Manto deposits of fluorspar, northern Coahuila, Mexico

Abstract

Over a million tons of fluorspar have been produced from the northern half of the State of Coahulia within the last 15 yr. Much of this production was obtained from manto-type deposits and although the relative importance of these deposits is decreasing, they still constitute a considerable reserve of fluorspar. The most extensive manto deposits in northern Coahuila, in the Buenavista-Encantada and El Tule districts, are stratigraphically restricted to the contact between the Lower Cretaceous Georgetown Limestone and overlying Del Rio shale and Buda Limestone. Individual mantos are long and narrow in plan and range up to several hundred feet in length and 100 ft in width. The mantos are commonly joined to give an over-all reticulate pattern, which coincides in direction with joints and faults in the limestone below. In cross section, the mantos are trough- or saucer-shaped, with the ore in the center of the trough ranging from 2 - 15 ft in thickness. The fluorspar is coarse-grained and typically grades 70-90% CaF 2 . The only gangue minerals are calcite and celestite. It is postulated that the deposits are hydrothermal and originated by introduction of F along deep-seated joints and faults in the Georgetown and underlying limestones, and that the fluorspar deposits were localized primarily by virtue of the impermeable nature of the Del Rio shale. The ultimate source of F is thought to be related to Tertiary volcanic activity.