Geology and tectonics of the southeastern portion of the Sierra de Guanajuato1
José Jorge Aranda-Gómez*Pablo Dávila-Harris
Luis Fernando Vassallo-MoralesUniversidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Querétaro, Qro., 76230, México
Martha GodchauxDepartment of Geology and Geography (retired), Mount Holyoke College, South Hadley, Massachusetts 01075, USA
Bill BonnichsenIdaho Geological Survey (retired), University of Idaho, Moscow, Idaho 82844 USA
Juventino Martínez-ReyesGerardo de Jesús Aguirre-Díaz
Universidad Nacional Autónoma de México, Centro de Geociencias, Campus Juriquilla, Querétaro, Qro., 76230, México
Maria Amabel Ortega-RiveraUniversidad Nacional Autónoma de México, Instituto de Geología, Estación Regional del Noroeste,
Hermosillo, Son., 83240 México
ABSTRACT
Guanajuato has a long history (450 years) of mineral exploitation and remark-able 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 con-ducted. 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 suc-cession 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
1This paper is an updated version of parts of a guidebook published by Aranda-Gómez et al. (2003).
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FOREWORD
The purpose of this fi eld trip is to show to some of the par-ticipants of the 108th Annual Meeting of the Cordilleran Sec-tion of the Geological Society of America several of the most relevant geological features in the Guanajuato mining district and its immediate surroundings. Guanajuato hosts world-class silver deposits, and in the past 30 years has become one of the most important gold producers in México. We hope that this visit will give the attendants a fair idea about the geological evolution of the area and its complex volcano-tectonic history.
This year’s fi eld trip will be done in three days and consists of 20 stops, most 20–30 minutes long. During Days 2 and 3 we will make short traverses that may take 2–3 hours in order to study and compare the volcano-sedimentary sequence in the Calde-rones formation, a unit that we believe is the key to understand the idea of the Guanajuato Caldera (Randall-R. et al., 1994). In the morning of the fi rst day, we will visit exposures of the Meso-zoic basal complex along the road that goes from the town of La Valenciana to Montaña de Cristo Rey (Cerro El Cubilete, Fig. 1). The town of La Luz (Fig. 1) is located near El Cubilete and our fi eld trip crosses La Luz Vein System. From El Cubilete we will go back to Guanajuato and take the road toward El Cubo mine to have a fi rst look of the units of the Cenozoic volcano-sedimentary sequence which unconformably overlies the basal complex. Dur-ing this fi rst day, we will pass several Cenozoic faults, some of them related to and others younger than the mineralization in the district. During the second and third days we will study in detail the volcanic units exposed in the eastern part of the mining dis-trict in order to recognize the different lithologies and understand the facies variations, which suggest the location of probable vent areas for the pyroclasts.
PART I. AN OUTLINE OF THE REGIONAL GEOLOGY OF THE SOUTHEASTERN PART OF THE SIERRA DE GUANAJUATO, MÉXICO
Sierra de Guanajuato is a N45°W-trending mountain range that extends over a distance greater than 80 km (Fig. 1). The southwestern front of the Sierra de Guanajuato (El Bajío Fault) is the boundary between two major physiographic, tectonic and volcanic provinces in central México (Fig. 2). South of El Bajío Fault is the Trans-Mexican Volcanic Belt, an active chain of subduction-related volcanoes and to the north is the Mesa Central (Aranda-Gómez et al., 1989), which we consider the southern-most part of the Basin and Range Tectonic Province (Henry and Aranda-Gómez, 1992). Volcanic outcrops at the Mesa Central are dominated by rocks associated to the Sierra Madre Occidental Volcanic province (Eocene–Miocene: McDowell and Keizer, 1977; Nieto-Samaniego et al., 1999; Ferrari et al., 2007), whereas Cenozoic volcanic rocks south of the fault are mostly related to the activity of the Trans-Mexican Volcanic Belt (Gómez-Tuena et al., 2007).
The rocks exposed in the Sierra de Guanajuato can be divided into two broad stratigraphic successions: the Basal com-plex, which includes Mesozoic rocks and an early Eocene gra-nitic batholith (K-Ar = 51 ± 1.3 Ma, Stein et al., 1994), and the Cenozoic volcano-sedimentary cover (Fig. 1). Contact between these successions is a marked angular unconformity as the basal complex was deformed by at least two pulses of compression during Early Cretaceous—early Eocene. All but one (i.e., La Perlita limestone) of the Mesozoic units in the basal complex have overprinted greenschist metamorphism and the proto-liths include: (1) several plutons of different ages (K-Ar ~157–108 Ma: Ortíz-Hernández and Martínez Reyes, 1993; U-Pb,
contributed signifi cant information to the concept of accretion of the Guerrero ter-rane 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. Imme-diately 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 fi rst 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 chloritiza-tion, 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).
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 137
Figure 1. Generalized geologic map of the Sierra de Guanajuato (modifi ed after Martínez-Reyes, 1992). The approximate location of the western fault of the Villa de Reyes Graben was taken from Tristán-González (1986). Field trip stops locations for the morning of the fi rst day are shown.
Figure 2. (A) Morphotectonic provinces in cen-tral Mexico (after Sedlock et al., 1993). The ap-proximate location of Sierra de Guanajuato is marked with a rectangle; León (L), Guanajuato (G), and Querétaro (Q) are shown for reference. The general distribution of late Cenozoic nor-mal faults of the southern portion of the Basin and Range province is also shown (modifi ed after Henry and Aranda-Gómez, 1992). Note that the road (Mex 45D) from Querétaro to Guanajuato (not shown) that we will travel dur-ing the fi rst day is located in the northernmost portion of the Trans-Mexican Volcanic Belt. On the way back, from Guanajuato to Querétaro, we will take road Mex 111 (not shown), where the landscape is akin to that at the southernmost region of the Mesa Central.
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zircon, granophyric tonalite = 144 ± 1.4 Ma: Martini et al., 2011), (2) dikes (U-Pb, zircon, dacite = 150.7 ± 0.8 Ma: Martini et al., 2011) with varied compositions (ultramafi c to felsic), and (3) submarine sedimentary and volcanic (U-Pb, zircon, rhyolite = 145.4 ± 1.1 Ma: Martini et al., 2011) rocks. All Mesozoic units were deformed by compressive stresses prior and/or during the early Eocene. The volcano-sedimentary cover consists of Eocene continental red beds and a thick sequence of mid-Tertiary (Oli-gocene-Miocene) volcanic rocks of predominantly felsic com-position. A notable exception is the Guanajuato mining district where volcanism was bimodal during the Oligocene (Aranda-Gómez et al., 2003). Widespread, fl at-lying gravel deposits con-taining mainly late Hemphillian to Blancan vertebrate faunas (e.g., Carranza-Castañeda and Miller, 2004; Flynn et al., 2005) are found in some regions in the southern portion of the Mesa Central, immediately north of the Sierra de Guanajuato. These gravels in turn are, partially covered in some places (e.g., at San Diego de la Unión, Gto.) by intraplate mafi c volcanic rocks (Aranda-Gómez et al., 1989, 2007). Southeast of El Bajío fault are exposed gravel deposits covered by 13.7 Ma andesitic lava fl ows (Aguirre-Díaz et al., 1997). Similar gravel and lava fl ows crop-out atop El Cubilete Mountain, which is one of the high-est points of the Sierra de Guanajuato. Both, the basal complex and the volcano-sedimentary cover have been affected by Ter-tiary (middle Eocene(?)–Pliocene) normal faults. It is unknown if Cenozoic extension occurred as a continuous episode or if nor-mal faulting was caused by several distinct pulses of extension.
STRATIGRAPHY
Basal Complex
The pre-Tertiary stratigraphy of the Sierra of Guanajuato consists of both volcano-plutonic and volcano-sedimentary asso-ciations, Late Jurassic–Early Cretaceous in age (Monod et al., 1990; Ortíz-Hernández et al., 1990; Lapierre et al., 1992). Both associations are interpreted as part of the Guerrero suspect ter-rane (Campa and Coney, 1983). A small remnant of shallow water limestone (Aptian–Albian) unconformably rests atop these older Mesozoic rocks at La Perlita NNW of the city of León (Fig. 1).
In the vicinity of the Guanajuato mining district the Meso-zoic volcano-plutonic sequence includes: (1) A thick (>1000 m) succession of pillowed and massive submarine lavas of basaltic composition in places interlayered with basaltic tuffs (K-Ar = 108.4 ± 5.6 Ma; Monod et al., 1990); (2) An anastomosed set of diabasic dikes emplaced in gabbroic (K-Ar = 112 Ma; Lapierre et al., 1992), dioritic, quartz-dioritic and/or tonalitic host rocks; (3) A pluton made of massive diorites (K-Ar = 122–120 Ma; Lapierre et al., 1992) with local hornblende rich pegmatitic segre-gations, which in places is cross-cut by basaltic dikes; and (4) An intrusive body composed by a leucocratic tonalite (K-Ar = 157–143 Ma; Lapierre et al., 1992) and granophyres, also intruded by diabase dike swarms. All these major units in the basal complex are stacked one on top of the other by major thrust faults. In the
fi eld the lowermost unit is the pillow-basalt and in turn this is covered by the dike complex followed by the massive diorite and leucocratic tonalite. An idealized reconstruction of the original succession (Fig. 3) has been proposed by Ortíz-Hernández and Martínez-Reyes (1993). Originally, the volcano-plutonic associa-tion was interpreted as the upper crust of an intraoceanic island arc sequence and it was referred to as the “Guanajuato Arc” (Fig. 1). As it is discussed below, alternative interpretations about the origin of these rocks were subsequently proposed.
Younger K-Ar ages have also been obtained from samples of the volcano-plutonic sequence, ranging between 108 and 66 Ma, but these are attributed to the greenschist facies metamorphism that affected all the Mesozoic rocks of Sierra de Guanajuato. The older ages obtained in the leucocratic tonalite are thought to be related to an excess of Ar relative to very low K contents (<0.6%). Thus, they are not considered signifi cant. Therefore, it is assumed that the volcano-plutonic sequence ranges in age from late Jurassic to early Cretaceous (Ortíz-Hernández and Martínez-Reyes, 1993).
The volcano-plutonic sequence is thrusted over a highly deformed volcano-sedimentary pelagic sequence which consists mainly of thinly bedded micritic limestone, black shale, chert, and sandstone interlayered with shale (Fig. 4). Basaltic pillow-lavas and hyaloclastites interbedded with the sediments are found in several places. K/Ar ages obtained in the basaltic rocks of the volcano-sedimentary sequence range between 93 and 85 Ma (Cenomanian-Santonian: Ortíz-Hernández and Martínez-Reyes, 1993). These results are also considered unreliable because of the ubiquitous hydrothermal alteration and/or the low-grade metamorphism in these rocks, which probably produced loss of radiogenic argon. Poorly preserved radiolarians, recovered from rocks in this sequence yield an uncertain Valanginian–Turonian age (Dávila-Alcocer and Martínez-Reyes, 1987). Nannofos-sils collected in limestone beds suggest a Tithonian–Berriasian age (Corona-Chávez, 1988). Thus, the volcano-sedimentary sequence is considered broadly contemporaneous to the volcano-plutonic association (Ortíz-Hernández and Martínez-Reyes, 1993). An idealized reconstruction of the original volcano- sedimentary succession is shown in Figure 4. This association was originally interpreted as sediments and lavas accumulated in an oceanic basin (Arperos Basin) located between an exotic island arc proceeding from the paleo-Pacifi c domain and North America craton.
A recent view about the stratigraphy of basal complex is that of Martini et al. (2011). They divided the volcano-sedimentary sequence of Martínez-Reyes (1992) into three units they call Esperanza, Arperos, and Paxtle petrotectonic assemblages. The Esperanza assemblage is subdivided into the Esperanza and Valenciana formations. Figure 7A in Martini et al. (this volume) depicts the lithologies in each unit and summarizes Martini et al.’s (2011) interpretation of the complex tectonic relations among these units.
