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Geology and tectonics of the southeastern portion of the Sierra de Guanajuato1

By
José Jorge Aranda-Gómez
José Jorge Aranda-Gómez
Universidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Querétaro, Qro., 76230, México
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Pablo Dávila-Harris
Pablo Dávila-Harris
Universidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Querétaro, Qro., 76230, México
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Luis Fernando Vassallo-Morales
Luis Fernando Vassallo-Morales
Universidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Querétaro, Qro., 76230, México
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Martha Godchaux
Martha Godchaux
Department of Geology and Geography (retired), Mount Holyoke College, South Hadley, Massachusetts 01075, USA
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Bill Bonnichsen
Bill Bonnichsen
Idaho Geological Survey (retired), University of Idaho, Moscow, Idaho 82844, USA
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Juventino Martínez-Reyes
Juventino Martínez-Reyes
Universidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Querétaro, Qro., 76230, México
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Gerardo de Jesús Aguirre-Díaz
Gerardo de Jesús Aguirre-Díaz
Universidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Querétaro, Qro., 76230, México
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Maria Amabel Ortega-Rivera
Maria Amabel Ortega-Rivera
Universidad Nacional Autónoma de México, Instituto de Geología, Estación Regional del Noroeste, Hermosillo, Son., 83240 México
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Published:
January 01, 2012

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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Contents

GSA Field Guide

The Southern Cordillera and Beyond

José Jorge Aranda-Gómez
José Jorge Aranda-Gómez
Universidad Nacional Autónoma de México Centro de Geociencias Campus Juriquilla Querétaro Qro. 76230 México
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Gustavo Tolson
Gustavo Tolson
Instituto de Geología Universidad Nacional Autónoma de México México D.F. 04510 México
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Roberto S. Molina-Garza
Roberto S. Molina-Garza
Universidad Nacional Autónoma de México Centro de Geociencias Campus Juriquilla Querétaro Qro. 76230 México
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Geological Society of America
Volume
25
ISBN electronic:
9780813756257
Publication date:
January 01, 2012

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