CHARACTERIZATION OF URANIUM MINERALS IN FRACTURES AT DEPTH FROM THE PB-1 WELL, PEÑA BLANCA URANIUM DISTRICT, CHIHUAHUA, MEXICO

The Nopal I uranium mine is part of the Peña Blanca uranium district, Chihuahua, Mexico. The Nopal I deposit is hosted in Tertiary rhyolite tuffs located in a semi-arid region over 200 m above the water table in an oxidizing, unsaturated zone. Many researchers recognized the similarities in the geologic and climatic conditions between the Nopal I deposit and the Yucca Mountain site, Nevada, formerly proposed as a geologic repository for high-level nuclear waste disposal. Therefore, the Nopal I deposits have been used as a natural analogue for the Yucca Mountain Project to provide additional ways of testing long term evolution models that support the Yucca Mountain total system performance assessment (TSPA). The main objective of this study is to investigate mineralogical relationships and uranium remobilization in fractures associated with the deposit. Primary uraninite within the main ore body and secondary uranium minerals associated with the deposit are fine-grained (<50 μM), which makes Nopal I geochronology difficult. Secondary ion mass spectrometry (SIMS) was used to obtain in situ U-Th micro-analysis of uranium minerals. Previously reported ages of uranium minerals from Nopal I range from less than one million years to over 30 Ma. New SIMS analysis of ²³⁴U/²³⁰Th disequilibrium ratios of secondary uranium minerals in fractures at depth indicate that uranium remobilization has occurred within the last one million years. Electron microprobe and x-ray diffraction analyses of these uranium minerals show that they are associated with silica and kaolinite, and less commonly iron oxyhydroxides.