According to Martini et al. (2011), within the context of the Guerrero terrane, the tectonic juxtaposition of the volcano- plutonic
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and volcano-sedimentary sequences has been interpreted in three different ways as a product of: (1) The closure of an extensive basin by the subduction of the Mezcalera plate (Dickinson and Lawton, 2001) and accretion of both the Guanajuato island arc and the Arperos basin sediments to the North American Craton during the mid-Cretaceous (Tardy et al., 1991, 1994: Lapierre et al., 1992): (2) accretion to the southern end of the North Ameri-can continent of a series of intraoceanic arcs as a consequence of subduction of the fl oor of small ocean basins (Talavera- Mendoza et al., 2007); and (3) the volcano-sedimentary and volcano- plutonic successions of the Guerrero terrane represent a west-facing North American continental arc, which suffered a great deal of extension with the consequent formation of a back-arc
oceanic basin during the Early Cretaceous. This back-arc basin subsequently was closed during Laramide shortening (Centeno-García et al., 2008).
Based on a study of the rocks of the basal complex exposed in the area between the Villa de Reyes graben and the city of Gua-najuato (Fig. 1), which includes geologic mapping, stratigraphic analysis, U-Pb (zircon) geochronology, and sandstone provenance studies, Martini et al. (2011) favor an interpretation of the Guer-rero terrane where a “detached slice of the Mexican leading-edge…drifted in the paleo-Pacifi c domain during back-arc exten-sion, and subsequently accreted back to the Mexican craton.”
NNW of the city of León (Fig. 1), at La Perlita, neritic lime-stone (Aptian-Albian) occurs in a small remnant unconformably
Wehrlite & serpentinite
Olivine clinopyroxenite
Gabbro
Diorite
Diabasic dikes emplacedin plutonic rocks that range in composition from gabbroto plagiogranite
Tertiary volcano-sedimentarysuccession (see Figure 5)
Angular unconformity
Martínez-Reyes (1992)nomenclature
Lithologies
San Juan de Otatespyroxenite
La Palma diorite
Cerro Pelón tonalite
La Luz formation
F
F
F
F
F
Figure 3. Reconstructed stratigraphy of the Guanajuato Arc. F = tectonic con-tact (thrust fault). Redrawn from Ortíz-Hernández and Martínez-Reyes (1993).
140 Aranda-Gómez et al.
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resting atop of the volcano-sedimentary sequence (Chiodi et al., 1988; Quintero-Legorreta, 1992). These limestones include oolitic limestones and calcareous breccias with abundant ammo-nites, brachiopods, lamellibranchs and gastropods. In addition to these fossils, reworked gastropods and pelecypodes (Ceritium bustamantii and Psilothyris occidentalis) of Neocomian-Aptian age, and one brachiopod (Peregrinella sp.) of lower Hauteriv-ian age are also present. Deformation and metamorphism of the underlying rocks pre-date the accumulation of this limestone, which is only gently folded.
The undeformed post-tectonic Comanja granite (K-Ar ~51 ± 1.3 Ma: Stein et al., 1994), which is exposed in the core of the Sierra as a chain of outcrops more than 50 km long (Fig. 1), was emplaced in the Mesozoic rocks of the basal complex. It is a calc-alkaline to mildly alkaline granite (Ortíz-Hernández and Martínez-Reyes, 1993), made of large potassium feldspar phe-nocrysts set in a medium to coarse grained matrix composed by quartz, plagioclase (albite-oligoclase) and biotite. The pluton is cut by numerous dikes of granitic pegmatites and aplites and by tourmaline veins. Near its contacts, the calcareous sediments of the Mesozoic volcano-sedimentary succession were trans-formed to skarns and many of the earlier structures, textures and mineral assemblages were obliterated. We obtained three Ar-
Sandstone
Slate
Micritic limestone
Conglomerate
Black slate
Chert
Diabase dike
Basaltic tuff
Hyaloclastite
Basaltic pillow lava
Massive lava0
500 m
Figure 4. Reconstructed stratigraphy of the volcano-sedimentary sequence accumulated in the Arperos Basin. Redrawn from Ortíz-Hernández and Martínez-Reyes (1993).
Ar ages of 51.5 ± 0.69 Ma (Kspar), 52.10 ± 0.49 Ma (bio), and 52.48 ± 0.63 Ma (bio) for the Comanja granite (Appendix A in the GSA Data Repository2).
Volcano-Sedimentary Cenozoic Cover
The basal complex is separated by an angular unconformity from the overlying formations (Fig. 5). In the Guanajuato mining district directly above the unconformity is a 1400–2000 m thick red bed sequence (Edwards, 1955; Buchanan, 1979) of thin- to thick-bedded, poorly sorted to well-sorted, siltstone, sandstone, and pebble- to boulder-conglomerates. Based on the overall lithology of the deposit and in vertebrate fossils Edwards (1955) and Fries et al. (1955) concluded that Guanajuato Red Conglom-erate (GRC) is an alluvial fan deposit accumulated at the base of mountain blocks that were being rapidly uplifted during the late Eocene to early Oligocene. Near the base of the clastic sequence the sediments are interbedded with andesitic lava fl ows (K-Ar, wr, ~49.3 ± 1.0 Ma: Aranda-Gómez and McDowell, 1998).
2GSA Data Repository item 2012103, Appendix A, is available at http://www.geosociety.org/pubs/ft2012.htm, or on request from [email protected].
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 141
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Chichíndaro formation (K-Ar = 30 - 32 Ma1)Rhyolite porphyry that forms domes and lava flows,along with associated ignimbrites and breccias.
Cedro formation (U-Pb, zircon = 32.7 - 33.6 Ma)2
Gray to black lava flows, in places interbedded withpyroclastic deposits. Total thickness 100-640 m;water affected near its base
Includes low- to medium grade ignimbrites, block and ash deposits, surge layers, ash-fall tuffs, re-workedtuffaceous deposited in water, tuff-breccias andmegabreccias. Displays pervasive chloritization. 200 m thick (?).
Felsic ignimbrite with san + qtz + bio phenocrysts (< 25%).In most places is poorly welded, but silicification turns it into a well indurated rock. It may form prominent cliffs withcrude vertical jointing. Up to 350 m thick.
.
Losero formationWater-laid epiclastic-volcanic deposit that passes upwardfrom medium to fine grained, well sorted sandstone to greenpyroclastic surge layers. 0 - 50 m thick.
Guanajuato conglomerate (latest early Eocene - early Oligocene)Boulder and pebble conglomerates, sandstones, and siltstoneswith sorting that varies from massive to thinnly layered. Abundantvolcanic clasts near its base. Plutonic fragments increase nearthe top. Near the base of the succession there are scarce andesitic lavas (K-Ar, w.r. = 49.3 + 1 Ma) . Near its top eartly Oligocene vertebrate fossils reported by Edwards (1955) and Ferrusquía (1987)
Angular unconformity
Basal complex (see figures 1, 3, and 4 for details)
Peregrina lava domeLight gray to pink, rhyodacite to dacite porphyry, with25 - 30 % phenocrysts of argillized feldspar and quartzset in an aphanitic groundmass. Conspicuous flow foliationwhich is nearly vertical in places. Marginal facies interfingewith Calderones formation.
Andesite dikesDeeply weathered and/or hydrothermally altered. Generally considered as Cedro feeders. Some of them interacted with wet Calderones materials producing hyaloclastite and hydrovolcanic rocks. At least in part contemporaneous with Calderones formation.
3
1= Nieto-Samaniego et al., 1996; 2 = Báez-López, 2012; 3 = Aranda-Gómez and McDowell, 1998
Figure 5. Generalized composite stratigraphic column of the Guanajuato mining district (modifi ed after Aranda-Gómez et al., 2003).
142 Aranda-Gómez et al.
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Conformably resting atop of the GRC thru a transitional contact is a very well-sorted, thin- to medium-bedded, green vol-caniclastic sandstone known as the Losero formation (Fig. 5). Its thickness varies from 0 to 50 m in the district but it usually is in the range of 10–20 m. Sedimentary features such as graded- and cross-bedding, ripple marks, scour and fi ll structures, and raindrop imprints are sometimes found in the deposit. Therefore, Losero formation was originally interpreted in epiclastic- volcanic sandstone accumulated in a shallow lake (Edwards, 1955). An alternative interpretation about the origin of the Losero forma-tion is that it is principally made up of subaerial pyroclastic surge deposits accumulated (and locally reworked) in a shallow lake; the pyroclastic nature, suggested mainly by very low-angle cross bedding, is more pronounced in the upper part of the Losero for-mation (Aranda-Gómez et al., 2003).
An erosional surface separates the Losero formation from the overlying Bufa formation (K-Ar, wr, = 37.0 ± 3.0 Ma: Gross, 1975; Ar-Ar, san = 33.58 ± 0.48 Ma, and U-Pb, zircon = 33.4 ± 0.4 Ma: Báez-López, 2012), which is a major (up to 350 m thick) ash-fl ow tuff with less than 25 percent by volume of phenocrysts of quartz, sanidine and minor plagioclase and biotite. Scattered in the deposit are lithic clasts of andesite and/or rhyolite, as well as fragments derived from the GRC. The thickness of this unit varies from 350 m (Fig. 5) at Las Torres (Buchanan, 1979) and El Cubo (Davis et al., 2009) mines to <10 m at the Sirena Hill, less than 5 km away from Las Torres. In the southeastern part of the district the ignimbrite shows conspicuous columnar jointing related to compaction, welding and/or silicifi cation.
The Calderones formation is a complex unit formed by an undetermined number of andesitic to dacitic ignimbrites locally interlayered with epiclastic-volcanic beds accumulated in a shallow lake. Calderones accumulated over an erosional sur-face developed on the underlying Bufa formation (Fig. 5) and, in general, it is easily identifi ed by its clastic nature, layering, and its light- to dark-green color due to pervasive chloritization. Calderones commonly displays shallow paleochannels, espe-cially in the proximal and medial locations. There are places where angular, aphanitic to porphyritic, chloritized andesitic lithic fragments, probably derived from the basal complex, make up to 75% of the deposit. In other places lithics derived from metasedimentary rocks in the basement are more abundant. Near the Peregrina dome (Fig. 6) some of the ignimbrites in the Calde-rones formation include abundant clasts derived from the growth and/or partial destruction of the domes; these pyroclastic units are interpreted as accumulated from “block and ash” pyroclas-tic fl ows. The formation is typically medium to thick bedded, and the grain size of the clasts ranges from fi ne sand to cobbles, although it is commonly in the range of pebbles and granules. In some places, the formation is thinly bedded and fi nely laminated. In the vicinity of El Cubo mine (Fig. 6), Calderones formation contains thin (3–5 m thick) to medium (up to 20–30 m thick) ash-fl ow tuffs. In some places, such as Arroyo Los Silvestres (Fig. 6; Stop 2.5) the base of each ignimbrite is a lithic-rich ash-fl ow tuff with large (up to 1 m in diameter) dacite clasts probably derived
from the Peregrina dome (Fig. 6; Stop 3.2) set in a vitroclastic matrix where glass shards and pumice fragments are altered to chlorite; above these basal horizons are massive to distinctly bed-ded welded tuffs where the pumice was fl attened and replaced by chlorite. In still other places, near the Cerro Alto de Villalpando (Fig. 6; Stops 3.3 and 3.4), Calderones is a heterolithologic tuff-breccia, with clast sizes ranging from a few millimeters to several meters in diameter, made of fragments of all lithologies seen in the basal complex, as well as fragments derived from the GRC, the underlying volcanic units, and Peregrina dacite. These tuff-breccias have variable amounts of ash in the matrix, and based on their contact relations are interpreted either as the fi lling of a ring dike or as intra-caldera collapse breccia (Aranda-Gómez et al., 2003). The interpretation of this tuff-breccia as the fi lling of a ring dike (Fig. 6; Stop 3.3) has been questioned by Davis et al. (2009) based on the lack of continuity of this lithology in the underground workings. Total thickness of the Calderones for-mation has been estimated between 200 and 250 m (Buchanan, 1979), but we note that due to the complex faulting in the area the unit maximum thickness is highly uncertain. Ar-Ar (sani-dine) and U-Pb (zircon) ages obtained by Báez-López (2012) are 31.33 ± 0.29 and 32.55 ± 0.2 Ma, respectively.
