The Sierra Pena Blanca district is a good example of uranium deposits related to volcanic rocks, which are here considered to be both the source of the uranium and the host rocks for its deposition. The mineralization occurs mainly in the lower units of a Cenozoic volcanic pile, the Nopal and Escuadra rhyolites, that overlie a Mesozoic calcareous basement. Three genetic types of mineralization have been identified: 1. A hydrothermal type, illustrated by the Nopal 1 deposit. The primary mineralization (relics of a pitchblende-pyrite association are observed) and the associated kaolinization of the rock are related to the circulation of hydrothermal fluids in breccias and fault systems. The present-day mineralization consists of mainly U+6 minerals (uranophane, weeksite, etc.) produced during later stages under oxidized conditions. 2. A mixed supergene-exhalative type, exemplified by the Las Margaritas deposit. The U-Mo minerals precipitated due to the mixing of oxidized, U-enriched ground waters with reduced, H2S-rich fluids in a tectonic valley. 3. A supergene type reflected by the Puerto 3 deposit. The U+6 mineralization (uranophane) is located between two impermeable layers and has a roll shape. The precipitation of this U+6 silicate, with kaolinite and quartz, is due to an SiO2 enrichment of the solutions at low temperature.
