A record of uranium-series transport at Nopal I, Sierra Peña Blanca, Mexico: Implications for natural uranium deposits and radioactive waste repositories

Abstract

Studies of uranium-series (U-series) disequilibria within and around ore deposits provide valuable information on the extent and timing of actinide mobility, via mineral-fluid interaction, over a range of spatial and temporal scales. Such information is useful in studies of analogs of high-level nuclear-waste repositories, as well as for mining and mineral extraction sites, locations of previous nuclear weapons testing, and legacy nuclear waste contamination. In this study we present isotope dilution mass spectrometry U-series measurements for fracture-fill materials (hematite, goethite, kaolinite, calcite, dolomite and quartz) from one such analog; the Nopal I uranium ore deposit situated at Peña Blanca in the Chihuahua region of northern Mexico. The ore deposit is located in fractured, unsaturated volcanic tuff and fracture-fill materials from surface fractures as well as fractures in a vertical drill core have been analyzed. High uranium concentrations in the fracture-fill materials (between 12 and 7700ppm) indicate uranium mobility and transport from the deposit. Uranium concentrations generally decrease with horizontal distance away from the deposit but in this deposit there is no trend with depth below the surface.Isotopic activity ratios, 238U–234U-230Th–232Th linear age trends, and U-Th-Pa model ages indicate an actinide mobility evolution that can be characterized as complex and varied. There is evidence of both open and closed system behavior for uranium, depending on both sample and time-scale. The 234U/238U activity ratios indicate U mobility from 1.2–0.2Ma. In the last 0.2Ma there is isotopic evidence for some U mobility but mostly U immobility. In almost all cases the Th‐Pa radionuclides studied here have been immobile for the past 200ka. Radium appears to have been recently (0.1–8ka) mobile due to ongoing fluid flow in the fractures. Overall, the mainly closed system behavior for U-Th-Pa over the past ~200ka provides an indicator of the geochemical immobility of these actinides over time-scales relevant for potential nuclear waste repositories situated in fractured, unsaturated tuff.