Near the Peregrina mine is exposed the dacitic Peregrina dome, which has complex contact relations with the Calderones formation as its pyroclastic block and ash-fl ows are intercalated with, and form part of the Calderones formation. In the area of the Peregrina Dam (Fig. 6; Stop 3.2) fl ow foliation in the lava fl ows defi ne a concentric pattern and it is nearly vertical at the dam. The domes are formed by a pale gray porphyritic dacite, with euhedral phenocrysts of plagioclase and quartz set in a devitri-fi ed groundmass. The outer portions of the domes are breccias formed by clasts derived from completely devitrifi ed carapaces.
Resting atop Calderones is the Cedro formation (Fig. 5), which is package of a gray to black, porphyritic, andesitic to basaltic, lava fl ows, in places interbedded with tuffs. Thickness estimates of this unit vary from 640 to 250 m (Echegoyén et al., 1970; Davis et al., 2009). The Calderones–Cedro contact is transitional at Loma El Venado (Fig. 6; Stop 2.3) and it consists of a succession of beds of very fi ne-grained green tuffs interlay-ered with dark brown tuffs and altered lava fl ows, which prob-ably interacted with water. This basal succession passes upward to subaerial lava fl ows that commonly display well developed spheroidal weathering that also suggest alteration with water. Two U-Pb (zircon) ages obtained in samples collected at the area located south of the tail ponds of El Cubo mine by Báez-López (2012) are 33.0 ± 0.25 and 33.2 ± 0.35 Ma.
Throughout the lower part of the Cenozoic volcanic sequence of the district occur dikes with a lithology similar to the Cedro andesites. These structures are regarded as feeders of the Calderones lava fl ows. The dike swarm has a NE to ENE trend and there is a hint of a radial array among them (Fig. 6). Some of the dikes emplaced in the distal part of the Calderones formation have peperites at their contacts which suggests interaction with water or with wet tuffs (Aranda-Gómez et al., 2003).
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2.1
Fie
ldtr
ip s
top
Pav
ed r
oad
Gra
vel r
oad
2 km
Gua
naju
ato
ca. 1
970
Key
Mod
fiied
afte
r E
cheg
oyén
et a
l., 1
970
II
II
III I
III I
III I
III I
I I
I III
II
I
I
I
I
I
I
I
I
I
I
I
I
2321
2326
274
268
261
1.7
1.8
1.9
2.1
2.2
2.3 2.4
2.5
3.1
3.2
3.3
3.4
El A
mpa
ro fa
ult
SC
F
Veta Madre fault
Veta
Mad
re fa
ult
La Leona fault
El Cubo fault
El C
ubo
faul
t
Villalpando fa
ult
El C
apul
ín fa
ult
Dolores faultLa Loca
fault
SEFC
C X
X
XXX
X
X
Ì
Ì
TM
CM
C
CB
V
LC
EC
CS
X
X
LP
CT
To PM
To CT
1.6
Figu
re 6
. Geo
logy
of
the
sout
heas
tern
par
t of
the
Sier
ra d
e G
uana
juat
o. M
odifi
ed f
rom
Ech
egoy
én e
t al.
(197
0); t
he n
orth
east
ern
bran
ch o
f Vet
a M
adre
aro
und
Cer
ro C
hich
índa
ro
(CC
), s
how
n w
ith d
ashe
s, w
as ta
ken
from
Mar
tínez
-Rey
es (
1992
). C
—to
wn
of C
alde
rone
s; C
B—
Cer
ro L
a B
ufa;
CM
—E
l Cub
o m
ine;
CS—
Cer
ro S
iren
a; C
T—
tow
n of
El C
ubo;
E
C—
tow
n of
El
Ced
ro;
LC
—L
a C
ueva
; L
P—L
etre
ro d
e Pe
regr
ina
junc
tion;
PM
—Pe
regr
ina
min
e; S
CF—
San
Cle
men
te f
ault;
SE
F—Sa
n E
useb
io f
ault;
TM
—To
rres
min
e; V
—L
oma
El V
enad
o.
144 Aranda-Gómez et al.
fl d025-07 1st pgs page 144
The youngest volcanic unit in the Guanajuato mining dis-trict is a porphyritic rhyolite (Fig. 5) which forms volcanic domes associated with low-volume ignimbrites and volcanic breccias. Its type locality is in the Cerro Chichíndaro, where a lava dome lies between two branches, and it is altered by the Veta Madre fault-vein. Somewhat similar volcanic structures are exposed at the top of Cerro Alto de Villalpando (Fig. 6) near the Calderones ring dike and in extensive outcrops elsewhere in the Sierra de Guanajuato, around the southern, eastern, and northern parts of the Guanajuato mining district (Davis et al., 2009). Similar vol-canic structures occur northeast of the district and throughout the northern part of the Sierra. In places, such as in the Sierra El Ocote, near Nuevo Valle (Fig. 1), these rhyolitic domes con-tain disseminated tin and “vapor phase” topaz in cavities located along the steeply inclined fl ow foliation. Gross (1975) reported a K-Ar age of 32.0 ± 1.0 Ma for the Chichíndaro rhyolite, while Nieto-Samaniego et al. (1996) dated rhyolitic domes that they considered Chichíndaro rhyolite at La Sauceda Graben (K-Ar: 30.8 ± 0.8 Ma) south of the district (Fig. 1), and in other rhyolite lava dome (K-Ar: 30. 1 ± 0.8 Ma) located to the north of the town of Santa Rosa (Fig. 1), immediately north of the district.
At the top of El Cubilete Mountain (Fig. 1; Stop 1.5), resting directly on rocks of the Mesozoic volcano-plutonic sequence, is a remnant of a gravel deposit crowned by an andes-itic lava fl ow. The gravels are mostly composed of well-rounded clasts derived from the mid-Tertiary volcanic sequence and by a small amount of fragments derived from the basal complex. The andesitic lava produced thermal alteration in the underly-ing sediments and an autobreccia occurs at its base. Vertical fracture sets form crude columns in the middle part of the fl ow. In the uppermost part of the outcrop, the andesite displays sub-horizontal platy jointing. Similar gravel deposits and andesitic lava fl ows are also found at El Bajío depression (Fig. 1), 600 m below the top of El Cubilete, in the downthrown block of the El Bajío Fault Zone. The K-Ar age of the andesite exposed atop El Cubilete is 13.7 Ma. Thus, the long-term average displacement rate at El Bajío fault system is in the order of 0.04 mm/yr if it is assumed that the fault still is active.
STRUCTURE
The structural grain in the southern part of Sierra de Guana-juato has a dominant NW-SE orientation which is defi ned by the schistosity in the low grade metamorphic rocks, the orientation of axial planes of folds in the basal complex, the overall outcrop pattern of the Comanja granitic batholith and the trend of some of the Cenozoic normal faults. Detailed analysis of the orienta-tion of microfold axes in the metasediments also shows a weaker NE-SW trend. Major NE-SW Cenozoic normal faults are also common in the area.
The contacts between the major sequences and among indi-vidual lithologic units in the basement often appear as subho-rizontal mylonitic zones, a few meters to tens of meters thick. These tectonic contacts are thrust planes considered by Monod
and co-workers (1990) as mid-Cretaceous in age. Overprinted on this early compressional event, that caused the formation of an early foliation with lineations oriented NS to NNE, is the late Cre-taceous to early Tertiary compressional Laramide deformation which produced folding with NW-orientated axial planes. New data obtained by Martini (Fitz et al., 2011) indicate that the early period of deformation occurred in the interval between the erup-tion of the felsic lavas of the Esperanza formation (145 Ma) and the accumulation of La Perlita neritic limestone (Aptian: ca. 125 Ma). The same researchers interpret three deformation pulses, the ear-liest produced ductile shear zones and isoclinal folds overturned to the SW. A second pulse caused structures with SE vergence. Finally, a third pulse overprinted the earlier structures and pro-duced cylindrical, symmetrical, upright, vertical open folds. The fi rst two pulses of deformation are attributed to the closure of the Arperos basin and accretion of the Guerrero terrane. The third pulse was caused by the early Tertiary Laramide deformation, which formed the fold and thrust belt of the Sierra Madre Orien-tal in eastern and northern México (Fig. 2).
The rocks of the cover only display structures related to extensional tectonics. There are two conspicuous sets of conju-gated faults throughout the southern end of the Mesa Central that in the Sierra de Guanajuato affect both the basal complex as well as the cover. The most important trend in the Sierra de Guana-juato is NW oriented, but in the region there are also important structures, such as the Villa de Reyes and La Sauceda Grabens, which strike NE and ENE, respectively (Figs. 1 and 7).
There is evidence that the extensional tectonics in the region began during the Eocene, contemporaneous to the deposit of the red beds of the Guanajuato Formation (Edwards, 1955), and may had continued at least until the Pliocene, affecting fl uviolacustrine sedi-ments with late Blancan vertebrate faunas (Carranza- Castañeda et al., 1994) in the San Miguel de Allende region (Fig. 7). It is probable that extension occurred in several periods of activity during this time (Aranda-Gómez and McDowell, 1998). Major NW-trending faulting was developed prior to the mineralization of the vein systems as most open space fi lling mineralization was emplaced in normal faults (Randall-Roberts, 2009a, 2009b). The K-Ar age of the mineralization determined in adularia in the veins is 27.4 ± 0.4 Ma (Gross, 1975). The Cenozoic fault pattern of the region cannot be attributed to a single period of deformation in
Figure 7. Regional geology of the area between Guanajuato (G) and San Luis Potosí (SLP). Note that between SLP and Sierra de Guanajua-to large portions of the area are covered by Paleogene volcanic rocks; most stratifi ed Eocene fanglomerates and Oligocene volcanics in that region are tilted to the NE. Inset shows a rose diagram of orientation of the Cenozoic faults in the San Luis Potosí and Guanajuato 1:250,000 quadrangles. Sections A–A′ and B–B′ are diagrammatic and intended only to show the Cenozoic faulting style and tilt domains. All Paleo-gene volcanic rocks were grouped in a single unit. Other abbrevia-tions used are: S—Salinas de Hidalgo; SF—San Felipe; DH—Dolores Hidalgo; L—León; SM—San Miguel de Allende; VM—Veta Madre; AF—Aldana Fault (after Aranda-Gómez and McDowell, 1998).
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 145
fl d025-07 1st pgs page 145
LD
X
F
SL
P
SI I
I I
II I I
I I
I I I I
22°N
21°N
M
PJ
S
Z
Gt
20
20 20
13
12
Key
Upp
er C
enoz
oic
cont
inen
tal s
edim
ents
Bas
altic
and
esite
s (M
ioce
ne)
Fel
sic
igni
mbr
ites
(Olig
ocen
e)
Rhy
oliti
c la
vas
(Olig
ocen
e)
Vol
cani
c ro
cks
(Eoc
ene)
Gua
naju
ato
cong
lom
erat
e
Com
anja
Gra
nite
(P
aleo
cene
)
Pre
-Cen
ozoi
c ba
sem
ent
Eoc
ene
cong
lom
erat
es (
isol
ated
out
crop
s)
Nor
mal
faul
t
Str
ike
and
dip
City
Tow
n
13
A'
A
EL
BA
JÍO
PL
AIN
Villa de Reyes Graben
Day
s 2-
3
Day
1
B'
B
102°
W10
1°W
100°
W
C18
00
2800
mas
l
mas
l
I I
I I
El B
ajío
Sie
rra
deG
uana
juat
oS
ierr
a de
S
an M
igue
lito
Vill
a de
Rey
es
Gra
ben
Qal
Qal
Qal
Qal
Tv
Tv
Tv
Tv
2400
1800
I I
I I
B'
B
AA
'
Mes Q
al
V
V
V
Vol
cano
, SM
AV
F
VV
La S
auce
da g
rabe
n
146 Aranda-Gómez et al.
fl d025-07 1st pgs page 146
which the orientation of stress components remained constant (Aranda-Gómez et al., 1989) as it is not possible, to establish a constant age relation among both sets of structures. It is pos-sible that the fault pattern was produced by successive periods of extension with different stress orientations. Reactivation of older fault planes during subsequent periods of deformation obliterated the age relations between both set of conjugated faults.
