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

摘要

The trend fractionation of the U-238 and U-235 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 parts per thousand, 2SD) measurements of the U-238/U-235 were carried out by MC-ICP-MS, using a U-233-U-236 double spike. The results involve the U-238/U-235 and U-234/U-238 ratios and the overall range of the delta U-238 variations determined in the rocks and waters (from -0.13 to -1.0 parts per thousand and from -0.22 to -0.59 parts per thousand, 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 parts per thousand enrichment of U dissolved in the water in the U-235 isotope. When the pitchblende is replaced by U(VI) minerals, the U-238 and U-235 isotopes also fractionate with similar to 0.3 parts per thousand enrichment of the younger U(VI) mineral phases in the light U-235 isotope. The U-238/U-235 and U-234/U-238 ratios are proved to correlate, and hence, the fractionation of the U-238 and U-235 isotopes and the enrichment of the aqueous phase in the light U-235 isotope proceed simultaneously with the well known shift in equilibrium the U-238-U-234 system with the accumulation of excess amounts of the U-234 in the aqueous phase. Uranium leaching from uranium minerals, which is associated with the enrichment of the aqueous phase in excess amounts of the U-234 isotope, can be viewed as a process that controls isotope