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Abstract

Guanajuato has a long history (450 years) of mineral exploitation and remarkable silver and gold production from a complex system of fault-veins. Despite this, it is only in the past 40 years that the systematic study of its geology has been conducted. Mid-Tertiary epithermal veins occur in all the Mesozoic and Paleogene rock units exposed in the mining district, and mineralization seems to be the result of the combination of several geologic factors, such as the occurrence of greenschists in the basal complex, a thick sequence of Early Paleogene red beds overlain by a thick succession of Oligocene volcanic rocks with the existence of one or more paleolakes when the volcanoes were active. The systematic study of the greenschists and associated plutonic and sedimentary rocks in the basal complex of Sierra de Guanajuato has contributed significant information to the concept of accretion of the Guerrero terrane to the SW end of the North American craton in the Early Cretaceous. Research on the Eocene red bed sequence suggests that early extension occurred creating fault patterns that later were reactivated during Neogene Basin and Range pulses. Immediately east of the city of Guanajuato, a thick volcanic sequence is exposed, with two pyroclastic units formed by felsic ignimbrites that almost certainly are related to a nearby caldera, which was active immediately prior to Ag-Au mineralization. The first activity pulse of the caldera produced the Bufa ignimbrite, a massive unit that displays very large thickness variations (300 to <10 m) in short distances, which we interpret as a signal that it may be an intracaldera deposit. The second explosive pulse originated the Calderones formation, a unit formed by an undetermined but large number of ignimbrites, surge deposits, layers with accretionary lapilli, and epiclastic-volcanic deposits. The Calderones formation is characterized by pervasive chloritization, which points out toward the presence of external water in the system, probably related to one or more shallow lakes within the caldera previously formed by the Bufa eruption. Lithofacies variations and stratigraphic arguments suggest that the Guanajuato caldera was probably located near the Cerro Alto de Villalpando and La Peregrina lava dome complex. Morphological and structural evidence of the caldera are masked by several pulses of younger normal faulting which affected the southern portion of the Mexican Basin and Range Province (i.e., Mesa Central).

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