Dip directions of Paleogene stratifi ed rocks in the southern portion of the Mesa Central (Stewart et al., 1998) are generally toward the NE (Fig. 7) and we interpret them as tilts caused mainly by NW-trending down to the SW listric normal faults (see inset in Fig. 7). A statistical analysis of tilt data in the GRC and
Calderones formation (Aranda-Gómez and McDowell, 1998) shows that these rocks are generally tilted toward the NE and the average dip of the GRC is signifi cantly larger than the inclination of the Oligocene ignimbrites (Figs. 8A and 8B), despite the fact that there is no signifi cant angular unconformity between these units. These features are consistent with growth faulting and/or formation of rollover folds, which produce a stratigraphic suc-cession where stratal dip progressively decrease upward. The sta-tistical analysis assumed that fanglomerates (i.e., the GRC) and ignimbrites were nearly horizontal (≤10° depositional dip) when deposited and subsequent tilting was related to extension, as there is no evidence of compressional deformation in these rocks.
2 %
4 %
6 %
8 %
10 %
12 %
14 %
16 %
10° 30°
A B
53/181 %
2 %
3 %
4 %
5 %
6 %
7 %
8 %
9 %
20°
40°
40/30
73/29
155/24
D
1 %
3 %
5 %
7 %
9 %
11 %
13 %
15 %
80/25
155/24
20°40°
C
1 %
3 %
5 %
7 %
9 %
20° 40°
42/29
N = 328 N = 43
Figure 8. Vein systems in the SE portion of the Sierra de Guanajuato. Simplifi ed after Buchanan (1979).
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 147
fl d025-07 1st pgs page 147
Figure 8 shows structural data compiled by Edwards (1955) and Echegoyén et al. (1970) in the GRC (n = 328) and Calderones formation (n = 43), respectively. Data is displayed separately in the fi gure as lines of dip direction projected onto the lower hemisphere of equal-area plots. Most beds in the GRC are tilted toward the NE and ENE (maxima at 040°/30° and 073°/29°, respectively) with a small cluster toward the SE (166°/24°), and most beds dip between 35° and 20°. In contrast, dip direc-tions in the Calderones formation concentrate in the NE quad-rant (063°/18°) with most beds inclined between 25° and 10°. A similar analysis separating the data taken in the lower and upper members of the conglomerate mapped by Edwards (1955) shows that the orientation of the tilted beds also varies signifi cantly. Data in the lower member has a maximum centered at 042°/29°, while dips in the upper member defi ne two maxima at 080°/25° and 155°/24°, respectively. These variations in the trend and tilt angles were interpreted by Aranda-Gómez and McDowell (1998) as evidence of three pulses of extension during the Paleogene. The fi rst pulse was caused by NE-SW extension and produced NW-trending normal faults. The second pulse of extension was oriented NW-SE and produced NE-trending normal faults, such as the Aldana and Amparo faults (Fig. 6). During the last period, extension was again NE-SW oriented and produced, and very likely reactivated NW-trending faults, such as the Veta Madre, La Leona, and El Cubo faults. At this point it is worth noting that there must be another signifi cant pulse of extension after the middle Miocene that originated the NW-trending El Bajío fault, which has a minimum throw of 600 m in the past 13.5 Ma.
MINERALIZATION
By far, the most important mineral deposits in the Sierra de Guanajuato are the epithermal Ag-Au veins. The district has a 450-year history of production. Recent data published by Ran-dall-Roberts (2009b) reports 214 metric tons of gold and 37,000 tons of silver extracted in the period 1701–2004. Production is from three NW-trending vein systems (La Luz, Veta Madre, and La Sierra) emplaced both in rocks of the basal complex and in the Cenozoic cover (Fig. 7). Ore “bonanzas” (spikes) within the veins are isolated, on strike and vertically, by barren vein seg-ments. Ore bodies lie at three different elevations (2350–2100, 2200–1700 and <1700 m above sea level) in the district. The min-eralogical assemblage of both the upper and middle ore horizons is: acanthite + adularia + pyrite + electrum + calcite + quartz. The deep ore paragenesis is chalcopyrite + galena + sphalerite + adularia + quartz + acanthite. However, up to now there is only one example of the deep mineralization known in the district at the Rayas mine located on the Veta Madre vein-fault. Buchanan (1979) proposed that shallow and intermediate ores as compared to the deep ore originated from fl uids from two different com-positions. The epithermal veins occupy normal faults; age of the mineralization is 27.4 Ma (Gross, 1975). Veta Madre can be traced on the surface for over 20 km, dips consistently 35–55° to the SW and has an estimated displacement of 1200 m near
Las Torres mine and of 1700 m at the Valenciana Mine. Histori-cally, the NW-trending normal faults in the mining district have been regarded as better exploration targets than those with dif-ferent trends (EW- and NE-trending). However, Randall-Roberts (2009a) points out that in El Cubo mine area, the NW-trending veins are Ag-rich and present decreasing values of Au at depth, while EW-trending veins have Au-Ag mineralization also with decreasing Au values at depth, and NE-trending veins are Au-rich and high values are more persistent at depth. In addition, places where these structures intersect generally produce bonanzas. The most recent bonanza at El Cubo mine is related to a set of incon-spicuous NE-trending fault-veins formerly interpreted as post-mineralization. Au production in the past 30 years totals nearly a metric ton per year (Randall-Roberts, 2009a).
Besides the epithermal veins near Guanajuato, Randall-Roberts (2009b) reported small stratiform sulfi de deposits in the Mesozoic volcano-sedimentary sequence near the town of Santa Rosa (Fig. 1). Likewise, there is Au mineralization in the granite exposed at the Comanja area (Fig. 1) and in the contact aureole of the pluton, small prospects of tungsten have been found (Fig. 1). The Tertiary volcanic rocks, especially the topaz-bearing rhyo-lites, contain small amounts of disseminated tin (e.g., at Sierra El Ocote, near Nuevo Valle; Fig. 1).
Near El Cubilete (Fig. 1) are steep-dipping tabular bodies where kaolinite has been mined. We suppose that these could be argillized Tertiary dikes emplaced in the basal complex.
PART II. ROAD LOG AND SITE DESCRIPTIONS
Day 1
During the morning of the fi rst day of the fi eld trip, we will study some of the lithologies present in the volcano-plutonic and volcano-sedimentary lithotectonic units of Sierra de Guanajuato basal complex. This information will become handy when we study the clast content in some of the ignimbrites exposed in the volcanic sequence of the Guanajuato mining district. Likewise, in some of the outcrops visited during this fi rst stage of the fi eld trip, we will observe some of the structural relationships among the lithotectonic units in the basement. We will also be able to observe some of the major Cenozoic structures in the region (i.e., Veta Madre, El Bajío Fault Zone and, in the distance, the NE-trending Aldana Fault and Villa de Reyes Graben, Fig. 1).
This quick overview will be used the discuss the geology and tectonics of Sierra de Guanajuato in terms of the major geologic and tectonic provinces that converge in central Mexico: the Sierra Madre Occidental volcanic fi eld (Paleogene), Trans- Mexican Volcanic Belt (Plio-Quaternary), Sierra Madre Oriental fold and thrust belt (late Mesozoic–early Paleogene), and the southern end of the Basin and Range province (Fig. 2).
From Querétaro to GuanajuatoThis day requires a drive of ~160 km to Guanajuato. We will
travel west along the toll road Mex 45D. In the outskirts of Silao
148 Aranda-Gómez et al.
fl d025-07 1st pgs page 148
we will turn right (north) and take Mex 110D to Guanajuato. The detailed road log begins at the Camino Real hotel in Guanajuato.
From Guanajuato to La Esperanza Damkm 0.0 Hotel Camino Real. Heading to the north on the
paved road to Dolores Hidalgo (Mex 110), the high-way passes through the early Tertiary red beds of the Guanajuato Red Conglomerate (GRC). A short distance before we reach La Valenciana church (km 1.9), looking toward the east there is a pan-oramic view of an outcrop of the Veta Madre fault-vein. The structure was mined at the surface, and the old works resemble triangular facets. Closer to the road, down in the stream, are the tail ponds of some of the mines in the area.
km. 2.4 La Valenciana town. To the right is a sixteenth cen-tury church built on the trace of the NE-trending Aldana fault, which puts in contact the Eocene red beds of the GRC and the Mesozoic rocks (Fig. 6).
Once we pass the town, the road continues uphill through a diabasic sheeted dike complex emplaced in dioritic and tonalitic host rocks, part of the Mesozoic volcano-plutonic sequence of the Guanajuato Arc (Fig. 1). The outcrops of the com-plex are located west of the road. At km 3.2, near the Camino de Guanajuato Hotel, the road crosses the trace of the Veta Madre fault, which puts into contact rocks of the volcano-plutonic sequence of the Guanajuato Arc and pelagic sediments of the Arperos Basin (Fig. 1). From this point on, the highway was built on intensely deformed rocks of the Arperos Basin, in this place composed of slate and thin-bedded limestone.
km 4.5 To the left is a small, fl at area where we will pull out of the road and have our fi rst stop. Use extreme caution at this stop, as the road is narrow and traffi c usually is heavy.
Stop 1.1a. Panoramic View of the Guanajuato Mining District(UTM 14Q266143, 2328854)
Here we will examine the lithology and structural features of a sequence of strongly deformed deep-water micritic limestones and interlayered shales exposed in the road cut. In the immediate surroundings are also exposed submarine lavas, and a fl ysch-type sequence of sandstone and shale that contains exotic blocks of granite of variable size which we interpret as olistoliths. The axes of the microfolds and the most conspicuous set of lineations in the metasediments have a NW trend. These fabrics were over-printed on more subtle NE-trending microfolds and lineations. Both pulses of deformation developed in the early Cretaceous (Martini et al., 2011).
Standing on top of a small hill formed by thin-bedded lime-stone of the Arperos Basin, we can observe a broad overview of
the most relevant rock units and Cenozoic structures exposed in the region.
To the south, Guanajuato City was built in a basin occupied by the red beds of the GRC. The southeastern part of the city is fl anked by near-vertical cliffs where the lowermost Tertiary vol-canic formations overlie the GRC (Fig. 5). Behind those moun-tains is the wide valley known as El Bajío. The depression in which Guanajuato lies is limited to the west by the NE-trending Aldana fault and to the north by the NW-trending Veta Madre fault (Figs. 6 and 7). Southeast of Stop 1.1 are the Chichíndaro and Sirena hills (Fig. 6), with their northeastern fl anks displaced by the Veta Madre fault, which in that place marks the tectonic limit between the Mesozoic and Cenozoic units. From this point, it is also worth observing the marked changes in thickness in some of the mid-Tertiary volcanic units, such as La Bufa forma-tion, which in the vertical cliffs south of Stop 1.1 exceeds 300 m and in the Sirena Hill is ≅10 m. This relationship may indicate: (1) accumulation of the Bufa ash-fl ow tuff on a very irregular surface as a consequence of active erosion and/or faulting imme-diately prior to the onset of volcanism; and/or (2) deposition of the Bufa ignimbrite as an intracaldera ash-fl ow.
To the east of us, the mountain range is also known as the Sierra de Santa Rosa, and it is partially covered by the Tertiary volcanic units that dip gently (~12°) toward the NE. Compared with the sharp southwestern limit of the sierra, controlled by El Bajío Fault, its northeastern boundary is poorly defi ned, and the volcanic rocks gradually merge with thick gravel deposits that partially fi ll the San Felipe–Dolores Hidalgo Basin (Fig. 7). The overall geomorphologic picture of the region suggests that the sierra is located in the southern end of a large block tilted to the NE during the Cenozoic. Analysis of tilt data in the Cenozoic layered rocks make us believe that normal faulting began in the middle Eocene and continued after the accumulation of the Ter-tiary volcanic sequence (<30 Ma, but >13.7 Ma)
To the north in the background is exposed the Cerro Pelón tonalite (Fig. 6), which stands out as the white ground without vegetation. In the foreground is Cerro El Plomo, which is made up of a terrigenous fl ysch-type sequence; and between that hill and us, at the bottom of the stream, is La Esperanza Dam.
