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

摘要

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.