ORIGIN OF URANIUM IN THE PEÑA BLANCA URANIUM DISTRICT, CHIHUAHUA, MEXICO

Uranium prospects were developed during the early 1960s in limestone and alluvium at the internal drainage basins of Chihuahua. More than 100 wells were drilled along the eastern side of the Sierra del Cuervo. The drilled wells in alluvium cover an area of 60 by 2 km. The first economic findings were made in limestone, but later more than 100 radiometric anomalies were identified in igneous rocks. Some of these radiometric anomalies were developed as deposits, e.g., La Domitila, Puerto 3, Margaritas, and Nopal I in the Peña Blanca Uranium District. Uranium exploration continued in the 1970s at other sites in Chihuahua. By 1980 several findings made it possible to interpret the origin of uranium-mineralizing fluids in the Peña Blanca Uranium District. The lithology of the Sierra Bloque del Nido to the west and Peña Blanca are similar. The vitrophyres at the base of the ash flow tuffs, with high anomalous uranium content, are devitrified and strongly altered to clays. These devitrified vitrophyres are related to welded ash flow tuffs, which were exposed to fluids, e.g., at the Tigre Creek. A stratigraphic continuity from Bloque del Nido to Peña Blanca allowed eastward migration of fluids because Bloque del Nido is topographically higher than the Sierra Peña Blanca. Igneous and hydrothermal activity migrated convection cells that were interrupted by the tectonic activity of the Rio Grande Rift southward towards Chihuahua. Evidence of these fluid activities is found at the Margaritas deposit and in the San Marcos Creek at the Bloque del Nido. This paper proposes a model of the continuity of the uranium mineralization from the west towards the east throughout the ash flow tuffs. It shows evidence of mineralization from the Bloque del Nido in the San Marcos Creek, and several sites inside of the Sacramento Basin, which is located between both Bloque del Nido and Peña Blanca, as well as the Cuervo Basin east of them. This work is being revised now because high alpha radioactivity has been detected in water of Chihuahua City.