To the west in the background is Cerro El Cubilete, crowned by a shrine topped with a large statue of Christ. The church was built on top of a subhorizontal andesite (13.7 Ma) and gravel deposits, which in turn are overlying the Mesozoic volcano-plutonic rocks of the Guanajuato Arc. Between El Cubilete and Stop 1.1 is a deforested hilly country with extensive outcrops of a sheeted dike complex (shown in Figure 6 as undifferenti-ated basal complex) and metalavas (greenschist facies) of the volcano-plutonic sequence (shown as Guanajuato arc in Figure 1 and as basal complex in Figure 6).
Stop 1.1b. Outcrop of Metasediments of the Arperos Basin and Thrust Contact with Cerro Pelón Tonalite
Complex contact relationships among various components of the Mesozoic basement, common in all parts of the Sierra de
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 149
fl d025-07 1st pgs page 149
Guanajuato, are displayed along the road cuts between coor-dinates (UTM 14Q266112, 2328952) and (UTM 14Q266403, 2328960). They also show some of the characteristic lithol-ogies of the Arperos basin (Tardy et al., 1991). The outcrop includes interbedded pelitic and carbonate sediments (i.e., the Esperanza Formation of Echegoyén et al., 1970), submarine andesite fl ows and tuffs, and tonalitic intrusives. In this outcrop weakly metamorphosed(?) sedimentary rocks are intensely deformed, displaying a well-developed schistosity. In the argil-laceous rocks, crenulations striking approximately N-S, asso-ciated with a pre-Laramide deformation, are observed. Dur-ing Laramide deformation, these structures were compressed, producing microfolds which are clearly observed along the outcrop. The axial planes of these microfolds have an aver-age strike of N35-40W and an average dip to the northeast of <45°. Other prominent structural features include incipi-ent shear zones in the cores of some of the larger folds, and lenses or boudins of weakly metamorphosed marls dispersed among the more phyllitic argillaceous layers. Another notable feature in the westernmost part of the outcrop is the presence of near-vertical extension fractures. These may be associated with mid-Tertiary Basin-Range extension.
Volcanic rocks metamorphosed to greenschist facies are found both in tectonic contact and in depositional contact with the calcareous metasediments. The protoliths of these rocks probably were mainly andesitic lava fl ows; however, metatuffs are also present. At this site, it is possible to observe a thrust contact in the Mesozoic sequence. We speculate that the thrust plane was gently folded, probably during the Laramide Orogeny. Along this thrust, a slice of leucotonalite (Cerro Pelón tonalite), which now has a mylonitic foliation, was transported over rocks of the volcano-sedimentary association described previously. Several Tertiary normal faults displace the thrust; in some places adjacent to these later faults, the thrust and the rocks of the road cut in general are drag-folded into nearly vertical positions.
Some of the basement rock lithologies observed in this road cut will be easily identifi able as clasts in the Guanajuato Red Conglomerate and also as accidental lithics in the volcanic rocks of the Mining District.
We will go back to La Valenciana and from there we will take the dirt road to Cerro El Cubilete.
End of the fi rst part of the road log.
From La Valenciana to Cerro El Cubiletekm 0.0 Church of La Valenciana. The gravel road to Cerro
El Cubilete begins at this point.km 0.25 To the left, the road to La Valenciana mine, which
is one of the most famous and ancient (1771) Ag bonanzas in the Guanajuato mining district. A few hundred meters ahead of the junction it is possible to observe a majestic building, a remnant of the great mining operations carried out by the Span-iards during the colonial period. The road passes through the sheeted dike complex (La Palma dio-
rite, Fig. 3). From here, until we reach the Cerro El Cubilete, the route will provide road cuts of some of the different units that form the Guanajuato Arc.
km 0.7 To the right is the recently rebuilt remains of the Guadalupe mine, which now hosts a hotel and golf course. The building has impressive elephant-like buttresses.
km 3.0 To the left is the village of Llanos de Santana, and at ~200 m to the right is the shaft of San Elías mine. The shaft is on the hanging wall of the Veta Madre vein.
km 4.7 To the right is the road that communicates to La Cebada mine owned by Endeavor Silver Corpora-tion. This is the westernmost active mine on Veta Madre Vein system. From this point, the road roughly follows the contact between the sheeted dike complex (La Palma diorite) and the Cerro Pelón tonalite (Fig. 6).
km 6.6 To the right is the road that communicates with Mesa Cuata village, and which passes through the top of Cerro Pelón, the type locality of Cerro Pelón tonalite. At this site the plagiogranite cut by diabase dikes can be observed. The road passes alternatively through the leucocratic tonalite and the sheeted dike complex (La Palma diorite). At some sites the hydrothermal alteration, as well as the deformation and intense weathering of the rock can be noted.
km 11 The road goes through the Cerro Pelón tonalite. To the left at a distance is the shrine on top of Cerro El Cubilete.
km 12.6 Stop 1.2.
Stop 1.2. Cerro Pelón Tonalite Criss-Crossed by Diabase Sills; Faulting and Associated Drag Folds(UTM 14Q0261867; 2331021)
Use extreme caution at this stop, as the road is narrow and traffi c includes ore-loaded heavy trucks, as well as a few buses and cars.
The road cut exposes the typical aspect of Cerro Pelón Tonal-ite crossed by diabase “dikes” in a spectacular geometric network (Fig. 9). These “dikes” are similar to the dikes in the sheeted dike complex (La Palma diorite), and have been interpreted as the feeders of the pillow lavas in the volcano-plutonic sequence of the Guanajuato Arc (Fig. 3). However, the nearly horizontal posi-tion of these mafi c bodies seems more consistent with a group of deformed sills. At this locality both the tabular intrusions and their host rocks are cut by several small-displacement normal faults, and one can try to use the geometry of the drag folds and other features to decipher the sense of motion on the faults.km 14.2 To the right is the La Luz village church.km 14.8 At the bottom of the ravine, to the right, is the Bola-
ñitos mine, owned by Endeavor Silver Corporation. Together with Golondrinas mine, it was one of the
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two active mines at La Luz Vein System in 2009 (Devlin and Hansen, 2009).
km 15.5 To the left is the road that goes to the Golondri-nas mine.
km 16.1 To the right is the road to Bolañitos mine; at a distance and in the same direction is El Gigante Mountain, which is covered by mid-Tertiary volcanic rocks.
km 17.2 Junction with the road to the old La Luz Mine.km 17.7 To the right is the road to the Asunción mine. Near
us is a small hill supporting antennas and which was formed by the sheeted dike complex (La Palma diorite). This outcrop is interpreted as a klippe resting on the pillow lavas of the volcano-plutonic sequence. The eastern border of this klippe is the La Luz vein-fault, which crosses the road at this point.
km 18.8 To the left is the La Luz graveyard, which is on another small klippe of the sheeted dike complex (La Palma diorite). In front of us is the Cerro El Cubilete, and to the right are the lowlands of “El Bajío Guanajuatense”; at its farthest limit is the city of León. The road continues on volcanic rocks of La Luz Basalt, which are nearly devoid of vegetation.
Stop 1.3. Submarine Lavas of the Volcano-Plutonic Association(UTM 14Q0256236; 2328009)
km 20.3 The object of this stop is to show the submarine basaltic pillow-lavas, their normal stratigraphic position, evident from the pillow structure, and the variable nature of the deformation, controlled in part by the lithology of the rock and in part by the presence of discrete shear zones. In this place the
penetrative foliation was preferentially developed in the inter-pillow matrix. Walking a short distance along the road cut in the direction of La Luz it is observed that the basalts were also affected by the deformation end transformed to chlorite-rich schists. In places, remnants of the basalt are pre-served as shear pods in the milonitic schists.
km 20.05 To the right is a dirt road to Los Lorenzos village.
Stop 1.4. Pyroclastic Rocks Metamorphosed to Chlorite Schists and Intensely Deformed(UTM 14Q0254905;2325895)
km 23.3 Along several hundred meters of road, we pass through chlorite schists which are interpreted as metamorphosed intermediate to mafi c tuffs in the upper part of the volcano-plutonic sequence. Defor-mation produced a schistosity which was in turn folded. Microfolds in the area have axial planes with a general NW-SE orientation.
km 24.8 Road junction: to the left is the road to the Guanajuato-Silao highway; to the right, the road goes uphill to the shrine atop El Cubilete.
km 24.8 Walls built close to the road cuts hide Early Mio-cene unconsolidated gravel and sand deposits, composed mainly of clasts derived from the mid-Tertiary volcanic sequence and a few fragments of the Mesozoic basal complex. The deposit displays crude bedding marked by grain size changes. Rest-ing on top of the gravels is a subaerial andesitic lava fl ow which caused thermal alteration in the gravels The base of the fl ow is a monolithologic volcanic autobreccia. In the middle part of the fl ow has verti-cal joints and uppermost part is characterized by platy jointing.
TonTonTonT alalalaa teteete
SoSoSo
TalTalTalus uus depdepdepososositt Road
Diabase
Tonalite
Soil
Talus deposit Road
Figure 9. Road cut showing diabase sills emplaced in the Cerro Pelón leucotonalite. Sills are cut by small dis-placement normal faults.
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Stop 1.5a. An Overview of the Sierra de Guanajuato and Surrounding Areas from the Top of Cerro El Cubilete(UTM 14Q0253836; 2325061)
At the top of El Cubilete (Fig. 1) was built a monument to Cristo Rey. This shrine is regarded a memorial to those that died during the Revolución Cristera, fought in the 1920s in the El Bajío region.
From this point, located 2590 m above sea level and 700 m over El Bajío valley, a splendid view can be seen in all directions of the Sierra de Guanajuato and its surroundings.
To the WNW is El Bajío, with the city of León in its western end. Farther to the right is the NW part of Sierra de Guanajuato; where the volcano-sedimentary rocks of the Arperos Basin have been intruded by the early Tertiary Comanja granite. Closer to us are the outcrops of the massive diorite, exposed near the Tuna Mansa village. These plutonic rocks were thrusted on top of the submarine lavas which outcrop at the Cerro El Cubilete. This tec-tonic contact lies in the down faulted block of the NE-trending, 27–25 Ma old Villa de Reyes Graben.
To the north are El Gigante and La Giganta Mountains, which are topped by Miocene(?) andesites similar to the one at the top of El Cubilete. The depression to the west of El Gigante and La Giganta is the Villa de Reyes Graben, a Basin and Range structure ~150 km long, which ends at El Bajío Fault Zone. The master fault in the eastern side of the graben puts in contact the massive diorite and the submarine lavas. At the center of the graben lies the village of Arperos, and farther north, inside the graben, is Sierra El Ocote, which is a large, mid-Tertiary, tin- and topaz-bearing rhyolitic lava dome (Fig. 1; near Nuevo Valle).
To the east of El Cubilete is the city of Guanajuato built in a depression bounded by the NE-trending Aldana Fault and the NW-trending Veta Madre and El Bajío fault zones. The highland northeast of the city of Guanajuato is the Sierra Santa Rosa, con-stituted mainly of mid-Tertiary felsic volcanic rocks. Beyond that mountain range is the Miocene Palo Huérfano Volcano, a large andesitic composite cone. San Miguel de Allende, a popular tour-ist trap, is located just north of the volcano. Looking in the same direction, it is a conspicuous ENE-WSW depression known as La Sauceda Graben, which is a Cenozoic structure that consti-tutes the southeastern limit of both the Sierra of Guanajuato and Veta Madre Vein System (Fig. 1).
To the SE, on a clear day, one can see near the far end of El Bajío lowland, some of the northernmost Pliocene volca-nic structures of the Trans-Mexican Volcanic Belt such as La Gavia and Culiacán continental lava shield volcanoes and La Batea cinder cone. In the immediate surroundings of La Batea is a cluster of young (≅200 ka) maar type volcanoes and cinder cones (Aranda-Gómez et al., 2010). These young volcanoes lie at least 100 km north of the active volcanic front of the Trans-Mexican Volcanic Belt and some of its lavas have compositions similar to the intraplate-type lavas of the Mesa Central (Blatter and Hammersley, 2010).
