U-Series disequilibrium in groundwater as a vector for U mineralisation

Groundwaters often exhibit ( (super 234) U/ (super 238) U) activity ratios greater than one as a result of fractionation between (super 234) U and (super 238) U nuclides during rock/water interactions. However, when groundwaters pass through high-grade uranium mineralisation, congruent dissolution of uranium minerals should impart a ( (super 234) U/ (super 238) U) activity ratio which is at or very close to secular equilibrium. This research characterises the uranium-series (U-series) disequilibria in groundwater surrounding a high-grade uranium deposit, and investigates the use of disequilibria in groundwater as a proxy for uranium exploration. U-Series isotopes have been analysed by isotope dilution MC-ICP-MS in groundwater samples along the groundwater flow path, within and surrounding the South Australian Four Mile and Pepegoona sediment-hosted uranium mineralised systems. Samples collected down-gradient of the Four Mile mineralisation have ( (super 234) U/ (super 238) U) activity ratios ranging from 1.12 proximal to mineralisation, up to 2.08 approximately 10 kms down-gradient from mineralisation. U concentrations range from 0.5 - 200 ppb, with highest concentrations found in samples collected in mineralisation. Groundwaters sampled within the high-grade mineralisation show ( (super 234) U/ (super 238) U) activity ratios close to unity (1.05), which is consistent with the congruent dissolution of uranium minerals. The observation of increasing disequilibrium with distance from mineralisation highlights the potential application of U-series isotopes as an indicator of high-grade uranium deposits. However, the extent and distribution of disequilibrium at the Pepegoona deposit is not entirely consistant with proximity to the mineralisation. Groundwaters samples within mineralisation exhibit much higher ( (super 234) U/ (super 238) U) activity ratios than at Four Mile, averaging 1.41. Samples collected down-gradient have typically lower U concentrations and ( (super 234) U/ (super 238) U) activity ratios approaching unity. This signature reflects the highly variable local geology of the Pepegoona deposit.1 page(s