Natural variations in uranium isotope ratios of uranium ore concentrates: Understanding the 238U/235U fractionation mechanism

Precise measurement of the U-238/U-235 ratio in geologic samples is now possible with modern techniques and mass spectrometers. Natural variations in this ratio have been shown in previous studies. In this study, data obtained from uranium ore concentrates of mining facilities around the world show clear evidence that the depositional redox environment in which uranium is precipitated is the primary factor affecting U-238/U-235 fractionation. Low-temperature uranium deposits are, on average, isotopically similar to 0.4 parts per thousand. heavier than uranium deposited at high temperatures or by non-redox processes. U-238/U-235 ratios coupled with U-235/U-234 ratios in the same sample provide evidence that the redox transition (U-VI -> U-IV) at low temperatures is the primary mechanism of U-238/U-235 fractionation and that aqueous alteration plays a very limited, if any, role in fractionation of the U-238/U-235 ratio. The isotopic variation of U is therefore a potential signature that can be used to trace the origin of uranium ore concentrate. (c) 2010 Elsevier B.V. All rights reserved.