted U(-V) and Cu(-Ag) deposits. Focusing primarily on the La Sal district, new mapping, core logging and petrography show that there are several different stages of carbonate formation: early U(-V) related cements are overgrown by multiple younger generations, and later-still are fault-related Cu(-Ag) carbonates. Carbonate concretions in Morrison Formation sandstones comprise ferroan dolomites, whereas tabular U(-V) deposits hosted in the same units contain paragenetically older calcites with a slight manganoan affinity. In contrast, carbonates in vein and stratabound Cu(-Ag) deposits can range from calcite to dolomite in composition, both in vein and in disseminated form. The range in compositions is consistent with differences in redox state and perhaps salinities of the causative fluids. U-Pb geochronology of ore-stage carbonates yields Early Cretaceous ages for U(-V) mineralization that are broadly congruent with earlier results obtained by different approaches (whole rock U-Pb, Rb-Sr) and the notion of near-surface mixing of hydrologic systems. Vein carbonates intergrown with hypogene Cu(-Ag) mineralization consistently yield middle Miocene to Pliocene carbonate U-Pb dates. This age range contrasts with previous geological inferences and two Re-Os dates on sulfides. Miocene and younger mineralization implies a change in the basin’s hydrology, possibly as a result of far-field tectonics (extension in the Basin and Range) or initial stages of exhumation.