Definition of overprinted metalogenic events in the Francisco I. Madero orebody

Abstract

The orebody Francisco I. Madero, located 15 km west from Zacatecas city, belongs to the Pb–Zn–Ag–(Cu) metalliferous province, known as the Silver Belt. The lithology of the area includes from bottom to top: a quartz–sericitic schist whose age has not been determined; a calcareous– pelitic sequence of nondetermined age; a Low Cretaceous volcanosedimentary sequence, quartzmon- zonitic–dioritic intrusive bodies, and a Eocene–Miocene rhyolitic sequence. The mineralization is related to the calcareous–pelitic sequence, and is found in lens-shaped bodies either massive or disseminated replacing the rock.

Two mineral associations were determined by means of a microscopic study. The first one corresponds to pyrite–pyrrothite with evidences of deformation, setting locating it in a mineralization event no younger than the Laramide deformation, whereas the Zn, Pb, Cu sulphides association that replace typical minerals of a calcic skarn belong to a overprinted late postlaramidic event. The complex mineralization of Pb, Zn, Cu sulphides, tellures and sulfotellures of Ag and Bi belong to a latest vetiform event associated to calcite and quartz with brecciated structure, with filled cavities and sligthly cockade texture. It is evident that both, associations and textures, show clearly two metallogenic event: the first one previous to the Laramide deformation and the second one late or post-laramidic; however, it is no possible to assign to the prelaramidic mineralization neither an origin nor a precise age.