Variations in ²³⁸U/²³⁵U in uranium ore deposits: Isotopic signatures of the U reduction process?

The ability to measure ²³⁸U/²³⁵U to high precision presents an important new opportunity to study the fate and transport of uranium in the environment. The ratio of ²³⁸U/²³⁵U was determined by multicollector-inductively coupled plasma-mass spectrometer in six uranium ore samples representing two different classes of deposits. Significant offsets in ²³⁸U/²³⁵U are observed between uranium ores precipitated from groundwaters at low temperature versus hydrothermal deposits precipitated at high temperatures, reinforcing an observation made previously but lacking the needed precision. Specifically, tabular sandstone-type uranium deposits were found to be depleted in ²³⁵U, with a total offset between low-temperature deposits and higher temperature deposits of =1.0‰. We attribute this offset to refl ect a temperature-dependent fractionation related to the nuclear field shift effect during chemical reduction of uranium in ambient temperature groundwaters.