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

• Published in: Petrology 2019, Vol.27 (4), p.407-424
• Main Authors: Chernyshev, I. V., Golubev, V. N., Chugaev, A. V., Mandzhieva, G. V., Gareev, B. I.
• Format: Article
• Language: English
• Subjects: Chemical composition; Distribution; Earth and Environmental Science; Earth Sciences; Enrichment; Fractionation; Geochemistry; Isotope composition; Isotope fractionation; Isotopes; Leaching; Light; Mineralization; Mineralogy; Minerals; Oxidation; Quarries; Ratios; Rocks; Uranium; Uranium isotopes; Volcanic rocks; Water analysis; Water sampling; Weathering
• ISSN: 0869-5911; 1556-2085
• DOI: 10.1134/S0869591119040027

The trend fractionation of the 238 U and 235 U 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 238 U/ 235 U were carried out by MC-ICP-MS, using a 233 U– 236 U double spike. The results involve the 238 U/ 235 U and 234 U/ 238 U ratios and the overall range of the δ 238 U 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 235 U isotope. When the pitchblende is replaced by U(VI) minerals, the 238 U and 235 U isotopes also fractionate with ~0.3‰ enrichment of the younger U(VI) mineral phases in the light 235 U isotope. The 238 U/ 235 U and 234 U/ 238 U ratios are proved to correlate, and hence, the fractionation of the 238 U and 235 U isotopes and the enrichment of the aqueous phase in the light 235 U isotope proceed simultaneously with the well known shift in equilibrium the 238 U– 234 U system with the accumulation of excess amounts of the 234 U in the aqueous phase. Uranium leaching from uranium minerals, which is associated with the enrichment of the aqueous phase in excess amounts of the 234 U isotope, can be viewed as a process that controls isotope fractionation in the 238 U– 235 U system. This should be taken into account in describing the fractionation mechanism of the 238 U and 235 U isotopes at U(IV) → U(VI) oxidation. The fractionation of the 238 U and 235 U isotopes, which results in the isotopic “lightening” of U in the aqueous phase, largely controlled the complicated distribution pattern of the 238 U/ 235 U 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 li