Immediately south of El Cubilete, bordering El Bajío depres-sion, is El Bajío fault zone; the trace of its master fault is located at the base of the mountain front, near La Ermita church. This fault was active in the late Cenozoic and has caused the down-ward displacement of El Bajío respect of the Sierra of Guana-juato. This is suggested by the Miocene sequence atop Cerro El Cubilete, which has been elevated more than 600 m with respect to its counterpart at El Bajío.
Stop 1.5b. Tertiary Gravels and Andesitic Lavas Crowning the Mesozoic Basement of the Sierra de Guanajuato(UTM: 14Q0254029, 23225138)
The Cristo Rey monument is built atop 13.7 Ma (Aguirre-Díaz et al., 1997) andesitic lavas. Underneath the Miocene andes-ite is exposed an unconsolidated fl uviolacustrine deposit com-posed principally of clasts of volcanic rocks derived from the mid-Tertiary units of the Sierra de Guanajuato. The most com-mon lithologies among the clasts are felsic ignimbrites, rhyolitic dome and fl ow rocks, and some andesites. Clasts derived from the Mesozoic basal complex are relatively rare. Clasts in this deposit are well rounded, and they are supported by coarse sand and silt matrix. The deposit has a faint stratifi cation, marked by subtle changes in grain size in the fi ner-grained clastic deposits, and by coarse gravel lenses. The estimated thickness of the gravel deposit is 150 m in its thickest part, but it thins against the Meso-zoic rocks. No fossils have yet been reported in this sedimentary sequence, but its age is bracketed by the age of the youngest fel-sic rocks known in Sierra de (around 25 Ma: Nieto-Samaniego et al., 1996), and the 13.7 Ma age of the andesite.
Resting unconformably on the fl uviolacustrine deposits is a thick subaerial andesitic lava fl ow, which caused thermal altera-tion at the contact with the gravel deposit. The base of the fl ow is an autobreccia which changes upward to a zone with well- developed columnar joints. Above this zone, without a marked interruption other than a prominent set of horizontal joints, is another thick andesite, very similar in texture to the earlier one. This one has a base with intense subhorizontal platy jointing which passes upward into a thick columnar zone in the middle part, and culminates with a zone of subhorizontal platy joint-ing in the uppermost part. In total, the andesite sums up to 70 m of thickness. Whether this andesite outcrop is formed by a very thick lava fl ow or by two lavas fl ows is not clear.
End of the second part of the road log.
From El Cubilete to Guanajuatokm 0.0 From Stop 1.5b we will follow the precipitous
road to El Bajío. Outcrops exposed at the road cuts show the volcano-plutonic sequence of the Guana-juato Arc.
km 4.0 About halfway down, we will see small adits exca-vated to mine kaolinite from argillized felsic dikes (Tertiary?). Another remarkable feature of this road is the very steep slope of the mountains and the
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presence of several abrupt changes in the slope that suggest step faulting along the El Bajío fault zone.
km 8.0 Near La Ermita Church, at the base of the mountain front, is the Aguas Buenas hot spring. The outcrops of the basal complex are replaced by extensive out-crops of the mid-Tertiary felsic volcanic rocks and of the Neogene sequence formed by the gravels and andesite seen in Stop 1.5b.
km 11.0 Junction with the Guanajuato-Silao toll road. We turn to the left. The freeway was built in sand and gravel deposits, similar to those atop El Cubilete.
Km 13.0 South of the village of El Capulín the gravel depos-its are covered by an andesitic lava fl ow. This is the same sequence we saw at the top of El Cubilete.
km 16.0 The freeway crosses the trace of the NW-trending Yerbabuena-Rodeo fault. The GRC is exposed in the road cuts and in the isolated hill located west of the road.
km 17.0 Toll house.End of the third part of the road log.
From Guanajuato to El Cubo Minekm 0.0 Junction Carretera Panorámica—Road to El Cubo.
Stop 1.6. Eocene Red Beds of the GRC(UTM: 14Q0267238, 2324332)
The road cuts expose the red beds of the GRC. This is a typical outcrop of the upper member of this unit. It is composed by well indurated pebble and/or cobble conglomerates. Crude coarse bedding in the deposit is defi ned by marked changes in the grain size and in the clast/matrix ratio. The coarser-grained layers are clast supported and the fragments are angular to suban-gular. Most of the clasts appear to be derived from the basal com-plex, and include fragments of basic metalavas, intrusive rocks and metasediments. However, we note that the lithology of some of the clasts (e.g., intermediate to felsic volcanic rocks) do not appear to have counter parts in the pre-Eocene outcrops known in the area. We concur with Edwards’(1955) interpretation that the lower member of the GRC is an immature, near source, allu-vial fan deposit derived from rapidly uplifting block mountain. Prior to the uplift, it is likely that the basal complex was par-tially to completely buried under a sedimentary and/or volcanic sequence. In a few places in the lower member of the GRC occur interlayered lava fl ows. This indicates volcanism contemporane-ous to the deposit. The age of this volcanic activity is broadly similar to the age of the Lower Volcanic Sequence in the Sierra Madre Occidental Volcanic Province. It has been proposed by Aguirre-Díaz and McDowell (1991) that this magmatic period may have been widespread in Mexico. In fact, there are reports of several isolated outcrops of Eocene volcanic rocks scattered in the Mesa Central.
In the outcrop are several NS-trending fractures surrounded by thin haloes of hydrothermal alteration, where the red beds
have been chloritized. In a few of them it is possible to fi nd thin veinlets of quartz and/or calcite.
Stop 1.7. GRC-Losero and Losero-Bufa Stratigraphic Contacts(UTM: 14Q0267866, 2324305)
km 0.5 These contacts are exposed both in the road cuts and in the cliffs across the canyon. The contact in both places is evident because of the marked change in the grain size and selection in the deposits, and the conspicuous changes in color and in bed thick-ness. The Losero epiclastic volcanic sandstone beds and surge layers are used as building stone in the city. Many of the façades in downtown Guanajuato are covered with fl agstones quarried in this area.
At the bottom of the ravine looking toward the SW is La Olla Dam. This old construction was built in the late 1700s, and for a long time was the main source of water for Guanajuato. It also played an important role in fl ood control (the city was partially destroyed in several occasions by fl ash fl oods).
Near the road cut it is evident that Bufa and Losero are sepa-rated by an erosional surface. Bedding in Losero is interrupted and in places near its base the ash-fl ow tuff contains fragments derived from the underlying formation. Whether this erosion rep-resents an important hiatus in the sequence or was caused by the turbulence at the base of a large pyroclastic fl ow is unknown. However, this is a common relationship throughout the district.
In this site the rhyolitic ash-fl ow tuff is massive and unwelded. A few hundred meters ahead, near our next stop, the road crosses a zone where the Bufa ignimbrite displays crude vertical columnar jointing. In that zone the pumice fragments are fl attened and the pyroclastic deposit has a weak eutaxitic foliation.
As we pointed out in Stop 1.1, Bufa formation has remark-able thickness variations in very short distances within the Gua-najuato mining district. In less than 5 km it varies from greater than 300 m (as in this area) to 0–10 m (Cerro Sirena). Outside the district, this unit has not been identifi ed. An ignimbrite exposed close to the Aldana–El Bajío fault junction, just north of Silao (Fig. 1) is thought to be part of the Cuatralba ignimbrite (~30 Ma), which covers a broad area west of the Villa de Reyes Graben. Likewise, the Calderones formation, which has a very distinct lithology, has not been found outside of the block par-tially limited by the La Sauceda Graben and the mineralized faults in La Sierra vein system (Fig. 7).km 1.1 Arroyo Los Rieles. We will park here and walk
The road cut shows that the base of the volcaniclastic, andes-itic to dacitic, Calderones formation locally occupies shallow
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channels carved in the upper part of the Bufa formation. There is a notable change in the thickness of the beds accumulated in the channel (Fig. 10); they thin out toward the border of the chan-nel. Lithology is very diffi cult to see here as the outcrop is cov-ered by dust. We will see good exposures of this contact in other stops. However, the erosional nature of the contact is very evident in this locality. Please note that as a consequence of the intense alteration Calderones formation presents, the composition of this unit is highly uncertain. Likewise, the lithostratigraphic unit is composed by many ash-fl ow units that we believe come from different sources. Therefore, it is possible that composition of primary clasts may not be uniform.km 2.6 Loma del Venado. Junction of the gravel roads that
lead to Las Torres and El Cubo mines and the dirt road to the Calderones Village. Las Torre mine was discovered in the early 1970s and for several decades was the largest mining operation in the district. The mine was closed for a while, but is again working.
km 3.0 Junction with road to Las Torres mine. Take road to the left.
km 4.5 To the east, at the bottom of the ravine are the tailing ponds of Las Torres. Looking in the same direction across the stream is an outcrop of the red beds of the GRC, crowned by Bufa formation. The Cenozoic sequence is repeated in this area by the Veta Madre vein-fault system. Immediately to the west of the road is a quarry excavated in the Cedro andesite. The barren rock is used to maintain the tailing ponds.
km 4.3 Park on one side of the road. Be careful of the cars, buses, and ore-loaded trucks.
Stop 1.9. Outcrop of the Eastern Branch of Veta Madre—Fault Contact between the Cedro and Calderones Formations with the GRC(UTM: 14Q0269486, 2324582)
According to Martínez-Reyes (1992) in this region Veta Madre splits into two branches that surround the Chichíndaro hill (Fig. 6). A recently built road cut shows a good outcrop of Veta Madre, which used to be covered by talus deposits. In a roughly EW-trending section, it is possible to see the tectonic contacts among the red beds and Calderones, and between Calderones and Cedro formations. The high angle (60–70°) normal faults are NS trending and the down-dropped blocks are to the west. Rocks in the outcrop appear in places intensely brecciated and altered. Please note that there are no clear signs of mineralization other than the intense alteration. The Chichíndaro hill is topped by rhyolitic lava dome ca. 30–32 Ma old. The rhyolites contain dis-seminated tin (J. Echegoyén, 1995, personal commun.). The age of the Veta Madre fault is bracketed by the age of the Chichíndaro rhyolite and the age of the mineralization in Veta Madre (K-Ar, adularia = ca. 27 Ma: Gross, 1975).
Day 2
We will leave our base hotel at 09:00 and from there we will take the road toward the center of the city. We will cross Guana-juato heading toward the Silao exit. Before arriving at the exit we will take the overpass to Pozuelos, in order to get onto the Pan-orámica in its southern part. We will pass the ISSSTE Hospital and stop at the CFE electrical substation.
Stop 2.1. An Overview of the Contact of the GRC with the Tertiary Volcanic Sequence and Discussion about Red Bed Syn-Sedimentary Faulting
From this saddle (UTM 14Q0266028; 2323830), we can take in a panoramic view of Cerro de La Bufa, the type locality of the Bufa Ignimbrite. The contact between the GRC and the overlying units, the Losero Formation and the Bufa Ignimbrite is clearly visible owing to the marked color contrast between the units—dark red for the GRC and light green and yellow-ish pink for the Losero-Bufa package. From here we can also observe the change in dip of the upper layers of the GRC with respect to its lower layers. The dip becomes gradually gentler upward, and at the top is even almost concordant with the sub-horizontal layers (~16°) of the Losero Formation. This gradual change in the dip was interpreted as a rollover fold in the Ter-tiary sequence by Aranda-Gómez and McDowell (1998). Fol-lowing Edwards (1955) these authors argue that the GRC and the volcanic sequence were accumulated at the same time that intense normal faulting was going on in the region.
From here we will go up to the contact area of the three stratigraphic units (UTM 14Q0266443; 2323665). Then, we will walk along the contact between the GRC and the Losero to a place known as La Cueva (UTM 14Q0265908; 2323168), that is a somewhat tabular excavation made in order to extract sheets and blocks of the Losero Formation. Losero by reason of its characteristic green color and fi nely laminated layers with graceful dune bedforms has been used as an ornamental stone in constructions around the region. The quarrying of the Losero is no longer going on at La Cueva, and now it is a small cha-pel traditionally visited during the Holy Week holidays. Owing to the workings of this quarry, the Losero is unusually well exposed, and it is possible to observe details of the sequence of surge deposits in the Losero on mutually perpendicular sur-faces, as well as the contact between the Losero surge beds and the Bufa Ignimbrite.
