A RECORD OF URANIUM-SERIES TRANSPORT IN FRACTURED, UNSATURATED TUFF AT NOPAL I, SIERRA PEñA BLANCA, MEXICO

Studies of U-series disequilibria near uranium deposits can provide valuable information on the timing of actinide mobility over a range of spatial and temporal scales. In this study, we characterize the geochemical evolution of the Nopal I uranium ore deposit (Sierra Peña Blanca, Chihuahua, Mexico) in terms of mineral-fluid interactions as well as the role that vertical fractures play in U transport and retention. The Nopal I deposit is a possible analog for actinide transport from a nuclear waste repository in fractured, unsaturated tuff and an arid climate.

Samples have been collected from a vertical drill core (PB-1b) that extends to a depth of 250 m below the surface. Three samples have been selected for initial study from the vertical fractures at depths of 8.5 m, 66.5 m and 191 m. Since a variety of minerals are present in the fine-grained fracture-fill materials, different colored fractions were selected from each sample. From these fractions we can evaluate U-Th isochrons for dating past actinide transport.

Samples from fractures were dissolved and spiked with ²²⁹Th and ²³³U tracers. U and Th were separated from the rock matrix by ion exchange chromatography, and high-precision U-Th concentration and isotopic ratios are being measured by MC-TIMS and MC-ICP-MS. Our preliminary data show that uranium concentrations range from ~0.1- 0.8 wt. % and U/Th weight ratios range from ~40-400. The ore deposit contains ~0.6 wt. % U, therefore our results suggest that U has been mobile up to 200 m depth in the past.

²³⁴U/²³⁸U activity ratios in the fracture-filling materials range from 0.66 to 2.44 and illustrate a complex evolution consistent with interaction between groundwater enriched in ²³⁴U and a tuff matrix that is depleted in ²³⁴U due to recoil-related leaching. Preliminary closed-system ages determined from U-Th isochrons are >200 ka. Although mineral-fluid interaction is ongoing and more complex open system models are possible, the simplest interpretation of these data is that the fracture filling materials have remained closed with respect to U and Th mobility for >200 ka. These results confirm our prior U-series chronology studies of horizontal fractures at this site and also provide a baseline for more detailed mineral work on actinide mobility and retention using LA-MC-ICP-MS.