MODELING OF CARNOTITE PRECIPITATION FROM OGALLALA AQUIFER GROUNDWATER, TEXAS PANHANDLE, USA

Economic deposits of carnotite (K₂(UO₂)₂(VO₄)₂·3H₂O) in near-surface settings are in Western Australia and in the Namib desert of Namibia. These types of carnotite deposits are associated with calcrete (calcareous-cemented surficial deposits), which form by evaporation of shallow, oxidizing uranium- and vanadium-bearing groundwater along the axis of large stable drainages with low gradients and in arid climates. Economic U deposits of this type are not known to occur in the United States. However, geologic, geochemical, and hydrologic conditions may have been favorable for the formation of calcrete-hosted deposits in the north Texas Panhandle during the Quaternary.

The United States Geological Survey (USGS) geochemical code PHREEQC was used to model the evolution of groundwater chemistry (sourced from the USGS, National Water Information System database) of 100 wells that tap the Ogalalla Aquifer. The purpose of this modeling is to (1) determine the effectiveness of evaporative concentration to precipitate carnotite, and (2) develop a conceptual model of carnotite formation that may lead to new discoveries. Modeling results indicate that a principal factor in achieving carnotite saturation during evaporation was the evolution of groundwater following calcite precipitation. Modeling results indicate that following the precipitation of calcite, if the concentration of calcium was greater than the carbonate alkalinity (2mCa⁺² > mHCO₃^- + 2mCO₃^-2) the groundwater would become saturated with respect to carnotite. If, following the precipitation of calcite, the concentration of calcium was less than the carbonate alkalinity (2mCa⁺² < mHCO₃^- + 2mCO₃^-2) the groundwater would not become saturated with respect to carnotite.