Behavior of the ²³⁸U, ²³⁵U, and ²³⁴U Isotopes at Weathering of Volcanic Rocks with U Mineralization: A Case Study at the Tulukuevskoe Deposit, Eastern Transbaikalia

The trend fractionation of the ²³⁸U and ²³⁵U isotopes and the extent of this process at the oxidative weathering of uranium minerals were evaluated by studying the variations in the U isotope composition of rocks, minerals, and fracture waters sampled in the quarry of the broadly known Tulukuevskoe uranium deposit in the Streltsovskoe ore field, eastern Transbaikalia. In the rock block in question, fine uranium minerals disseminated in the rocks were weathered under the effect of oxidizing fracture waters. Uranium isotope composition was measured in 22 water samples, eleven samples of the mineralized rocks, and eight uranium minerals. High-precision (±0.07‰, 2SD) measurements of the ²³⁸U/²³⁵U were carried out by MC-ICP-MS, using a ²³³U–²³⁶U double spike. The results involve the ²³⁸U/²³⁵U and ²³⁴U/²³⁸U ratios and the overall range of the δ²³⁸U variations determined in the rocks and waters (from –0.13 to –1.0‰ and from –0.22 to –0.59‰, respectively). Interaction between the waters and rocks induces U(IV) → U(VI) oxidation, U(VI) transfer into the aqueous phase, and 0.15–0.28‰ enrichment of U dissolved in the water in the ²³⁵U isotope. When the pitchblende is replaced by U(VI) minerals, the ²³⁸U and ²³⁵U isotopes also fractionate with ~0.3‰ enrichment of the younger U(VI) mineral phases in the light ²³⁵U isotope. The ²³⁸U/²³⁵U and ²³⁴U/²³⁸U ratios are proved to correlate, and hence, the fractionation of the ²³⁸U and ²³⁵U isotopes and the enrichment of the aqueous phase in the light ²³⁵U isotope proceed simultaneously with the well known shift in equilibrium the ²³⁸U–²³⁴U system with the accumulation of excess amounts of the ²³⁴U in the aqueous phase. Uranium leaching from uranium minerals, which is associated with the enrichment of the aqueous phase in excess amounts of the ²³⁴U isotope, can be viewed as a process that controls isotope fractionation in the ²³⁸U–²³⁵U system. This should be taken into account in describing the fractionation mechanism of the ²³⁸U and ²³⁵U isotopes at U(IV) → U(VI) oxidation. The fractionation of the ²³⁸U and ²³⁵U isotopes, which results in the isotopic “lightening” of U in the aqueous phase, largely controlled the complicated distribution pattern of the ²³⁸U/²³⁵U ratio in the quarry. In addition to isotope fractionation, this distribution was likely also affected by isotope exchange between uranium dissolved in the water and uranium in the finely dispersed minerals. The isotopically light uranium of the water could participate in forming U(VI) minerals at lower levels of the quarry.