Natural immobilization of uranium by phosphate mineralization in an oxidizing saprolite–soil profile: chemical weathering of the Coles Hill uranium deposit, Virginia

Abstract

Geochemical and mineralogical studies of the 15- to 20-m-thick soil and saprolite profile developed over the Coles Hill uranium deposit demonstrate that uranium transport may be inhibited or naturally attenuated by the precipitation of U(VI) phosphate minerals in oxidizing and saturated soil environments. This study examines geochemical conditions in which these secondary uranium phosphates are formed and remain stable for hundreds of thousands of years. We also describe geochemical conditions that may lead to their dissolution. The lower part of the weathering profile, which consists of groundwater saturated saprolite, contains up to 1300 mg/kg uranium (within the solid). This concentration is approximately 1.5 times greater than the average ore grade of the deposit, indicating that the saturated saprolites are enriched relative to the underlying primary ore. Uranium within this zone is dominantly associated with U(VI) phosphates of the meta-autunite mineral group. Groundwaters from this zone contain less than 14 μg/l uranium suggesting that the U(VI) phosphate minerals present within the Coles Hill saprolites are capable of buffering dissolved uranium concentrations to values significantly lower (by a factor of 2 or more) than the US-EPA maximum contaminant level of 30 μg/l. Above the water table, U(VI) phosphates of the type found in the saturated saprolite zone are not present, and the soil uranium concentration drops to an average value of 250 mg/kg. Thus, the zone of water table fluctuation represents a major transition zone in uranium mineralogy. Mineralogical characterization of the upper part of the profile suggests that geochemical conditions in the unsaturated zone are not conducive to uranium stabilization by meta-autunite group minerals. Uranium within the vadose zone, however, is associated with phosphate (e.g. Al phosphate minerals and sorption with phosphate to ferric oxyhydroxides). As a whole, the Coles Hill soil profile is interpreted to have reached a steady state with respect to uranium transport and immobilization. Results of this study suggest that uranium is leached from the unsaturated part of the profile (meta-autunite unstable) and reprecipitated below the water table where the measured activity ratio of dissolved phosphate to carbonate is higher. Based on estimates of weathering rates for this region, the processes responsible for the dispersal of uranium within the Coles Hill system are estimated to have been active on a time scale of hundreds of thousands of years. This system thus represents an excellent natural laboratory for understanding the long-term mineralogical and geochemical processes involved in the coupling of uranium and phosphorus in natural, fluid-rich systems.