Stop 2.2. Lithologies of the Upper Part of the GRC, the Losero Formation, and the Base of the Bufa Ignimbrite
The GRC-Losero transition: At this section, from (UTM 14Q0266443; 2323665) to (UTM 14Q0265908; 232316)) along a trail, we have a well-exposed section which shows the transition from the GRC to the Losero Formation and the contact between the Losero Formation and the Bufa Ignimbrite. The upper layers of the GRC are strata of fi ne gravel and coarse sands with rather thin bedding (10–20 cm) which give way upward to red siltstone.
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BB C
D E
F G
B CC
DD EE
A
FF GG
Figure 10. Different aspects of the Calderones formation. (A) Erosional contact between the Bufa and Calderones for-mations, exposed on a road cut near arroyo Los Rieles (UTM 14Q268409, 2324293). (B) Mixed interval composed by epiclastic-volcanic beds interlayered with surge deposits (UTM14Q268738, 2321817). (C) Surge sequence at the base of the capping ignimbrite at Cerro El Coronel. (D) Monolithologic lithic rich bed at the base of one ignimbrites exposed at arroyo Los Silvestres (UTM 14Q271643, 2324569). (E) Angular fragments of dacite surrounded by a tuffaceous matrix. (F) Lithic rich breccia with diffuse bedding exposed at the base of the capping ignimbrites at Cerro La Loca. (G) Ballistic fragment and bomb sag at the surge sequence exposed at the base of the capping ignimbrites at Cerro La Loca.
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The sequence continues with a mixed interval of green and red siltstone with cross-bedding, which changes in an almost imper-ceptible way to the entirely green layers of the Losero. Therefore the contact between the GRC and the Losero is transitional and concordant at this point.
Characteristics and origin of the Losero: Along the out-crops on this trail, the Losero has a variable thickness, from 0 to 10 m. It consists of epiclastic deposits of well-sorted sands and silts and rhythmic successions of surge layers. These deposits form thin strata with fi ne internal laminations. The surge layers have very low angle cross stratifi cation. The surge deposits are composed of pyroclastic material that varies from very fi ne to coarse ash. The uppermost part of the Losero includes some very fi ne ash deposits with normal grading. We think that many of the pyroclastic deposits described above accumulated in water, pos-sibly in an ephemeral shallow lake. This interpretation is based on the presence of layers of well-sorted sandstone and siltstone and pervasive chloritization in the deposit. This interpretation is congruent with the lithology of the uppermost portion of the GRC and the transitional contact observed; however, it is also possible that the apparently detrital layers are in fact the planar facies associated with sandwave facies in a surge deposit (Wohletz and Sheridan, 1979).
The Losero-Bufa contact: The Bufa ignimbrite in many places seems to overlie the Losero concordantly, but in others it is easy to see layers of the Losero truncated by the base of the ignimbrite. One can also see surfaces with raindrop pits in the upper layers of the Losero, which implies that there was a time lapse between the activity that produced the Losero and that which produced the Bufa ignimbrite. On the other hand, there are no reworked deposits or paleosols between the two units. Thus we infer that the Bufa ignimbrite was emplaced shortly after the accumulation of the Losero surge layers. In fact, it is possible that the Losero and the Bufa are both products of the same source and that the Losero represents the initial phases of the paroxysmal eruption that produced the Bufa ignimbrite. The contact forms a planar surface with an average attitude of N30W, 18NE.
Characteristics of the lower portion of the Bufa ignim-brite: In these outcrops at La Cueva the Bufa ignimbrite is ~300 m thick. We will only be able to study the lowermost part of the unit, which has a fairly low degree of welding. In the lower part of the Bufa, just above the contact with the Losero, is a zone ~2 m thick which is rich in lithics and has green fi amme. It changes gradually upward to a partially welded, lithic-poor zone several meters thick. This zone is overlain by a zone with abundant hollow pits, which range in size from golf balls to soccer balls. These pits are interpreted as the result from differ-ential erosion of pumice clasts with respect to matrix, the pum-ices in this case being more easily eroded. Above the pitted layer, the ignimbrite changes to a silicifi ed zone gray in color with black spots and patches of iron oxide. Silicifi cation is per-vasive in this zone, transforming it into erosion-resistant rock. The original texture of the silicifi ed zone was totally obliter-
ated, and secondary quartz is abundant. As primary minerals the ignimbrite contains fairly abundant euhedral biotite, sani-dine, and quartz phenocrysts.
As was mentioned at Stop 1.1a, the Bufa Ignimbrite has great lateral variations in thickness within the district, and out-side of the district it seems to be absent. We speculate that one or more of the curvilinear faults that separate this outcrop from Cerro de Sirena (the Amparo and San Clemente faults (Fig. 6) and the northeastern branch of the Veta Madre) form part of the northern margin of a caldera associated with the Bufa eruption, although in a strict sense the source(s) of the Bufa are not known.
We will return to the vehicles and take the Panorámica High-way and drive east toward La Olla Dam. This construction dates from the 1700s, and for many years it was the only source of water for the city. It also played an important role in controlling fl oods (the city was destroyed several times by fl ooding). Upon arriving at the dam, we will continue until we take the road to El Cubo mine.km 0.0 We will take the dirt road toward El Cubo Mine.km 2.6 Junction with the road to the Calderones Village at
Loma El Venado. We will park here and walk back along the road to see the Calderones–Cedro contact along the road cuts.
Stop 2.3. Contact between the Calderones Pyroclastic Rocks and the Cedro Andesite(UTM 14Q0268833; 2323404)
About 100 m west of the bus stop, the contact between the two units is exposed in the road cuts. In an interval of ~12 m, it is possible to see that at the top of the Calderones is a ~3 m thick succession of thin layers of relatively crystal-rich tuffs with highly vesiculated and intensely palagonitized glassy matrix. These yel-lowish brown layers are interstratifi ed with the more typical fi ne-grained green layers in the lower part of the transitional interval. Just at the base of the Cedro lava fl ows is a horizon of very thin layers with well-formed desiccation cracks. Above these layers rests an andesite fl ow with well-developed spheroidal weather-ing and rounded structures that resemble small pillows, which changes gradually upward into massive andesite. These features at the fl ow base are interpreted as evidence that the lava inter-acted with water. In a small quarry located to the north of the bus stop it is possible to see that the fi rst andesite fl ow at the base of the Cedro is overlain by another fl ow with characteristics similar to those described above.
From the bus stop we will follow a dirt road that head south toward the Calderones town (Fig. 6). We will cross the town and follow the road toward the Humboldt shaft where we will park and leave the trucks in order to make a 1.0 km long traverse from the Bufa–Calderones contact (UTM 14Q267894, 2321703) to the base of a vertical cliff near the top of Cerro El Coronel (UTM 14Q269113, 2321588).km 5.0 Humboldt shaft (UTM 14Q268810, 2321667).
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Stop 2.4. A Stratigraphic Section across Intermediate to Distal Facies of the Calderones Ignimbrites and Evidence of “Wet” Eruptions
The features observed along the traverse are summarized in Figure 11. The stratigraphic log covers from the eroded top of the Bufa ignimbrite (UTM 14Q0268320, 2321977), where a thin dike of andesite may be seen, to the relatively thick ignim-brites that cap Cerro El Coronel (UTM 14Q0269116, 2321594). At fi rst sight, it is evident that the Calderones formation is made by an undetermined, but relatively large number of ignimbrites, surge beds, and epiclastic-volcanic horizons. Lithic-rich inter-vals within the ignimbrites are common and the lithology and origin of lithics is variable. In some places are more evident clasts derived from the basal complex or from the GRC. Other horizons include fragments of felsic volcanic rocks that we believe come from the Peregrina lava dome. Distinguishing features through-out the succession are: (1) the pervasive chloritization of the pyroclastic rocks, which in most places have characteristic green color, although there are intervals where rocks have a purple col-oration; collapsed pumice, however, is almost invariably seen as medium- to dark-green fi amme; (2) the fact that most ignimbrites display internal layering that varies from distinct to diffuse; and (3) a distinctly layered succession epiclastic-volcanic sandstones with normal graded bedding and marked internal laminations (Fig. 10B) near the road.
Near the top of the section, just below of the cliff-forming, relatively voluminous massive ignimbrite that caps Cerro El Coronel, there are numerous ignimbrites with spectacularly fl at-tened fi amme in ashy matrix material that are interbedded with increasingly thicker intervals of surge deposits. The thicknesses of the layers become smaller near the top of the section. The fi nal two meters of these deposits beneath the summit cliff of Cerro Coronel display a wide variety of structures and a delicate style of the lamination. These layers are interpreted as a series of pyroclastic surges (Fig. 10C), which probably preceded the emplacement of the capping ignimbrite. This uppermost ignim-brite in the section is formed at least by two fl ow units. Another characteristic is the high lithic content of this deposit (more than 30% of the deposit in some places). As for the lithologic nature of those fragments the overwhelming majority are of felsic rock with fi ne fl ow banding.
We believe that many of the observed features indicate an important role of external water in the formation of parts of the Cerro El Coronel sequence. The fact that clasts in the lithic-rich portions of the pyroclastic succession rarely exceed 10 cm in diameter, and in comparison with other localities that we will study in the fi eld trip, indicates that this may be considered a relatively distal portion of the ash-fl ows that form the Calderones formation.
On the way to the top of Cerro El Coronel we will see two andesite dikes. At least one of them appears to be emplaced in a syn-volcanic fault and other one, exposed just east of the road, displays textural features and alteration that we interpret as evi-dence of its interaction with wet pyroclastic rocks.
We will drive back to El Cubo road. At the road junction of Loma El Venado (Fig. 6) we will turn right toward El Cubo mine.km 7.4 J unction with the road to Las Torres mine at Loma
El Venado. We will pass the junction to Las Torres mine and head northeast to the junction with the road to the Peregrina mine (Fig. 6; LP locality).
km 10.8 Junction to the Peregrina mine: take the right fork heading east to El Cubo mine.
km 12.3 Arroyo Los Silvestres: park on the south side of the road. Walk downstream along the arroyo bed.
Stop 2.5. Arroyo de Los Silvestres: Boulder beds, Pyroclastic Flow Layers and Surge Layers in the Medial Facies of Calderones(UTM: 14Q0271688, 2324734)
At this location we are in a major graben bounded by the NW-trending La Leona and El Cubo faults (Fig. 6). This struc-ture may be syn-volcanic, although both faults have had further displacement after the Cedro lava fl ows were emplaced. Evi-dence for such movement can be seen in the drag folding of the Calderones beds along the La Leona fault and in the fact that the Cedro Andesite was tilted to the NE along El Cubo fault. Evidence of syn-volcanic activity is suggested by an andesite dike (presumably a feeder for the Cedro formation) that cuts La Leona fault (Fig. 6).
We will walk ~400 m downstream in order to look at a rep-resentative portion of the medial facies of Calderones. Along this section there are a number of repetitions of the typical sequence, which contains (from the base upward): surge beds overlain by thick layers (up to three meters) with large angular to subangular boulders (up to 1 m long) set in variable amounts of tuffaceous matrix (Figs. 10D and 10E). These, in turn, are succeeded by ash-fl ow deposits with abundant, conspicuously fl attened and ramped, pumices and abundant small lithics of various rock types; each sequence is capped by a package of very fi ne-grained surge layers with unusually low-angle cross-bedding. As we go downstream, the boulder layers at the bases of each ignimbrite vary from nearly monolithologic accumulations of dacitic to rhyodacitic dome rocks to mixed assemblages containing more and larger GRC boulders and a signifi cant quantity of andesite (Mesozoic La Luz meta-andesite and/or GRC hydrothermally altered andesite).
