Geology of the Pico Etéreo area, Municipality of Acuña, Coahuila

Abstract

The mapped area is located in northern Coahuila, between the arcs formed by the Serranía del Burro and the Sierra del Carmen, where numerous gently folded Laramide-age anticlines are cut by normal faults. Cretaceous sedimentary rocks, approximately 1,000 m thick, rest on a basement of pre-Mesozoic schists. The Tertiary system is almost entirely composed of sodium-rich intrusive igneous rocks, including gabbro, olivine basalt, andesite, phonolite, latite, quartz monzonite, sodium syenite, trachyte, and rhyolite. The only Tertiary sedimentary unit that outcrops is a non-marine sandstone. The Quaternary system includes colluvium, gravel, and alluvium. The La Cueva Intrusive Complex and the La Cueva Dome are the result of successive intrusions: 1) quartz monzonite, 2) a differentiation series from gabbro to sodium syenite, and 3) sodium microsyenite.
Near Pico Etéreo, five prominent asymmetric domes are associated with arched dikes. The dome structure resulted from a laccolithic intrusion along an arched dike, elevating the rocks on the concave side of the dike higher than the other side. Near La Salada, the arched dike projects above the adjacent laccolith level. In the Amezcua Canyon, an arched dike can be seen emerging with the exposed edge of the laccolith. A volcanic chimney breccia includes fragments of schists dragged from the basement rocks, at least a kilometer deep beneath the present surface. In the Malabrigo mine, a significant mineralized body occurs in a cavernous zone along a contact between rhyolite and "Georgetown" limestone.
Field associations suggest that the intrusive rocks may have derived from the same regional magma, but the proportion of felsic to mafic rocks is extraordinarily high. The large volume of rhyolitic rocks may result from the dilution of ascending gabbroic magma through assimilation of low-melting components from the basement rocks. The silica-poor rocks may have isolated in a subsidiary magma chamber, where they underwent divergent differentiation.
The mapped area is within one of Mexico's most important fluorite districts. A major mineralized body at the Malabrigo mine is found in a cavernous zone along the contact between rhyolite and "Georgetown" limestone, the upper massive limestone member of the Devils River Limestone. The area is highly promising for the discovery of new mineralized bodies and the extension of known mineralized bodies.