The Guanajuato Mining District (GMD) is located at the southern end of the
Sierra Madre Occidental Eocene-Oligocene volcanic province and between the Sierra
Madre Oriental and the Trans-Mexican volcanic belt (Clark et al., 1982). The GMD is
part of a large northwest trending belt of silver-lead-zinc deposits that parallels the
eastern flanks of the Sierra Madre Occidental (Randall et al., 1994). Aranda-Gómez et al.
(2003) divide the rocks in this area into a “basal complex” and “cover rocks”. The basal
complex consists of metamorphosed marine sediments of Mesozoic to early Tertiary age,
while the Cenozoic cover rocks are composed of continental sediments and subaerial
volcanic rocks. The Cenozoic volcanism has been divided into seven pulses (Aranda-
Gómez et al., 2003), ranging in age from about 51 Ma to 8 Ma. Felsic to intermediate
volcanism that occurred from 37 to 27 Ma produced the rock units and structures that
host the mineral deposits (Godchaux et al., 2003).
The regional geology in the GMD has been studied in detail by Edwards (1955),
Echegoyen-Sanchez (1964), Taylor (1971), Gross (1975), Buchanan (1979) and Randall
et al. (1994). The lowermost unit present in the area is the Esperanza Formation (Fig. 4)
composed of carbonaceous and calcareous mudstones, shales, limestones and andesitic to
basaltic flows that have all been metamorphosed to phyllite, slate and marble (Wandke
and Martinez, 1928; Randall et al., 1994; Stewart, 2006). The thickness of this unit
exceeds 600 m in the GMD and it has been assigned a Cretaceous age based on radiolaria
(Dávila and Martínez-Reyes, 1987).