Ore-Forming Process and Redox Zoning of Sandstone-Type U Deposits: Evidence from U Series Disequilibrium

Uranium series disequilibrium techniques were used to provide information on the redox zoning and ore-forming processes of the Kujie'ertai uranium deposit, Yili basin, Xinjiang, NW China. The uranium and thorium contents and the activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²³⁰Th/²³⁸U for about 26 samples from the deposit were measured using ICP-MS and α-spectrometer respectively. The characteristics of uranium and thorium isotopes for different redox zones are significantly different, which can serve as indicators for locating ore-bodies of the sandstone-type uranium deposit. The U and Th concentrations and Th/U ratios of rocks are 12.4 μg/g, 4.5 μg/g and 0.48 for the strongly oxidized zone, 20.4 μg/g, (5.0 μg/g) and 0.38 for the weakly oxidized zone, 169.7 μg/g, 4.7 μg/g and 0.07 for the transitional zone (the mineralized zone), 6.8 μg/g, 3.7 μg/g and 0.87 for the reduced zone (the unaltered zone) respectively. These different redox zones also present different ratios of radioactive nuclides for various samples. The ²³⁴U/²³⁸U ratios are mostly greater than 1 for the strongly oxidized zone, partly greater or less than 1 and partly close to 1 for the redox front (the mineralized zone), and predominantly greater than unity for the reduced zone (the unaltered zone) respectively. While the ratios of ²³⁰Th/²³⁴U and ²³⁰Th/²³⁸U are greater than or close to unity for the oxidized zone, partly greater than 1 and partly less than or close to 1 for the redox front (the mineralized zone), and greater than or close to unity for the reduced zone (the mineralized zone). The uranium ore-formation processes in the deposit are traced effectively by U series disequilibrium.