Uranium-Series Disequilibria In The Groundwater Of The Shihongtan Sandstone-Hosted Uranium Deposit, Nw China

Uranium (U) concentration and the activities of U-238, U-234, and Th-230 were determined for groundwaters, spring waters, and lake water collected from the Shihongtan sandstone-hosted U ore district and in the surrounding area, NW China. The results show that the groundwaters from the oxidizing aquifer with high dissolved oxygen concentration (O-2) and oxidation-reduction potential (Eh) are enriched in U. The high U concentration of groundwaters may be due to the interaction between these oxidizing groundwaters and U ore bodies, which would result in U that is not in secular equilibrium. Uranium is re-precipitated as uraninite on weathered surfaces and organic material, forming localized ore bodies in the sandstone-hosted aquifer. The U-234/U-238, Th-230/U-234, and Th-230/U-238 activity ratios (ARs) for most water samples show obvious deviations from secular equilibrium (0.27-2.86), indicating the presence of water-rock/ore interactions during the last 1.7 Ma and probably longer. The U-234/U-238 AR generally increases with decreasing Uconcentrations in the groundwaters, suggesting that mixing of two water sources may occur in the aquifer. This is consistent with the fact that most of the U ore bodies in the deposit have a tabular shape originati from mixing between a relatively saline fluid and a more rapidly flowing U-bearing meteoric water.