Geology of Mina Plomosas area, Chihuahua, Mexico

Three previously unrecognized outcrops of pre-Carboniferous rock in the Mina Plomosas - Placer de Guadalupe area have a total area of slightly less than four square miles. Six units, ranging in age from Ordovician to Jurassic, are mapped within the Plomosas Formation named by Burrows in 1909; the total thickness is about 8,000 feet. The lower 2,000-foot section of Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian rock, composed predominantly of limestone, resembles contemporaneous formations in west Texas. The 2,000-foot to 3,000-foot section between Pennsylvanian and Late Jurassic rock, composed predominantly of siltstone and conglomerate, consists of a 500-foot to 1,000-foot Permian (Wolfcampian-Leonardian) sequence overlain by beds of undetermined age. The Permian sequence includes a reef. The 3,000-foot section of Late Jurassic rock is composed of shale, sandstone, and limestone. The area lay to the north of the main Ouachita trend, and Paleozoic tectonism was less intense than in the Marathon uplift. Nevertheless, angular unconformities within the Permian sequence indicate Permian movements. Laramide tectonism was extreme, with major overthrusting toward the west. Two large sections are upside-down. Crustal shortening on the order of ten miles can be proved near Mina Plomosas. Probably the sericite and microspar (finely recrystallized limestone), in the Paleozoic and Jurassic rocks, are products of incipient metamorphism resulting primarily from Laramide deformation. Mina Plomosas is an important lead-zinc mine. A little placer gold is found at Placer de Guadalupe. All mineralization in the Placer de Guadalupe - Mina Plomosas area is thought to be Tertiary in age. The petroleum potential of Chihuahua is still untested.