Geochemical evolution of uraniferous soda lakes in Eastern Mongolia



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Extremely high concentrations of U were discovered in hypersaline soda lakes in eastern Mongolia. The lakes are small, shallow (<1km², <1m), and terminal with variable salinity. The origin and fate of U in these lakes was investigated using geochemical analyses and modeling, using samples collected from lakes and lake pore waters, wells and a stream. Samples were analyzed for Sr and U isotopes, cations, trace metals, anions, total inorganic carbon (TIC), and unstable field parameters. A representative groundwater in the field area is dilute and alkaline, with pH=7.9, 10 mmol L⁻¹ of TIC and 5 mmol L⁻¹ Cl⁻. In contrast, a representative lake water is pH(similar to)10 with TIC and Cl⁻ each more than 1000 mmol L⁻¹. Uranium concentrations in lake waters range from 0.24 to more than 62.5 (Greek small letter mu)mol L⁻¹, possibly making these lakes the highest naturally occurring U concentrations ever reported in a natural water. Groundwater concentrations of U range from 0.03 and 0.43 (Greek small letter mu)mol L⁻¹. The U is natural and derived from groundwater discharging to stable closed basin lakes. Waters are concentrated by evaporation and U(VI) is chelated by CO₃⁻² to form the highly soluble UO₂(CO₃)₃⁻⁴. Two sets of well waters with corresponding lake discharge waters were analyzed for U isotopes. Unnatural concentrations of ²³⁵U were tested for, the presence of which would indicate fallout-derived U. The average of four samples was ²³⁸U/²³⁵U=136(plus or minus)2 indicating that the U is naturally derived. (Greek small letter delta)²³⁴U between one well and its corresponding discharge lake had similar (Greek small letter delta)²³⁴U values ([Greek small letter delta]²³⁴U=837.6- 858.5). The other sample pair however, revealed significant differences between the well and its discharge lake ([Greek small letter delta] ²³⁴U=303.6-1530). This suggests nuclide recoil is significantly enriching one of the lakes with ²³⁴U beyond secular equilibrium during alpha-decay of ²³⁸U in lake sediments or along the groundwater flow path. Modeled evaporation of lakes demonstrates that a U-mineral phase is likely to precipitate during evaporation. Strontium isotopes varied in groundwaters between ⁸⁷Sr/⁸⁶Sr= 0.706192-0.709776 and in lakes ⁸⁷Sr/⁸⁶Sr=0.708702-0.709432. Cretaceous mafic rocks likely account for low ⁸⁷Sr/⁸⁶Sr values while Cretaceous alkaline rhyolites account for the high ⁸⁷Sr/⁸⁶Sr values. High concentrations of U, Na, Cl⁻, and K correlate to radiogenic Sr in lake waters indicating that U is sourced from local Cretaceous alkaline rhyolites.