Variations in the uranium isotopic compositions of uranium ores from different types of uranium deposits

Variations in U-238/U-235 and U-234/U-238 ratios were measured in uranium minerals from a spectrum of uranium deposit types, as well as diagenetic phosphates in uranium-rich basins and peraluminous rhyolites and associated autunite mineralisation from Macusani Meseta, Peru. Mean delta U-238 values of uranium minerals relative to NBL CRM 112-A are 0.02 parts per thousand for metasomatic deposits, 0.16 parts per thousand for intrusive, 0.18 parts per thousand for calcrete, 0.18 parts per thousand for volcanic, 0.29 parts per thousand for quartz-pebble conglomerate, 0.29 parts per thousand for sandstone-hosted, 0.44 parts per thousand for unconformity-type, and 0.56 parts per thousand for vein, with a total range in delta U-238 values from -0.30 parts per thousand to 1.52 parts per thousand. Uranium mineralisation associated with igneous systems, including low-temperature calcretes that are sourced from U-rich minerals in igneous systems, have low delta U-238 values of ca. 0.1 parts per thousand, near those of their igneous sources, whereas uranium minerals in basin-hosted deposits have higher and more variable values. High-grade unconformity-related deposits have delta U-238 values around 0.2 parts per thousand, whereas lower grade unconformity-type deposits in the Athabasca, Kombolgie and Otish basins have higher delta U-238 values. The delta U-234 values for most samples are around 0 parts per thousand, in secular equilibrium, but some samples have delta U-234 values much lower or higher than 0 parts per thousand associated with addition or removal of U-234 during the past 2.5 Ma. These delta U-238 and delta U-234 values suggest that there are at least two different mechanisms responsible for U-238/U-235 and U-234/U-238 variations. The U-234/U-238 disequilibria ratios indicate recent fluid interaction with the uranium minerals and preferential migration of U-234. Fractionation between U-235 and U-238 is a result of nuclear-field effects with enrichment of U-238 in the reduced insoluble species (mostly UO2) and U-235 in oxidised mobile species as uranyl ion, UO22+, and its complexes. Therefore, isotopic fractionation effects should be reflected in U-238/U-235 ratios in uranium ore minerals formed either by reduction of uranium to UO2 or chemical precipitation in the form of U6+ minerals. The delta U-238 values of uranium ore minerals from a variety of deposits are controlled by the isotopic signature of the uranium source, the efficiency of uranium reduction in the case of UO2 systems, and the degree to which uranium was previously removed from the fluid, with less influence from temperature of ore formation and later alteration of the ore. Uranium isotopes are potentially superb tracers of redox in natural systems. Crown copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.