The Pena Blanca Range of central Chihuahua is the site of Mexico's largest uranium deposit. The exposure at Pena Blanca consists of Tertiary silicic pyroclastics overlying middle Cretaceous (Albian and Cenomanian) limestones. The uranium is present predominantly in the basal unit of the pyroclastics, at or near the contact with the limestones. The limestones make up a large

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rudistid reef, with extensive fore-reef and lagoon facies, situated at the edge of the Chihuahua trough. Younger basin limestones overlap the lower edges of the fore-reef slope. The reef is of Albian age and shares faunal and lithologic characteristics with both the El Abra Formation of Mexico and the Edwards Formation of Texas. Lagoon, back-reef, requienid rudistid mounds, near back-reef carbonate sand, caprinid and radiolitid rudistid-reef core, fore-reef carbonate sand, and fore-reef debris slope facies are all evident in outcrop. In the reef core, rudistids predominate over all other reef-forming organisms. The basin limestones include the rhythmic Cuesta del Cura Formation, upper Tamaulipas Formation, and Aurora Formation. Unlike the typical petroleum-exploration target, this ree has virtually no porosity. The abundance of carbonate mud and diagenetic calcite cement has occluded all available pore space.

Uranium mineralization is localized at and above the boundary between the pyroclastics and the limestone. The impermeable limestones may have formed both a barrier to mineralizing solutions and a reaction site for mineralization. Reaction of the uranium-bearing carbon dioxide solutions with the limestones could have resulted in uranium precipitation. In addition, hydrocarbons from the basin and reef-slope limestones may have provided a reducing environment that enhanced this precipitation. The Pena Blanca deposit demonstrates the presence of economically significant uranium resources in volcanic terranes.

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