Geology of the Pico Etéreo area, Município de Acuña, Coahuila, Mexico
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The map area lies between the Serrania del Burro and Sierra del Carmen arches. Many gently folded anticlines of Laramide age are broken by normal faults. Approximately 3,200 feet of Cretaceous sedimentary rock, dominantly limestone, rests on a basement of pre-Mesozoic schist. The Tertiary System consists almost entirely of sodic intrusive igneous rocks and includes the following types: gabbro, olivine basalt, andesite, phonolite, latite, quartz monzonite, sodic syenite, trachyte, and rhyolite. The only Tertiary sedimentary unit exposed is a non-marine sandstone. Rocks of the Quaternary System include colluvium, gravel, and alluvium. La Cueva intrusive complex and La Cueva dome resulted from successive intrusion of: (l) quartz monzonite, (2) a differentiated series ranging from gabbro to sodic syenite, and (3) sodic microsyenite. Near Pico Etereo five prominent asymmetric domes are associated with arcuate dikes. The domal structure resulted from laccolithic intrusion along an arcuate dike, lifting rocks on the concave side of the dike higher than those on the other side. At La Salada the arcuate dike rock projects above the level of the adjacent laccolith. In Amezcua Canyon an arcuate dike can be seen to merge with the exposed edge of the laccolith. A volcanic pipe agglomerate includes fragments of schist torn from the basement rocks at least 3,000 feet below the present surface. Field associations suggest that the intrusive rocks may have been derived from the same regional magma, but the proportion of felsic to mafic rocks is unusually high. The large volume of rhyolitic rock may be the result of dilution of rising gabbroic magma by assimilation of low-melting constituents of the basement rocks. The silica-deficient rocks may have been isolated in a submagma chamber, where they underwent a divergent trend of differentiation. The map area lies within one of the most important fluorspar districts in Mexico. A major orebody in a cavernous zone in the Malabrigo mine lies along a contact between rhyolite and "Georgetown" limestone. The so-called "Georgetown" is the upper massive limestone member of the Devils River Limestone. Measured, indicated, and inferred reserves of the mine total 200,000 tons. The area bears great promise for the discovery of new orebodies and the extension of known orebodies.