The uraniferous mineralizations of the Nopal I deposit (Sierra Pena Blanca, Chihuahua, Mexico) are related to a breccia pipe in a tertiary vitroclastic tuff (Nopal Formation). Detailed mineralogical and fluid inclusion studies led to the discovery of a primary mineralization stage of tetravalent uranium as part of an ilmenite-hematite phase. This stage occurs soon after the deposition of the tuff and is related to H2O–CO2–N2 fluids, similar to those of the vapor phase, under temperatures ranging from 300 to 350°C. The well developed kaolini-zation of the Nopal tuff is associated with a second tetravalent uranium stage (pitchblendepyrite association). Fluids are aqueous and their temperature ranges from 250 to 200°C. The precipitation of pitchblende with pyrite within the pipe is due to a H2S activity increase strictly limited to this structure. The remainder of mineralizing events, either hydrothermal or supergene, led to the hexavalent uranium minerals that prevail today.

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