Fractionation of 238U/235U by reduction during low temperature uranium mineralisation processes

Investigations of 'stable' uranium isotope fractionation during low temperature, redox transformations may provide new insights into the usefulness of the U-238/U-235 isotope system as a tracer of palaeoredox processes. Sandstone-hosted uranium deposits accumulate at an oxidation/reduction interface within an aquifer from the low temperature reduction of soluble U(VI) complexes in groundwaters, forming insoluble U(IV) minerals. This setting provides an ideal environment in which to investigate the effects of redox transformations on U-238/U-235 fractionation. Here we present the first coupled measurements of U-238/U-235 isotopic compositions and U concentrations for groundwaters and mineralised sediment samples from the same redox system in the vicinity of the high-grade Pepegoona sandstone-hosted uranium deposit, Australia. The mineralised sediment samples display extremely variable U-238/U-235 ratios (herein expressed as delta U-238(CRM145). the per-mil deviation from the international NBL standard CRM145). The majority of mineralised sediment samples have delta U-238(CRM145) values between -1.30 +/- 0.05 and 0.55 +/- 0.12 parts per thousand, spanning a ca. 2 parts per thousand range. However, one sample has an unusually light isotopic composition of -4.13 +/- 0.05 parts per thousand, which suggests a total range of U isotopic variability of up to ca. 5 parts per thousand, the largest variation found thus far in a single natural redox system. The U-238/U-235 isotopic signature of the mineralised sediments becomes progressively heavier (enriched in U-238) along the groundwater flow path. The groundwaters show a greater than 2 parts per thousand variation in their 238U/235U ratios, ranging from delta U-238(CRM145) values of -2.39 +/- 0.07 to -0.71 +/- 0.05 parts per thousand. The majority of the groundwater data exhibit a clear systematic relationship between U-238/U-235 isotopic composition and U concentration; samples with the lowest U concentrations have the lowest U-238/U-235 ratios. The preferential incorporation of U-238 during reduction of U(VI) to U(IV) and precipitation of uranium minerals leaves the groundwaters enriched in U-235, resulting in a progressive shift in U-238/U-235 towards lighter values in the aqueous phase as U is removed. These data can be modelled by a closed system Rayleigh fractionation model, with a fractionation factor (alpha, representing the U-238/U-235 composition of the groundwater relative to the solid uranium minerals) ranging from similar to 0.9996 to 1.0000, with the majority of datapoints ranging from alpha values of 0.9998 to 0.9999. The sense and magnitude of the results of this study imply that U-238/U-235 fractionation is likely to be controlled by volume-dependent nuclear field shift effects during the reduction of U(VI) to U(IV) during mineralisation processes. These findings support the use of the U-238/U-235 isotopic system as a tracer to constrain the nature and timing of palaeoredox conditions. (C) 2013 Elsevier B.V. All rights reserved.