Dispersion geochemistry at different scales at the Nopal I deposit, Peña Blanca Uranium District, Chihuahua, Mexico

Katrina Eileen Pekar-Carpenter, University of Texas at El Paso


The Nopal I mine in the Peña Blanca Uranium District, Chihuahua, Mexico is a natural analog for the geologic storage of nuclear waste. This dissertation presents data from two sites within Nopal I: 1) the PB-1 core, which was drilled along the extent of uranium mineralization in the Nopal I mine; and 2) a high-grade stockpile boulder from the mineralized Nopal tuff. The PB-1 core allowed an investigation into the mineralogical and geochemical changes that are important to research on a nuclear waste repository. Five zones were determined to control these changes: 1) Zone 1, the primary uranium mineralization (quartz-uraninite-pyrite-kaolinite-ilmenite); 2) Zone 2, the oxidized equivalent to zone 1; 3) Zones 3a and 3b, which are redox-transition zones; 4) Zone 4, or the Pozos Uranium Anomaly, which was a reducing zone for uranium and other redox sensitive elements; and 5) Zone 5, the Cretaceous limestone, which serves as a barrier to a majority of radionuclide transport. SIMS U-series investigation of uranium minerals showed that the Pozos Uranium Anomaly formed 375 ka-1 Ma, and unlike the primary uranium assemblage, the younger uranium minerals were associated with anatase and kaolinite, indicating their adsorptive capabilities in a potential repository setting. The mineralized boulder was analyzed using qualitative and quantitative methods to determine the mode of radionuclide release from the boulder to the soil, the weathering rate, and the transport of radionuclides within the soil from the boulder. The weathering rate of uranium from the boulder (12-34 mg U/m2/day) is ~150 times higher than in laboratory experiments using UO2 fuel pellets, which may be more representative of the effect uncontrolled, intermittent environmental changes have on weathering. Radionuclide transport from the boulder and within the soil occurs by diffusion, dispersion and filtration, but the amount each contributes is still undetermined.

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Recommended Citation

Pekar-Carpenter, Katrina Eileen, "Dispersion geochemistry at different scales at the Nopal I deposit, Peña Blanca Uranium District, Chihuahua, Mexico" (2011). ETD Collection for University of Texas, El Paso. AAI3490041.