Geochemical evolution of uraniferous soda lakes in Eastern Mongolia

Extremely high concentrations of uranium (U) were discovered in shallow, groundwater-fed hyperalkaline soda lakes in Eastern Mongolia. A representative ground water sample in this area is dilute and alkaline, pH = 7.9, with 10 mM TIC and 5 mM Cl-. In contrast, a representative lake water sample is pH similar to 10 with TIC and Cl each more than 1,000 mM. Groundwater concentrations of U range from 0.03 to 0.43 mu M L-1. Lake water U ranges from 0.24 to >62.5 mu M, possibly the highest naturally occurring U concentrations ever reported in surface water. Strontium isotopes Sr-87/Sr-86 varied in groundwaters from 0.706192 to 0.709776 and in lakes Sr-87/Sr-86 varied from 0.708702 to 0.709432. High concentrations of U, Na, Cl-, and K correlate to radiogenic Sr in lake waters suggesting that U is sourced from local Cretaceous alkaline rhyolites. Uranium-rich groundwaters are concentrated by evaporation and U(VI) is chelated by CO3-2 to form the highly soluble UO2(CO3)(3)(-4). Modeled evaporation of lakes suggests that a U-mineral phase is likely to precipitate during evaporation.