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

Precise measurement of the 238U/235U 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 238U/235U fractionation. Low-temperature uranium deposits are, on average, isotopically ∼ 0.4‰ heavier than uranium deposited at high temperatures or by non-redox processes. 238U/235U ratios coupled with 235U/234U ratios in the same sample provide evidence that the redox transition (UVI → UIV) at low temperatures is the primary mechanism of 238U/235U fractionation and that aqueous alteration plays a very limited, if any, role in fractionation of the 238U/235U ratio. The isotopic variation of U is therefore a potential signature that can be used to trace the origin of uranium ore concentrate.