THE GUANAJUATO MINING DISTRICT CALDERA, MEXICO

The Guanajuato Mining District (GMD), Guanajuato, Mexico, is a world-class fissure-vein deposit famous for its prodigious silver production. The GMD is dominated by Cenozoic volcanic rocks that erupted in the Eocene, after Laramide shortening. In the GMD, a sequence of Oligocene volcanic rocks, which are host to extremely productive silver northwest-striking fissure veins, had been hypothesized in 1994 by the junior authors, to be associated with a caldera-forming volcanic-plutonic center. Detailed mapping of the GMD now shows that faulting, volcanic rock deposition and mineralization, fit reasonably well into the Valles-Type model, and that Guanajuato is a classic example of an epithermal precious metal deposit associated with a caldera. Previously, geologic interpretations of the GMD were difficult, because the southern portion of the GMD remained unmapped. Consequently, a detailed geologic map, concentrated in the unmapped southern GMD was generated during six months of surface and some subsurface mapping, from 1999-2001. This map was then integrated with a compilation of geologic maps in the northern part of the district, to create a 1:20,000 scale geologic map of the GMD. The district-wide map, along with structure sections, revealed excellent data to support the GMD caldera hypothesis. The caldera interpretation is supported by the presence of a peripheral three-quarter ring of rhyolite domes intruded along bounding faults, Oligocene volcanic formations ponded within this ring, megabreccia, topographic rim, and caldera floor. Furthermore, more detailed stratigraphic and structural evaluation in the GMD, support a trapdoor-collapse structure that was hinged on the western margin of the GMD. Eastern-dipping beds in the central and eastern parts of the GMD, location of megabreccia confined to the eastern part of the caldera, size reduction of megabreccia from the eastern caldera edge moving west, westward thinning of caldera-fill units including the megabreccia unit, anticlinal structures on the western side of the district, and the presence of major domal intrusions on the eastern edge and absence on the western edge of GMD, support a trapdoor collapse. In addition, the stratigraphic position of the megabreccia over the first caldera-fill units, suggests a two-stage collapse within the structure.