Day 3
We will leave our base hotel at 09:00 and from there we will go back to Arroyo Los Silvestres, the site of yesterday’s Stop 2.5
Figure 11. Stratigraphic log of an intermediate to distal section of the Calderones formation exposed at Cerro El Coronel. Not to scale; units thickness were estimated. Key of abbreviations used to describe the lithofacies modifi ed after Branney and Kokelaar (2002).
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 157
km 0.0 Arroyo Los Silvestres. Road log begins.km 0.3 Junction of road to El Cubo mine and road that
joins the Peregrina mine (PM) and town of El Cubo (CT, Fig. 6). Keep on the road to El Cubo mine.
km 0.9 We will park and leave the trucks in order to make a 400 m long traverse along a relatively steep stream, from the top of the Bufa ignimbrite (UTM 14Q272542, 2324966) to the base of thick ignimbrites capping Cerro La Loca (UTM 14Q272715, 2324966)
Stop 3.1a. Cerro de La Loca: Proximal Facies of the Calderones Sequence—Facies Changes from Thin Ignimbrites and Interbedded Breccias Upward to Thick Capping Ignimbrites
The features observed along the traverse are summarized in Figure 12; immediately north of the road is exposed the top of a poorly welded, massive, felsic ignimbrite which contains quartz, feldspar, and biotite phenocrysts. Echegoyén et al. (1970) mapped other outcrops of Bufa ignimbrite in the footwall block of El Cubo fault (Fig. 6). This ignimbrite presents irregular sub-horizontal joints roughly parallel to the contact with the overly-ing Calderones formation, a fact that we interpret as evidence of weathering of the top of Bufa, prior to Calderones accumulation.
At the base of the Calderones formation is exposed a thin to medium bedded (20–40 cm) pebble conglomerate made of rounded to sub-rounded clasts (2–4 cm). This sequence changes upward to thin bedded (5–10 cm) succession of sandstone and conglomeratic sandstone strata separated by clay partings. Finally, the sequence changes to very thin beds of distinctly lami-nated, very fi ne grained, sandstones. These strata at the base of Calderones are exposed along the cuts on the road to El Cubo where they display wavy shapes, suggestive of very open fold-ing, draping, or differential compaction over an irregular sur-face(?). These beds at the base of Calderones lack cross-bedding and some of them have rather poorly developed normal graded bedding. We think these beds are best described as a rhythmic sequence of mixed fl uvial and reworked(?) pyroclastics, which were deposited in very shallow water. The sequence passes upward thru and abrupt contact to a relatively thick, massive, crystal- and lithic-rich, moderately welded ignimbrite, in places with 4–5 mm long fi amme, which are always replaced by chlo-rite. Exposures in the area immediately to the NW of the arroyo bed show at least one grain size change associated with “splay and fade” bedding that suggests more than one fl ow unit within this particular ignimbrite.
Closer to the massive ignimbrite sequence capping the top of the hill there are thin to medium-thickness (15–30 cm) pyroclastic fl ow deposits, which include surges, minor ignimbrites and brec-cias of uncertain origin, possibly medial portions of block-and-ash-fl ows. Some of these beds include easily recognizable accre-tionary lapilli. In this section, as in most parts of the Calderones Formation, almost all the deposits contain fi amme converted to dark green chlorite. There are many interesting color variations
in these beds—in some layers only the shard-rich matrix and the fi amme are green; other layers have green fragments in a grayish matrix, and still other layers have both green lithic fragments and green fi amme/matrix. This series of beds continues for several meters, until a contact is reached with a package of thin cross-bedded layers accumulated almost exclusively from pyroclastic surges. The surge layers are overlain by a series of green thinly bedded sandstones which are in turn overlain by another package of surge-bedded tuffs, rich in large (up to 35 cm) angular lithic fragments (Fig. 10F) of various types (GRC, vein quartz, granite, etc.). We observed at least one ballistic fragment, 40 cm long, which produced a sag in the underlying beds (Fig. 10G).
The pyroclastic rocks that make up the summit of Cerro de La Loca are a group of thick ignimbrites which form the culminating sequence of this part of Calderones. We interpret this entire package as a single cooling unit, accumulated in at least four pulses of emplacement. The degree of welding in the capping ignimbrites is comparable to the strongest welding observed in any part of the Calderones. Each of the emplacement units has different lithic fragments; for example, the lower most emplacement unit contains sparse small fragments of limestone as well as many slightly larger angular fragments of Peregrina dacite, while the upper three units have fragments of phyllite but apparently lack limestone. The third emplacement unit, up from the base, has scarce lithics of small size. The uppermost unit is characterized by an abundance of very small lithics that varies little from base to top. It is perhaps the unit with the high-est lithic content of the four units that make up this composite ignimbrite. The lithics are principally of reddish-brown lavas and glassy white lavas. Pumice is not apparent, but it may have been completely masked by secondary alteration. In total, the thicknesses of the four ignimbrite emplacement units add up to ~50 m. Our interpretation is that these ignimbrites were erupted during the paroxysmal phase of the eruptions that formed the Calderones sequence.
Stop 3.1b. Interaction between Dikes and Bedded Deposits(UTM 14Q272802, 2324933)
Calderones is cut in many places by andesitic dikes that are thought to be equivalents, and probably feeders, of the Cedro Andesite. As a whole the andesitic dikes defi ne a rough radial pattern that points toward the Cerro Alto de Villalpando region where Stop 3.3 is located (Fig. 6).
The interaction between the dikes and the apparently still water-bearing pyroclastic deposits of the Calderones locally gave rise to a second generation of pyroclastic products. We can observe this phenomenon in the saddle between Cerro La Loca and its neighbor to the southeast, where deposits of ash and brec-cias appear to lie unconformably over part of the Calderones sequence adjacent to an andesitic dike. The dike also produced thermal alteration in the surrounding layers of Calderones, indu-rating them and making them more resistant to erosion than the parts not altered by the dike. The result of this process is that
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 159
fl d025-07 1st pgs page 159
Log GTO-12Cerro La LocaGuanajuato
ash
2.0
2.0
2.0
64.0
64.0
64.0
mm
≥40 m mT (spotted)
base notexposed
emLT
dsemLT
dsT
Pervasivechloritization
lmLT
lmLT
//sTpebbly layers
//sT to laxsT
dseLT
mLT
mLBr
mLBr
xs- to mLBr
//sT
//sTlmT
xsTmLmTacc
lmT
lmLT
dsLT
//sT
//sT
mT-emLT-lmLTlaxsT
top eroded
paleosol(?)not exposed
not exposed
xsT
Flow-unit boundary(?)
andesitic dike/faultwith altered sides
scour
mT
mT
mT
mT (fine-grained, sands/silts)
sT
BufaFormation
~150 m
CalderonesFormation
30 m
4.0 m
10 m
30 m
~50 m
coar
se a
sh0.
032
>64
.0>
64.0
>64
.0
0.00
4
lapi
lli
bloc
ks
Figure 12. Stratigraphic log of a proximal section of the Calderones formation exposed at Cerro El Coro-nel. The lowermost part of Calderones is interpreted as a mixed epiclastic and pyroclastic deposit accumu-lated in shallow water. Not to scale; units thickness were estimated. See Figure 11 for a key to abbrevia-tions used to describe the lithofacies
160 Aranda-Gómez et al.
fl d025-07 1st pgs page 160
tabular erosional forms of the baked Calderones stand up above ground level on both sides of the deeply eroded dikes.
We will go back to the truck and drive back toward the inter-section between the roads to El Cubo and La Peregrina mine (LP in Fig. 6). We will turn to the right and park near the Peregrina Dam to study the lithology of the Peregrina lava dome.km 3.2 Letrero de Peregrina junction.km 4.8 Park on the side of the road that leads to the Pereg-
rina mine.
Stop 3.2. Overview of the Peregrina Lava Dome near the Peregrina Dam(UTM 14Q271490, 2326468)
From this vantage point we can see a good panoramic view of the Peregrina Dam and the northwestern part of the Peregrina Dome Complex. We can also study outcrops of the dacitic phase of the Peregrina with its characteristic fl ow banding. The contact between the Peregrina domes and Calderones tuff-breccias and other proximal deposits, seems to be a complex and repetitive one, as the pyroclastic block and ash-fl ows produced by the Per-egrina domes are intercalated with, and form part of, the Calde-rones sequence. South of here, many of the fl ow units observed in the Calderones are rich in angular lithic fragments of this dacitic phase of the Peregrina, as we saw yesterday evening at Arroyo Los Silvestres (Stop 2.5; Figs. 10D and 10E).
Looking toward the Peregrina Dam, you can see two small hills on the skyline. In the outcrops at the sides and tops of these hills, fl ow foliation typical of domes can be seen; this fl ow- banding is nearly vertical around the Peregrina Dam.
From this point we will be guided by a geologist of Aurico Gold to an outcrop located on a nearly abandoned road that leads toward the top of Cerro Alto de Villalpando. We may have to walk to the stop as the road is steep and in poor condition. Keep on the road as there is a place where subsidence into an old mine has occurred! This outcrop (Stop 3.3) as well as next one (Stop 3.4) are located inside of Aurico Gold property near some of the mine installations. We will have to cross a control gate.
Stop 3.3. Cerro Alto de Villalpando: Ring Dike with Calderones Vent Facies and a Panoramic View of the Calderones Sequence from its Principal Point of Origin
In this long road cut, between (UTM 14Q273388, 2326334) and (UTM 14Q 273643, 2326300), just beneath the summit of the hill, we can observe a large ring dike which is composed of Calderones tuff and tuff-breccia. The dike cuts a dacitic to rhyo-litic dome that forms the greater part of Cerro Alto de Villalpando and that we consider part of the Peregrina dome complex. Over-lying the Peregrina dome, and likely cutting it is the Chichíndaro Rhyolite, which forms the highest part of the hill. The road cuts obliquely across the ring dike, affording us an excellent exposure of its contacts and its interior along a traverse of several hundred meters; the true width of the dike is at least 50 m. The contact between the ring dike and the Peregrina dacite near where we
left the vehicles is subvertical and irregular, with well-developed shear surfaces both within the dike and in the host rock. In other locations we have seen breccias and cataclastic rocks in zones up to 8 m wide along the dike margin. These zones are made up of well-preserved fragments of the Peregrina dome rocks in matri-ces of Calderones tuff. The fragments in the interior of the dike are heterolithologic, including clasts derived from the Mesozoic basement, such as phyllite, argillite, quartzite, meta-andesite, and calcareous rocks, as well as clasts derived from the Cenozoic cover, such as the GRC and altered rhyolitic to dacitic rocks, pre-sumably from the Peregrina domes. All these clast types exhibit considerable size variation, ranging from a few millimeters to several meters in diameter. Extremely large blocks of phyllite of the Esperanza Formation are present in the dike, suggesting that this formation is present at shallow depths below the sur-face; one such block can be seen behind the concrete platform at (UTM 14Q 273643, 2326300). This pyroclastic dike probably served as the principal venting structure for the Calderones For-mation, and it can be interpreted as a partial-ring fracture bound-ing a caldera. Because its outcrop is limited to the northeastern quadrant of the putative circular boundary, a partial or trap door, morphology is suggested for the Calderones caldera (Fig. 6).
After examining the dike, we will avail ourselves of the excellent panoramic view, looking westward, of the Calderones sequence. Immediately below the road is an area with a notice-able reddish brown color, which forms a band of low, rounded hills in the vicinity of the Tiro de San Lorenzo de Villalpando. These hillocks correspond to the intra-caldera facies of the Calde-rones sequence, which we interpret as a collapse megabreccia made up of enormous clasts of Esperanza Formation (phyllites and schists) in a scant matrix of Calderones tuff. In addition to the more abundant Esperanza-dominated megabreccia, there are isolated outcrops of Peregrina-dominated megabreccia. We will be looking at these outcrops at the next stop.
In the middle distance, beyond the megabreccia, we can see a prominent ridge with several summits held up by thick, apparently massive, layers. The largest and highest of these summits is Cerro de La Loca, which we visited in Stop 3.1. Cerro de La Loca itself, like the entire La Loca ridge, is made up of the proximal facies of the Calderones sequence, with a series of relatively thick and voluminous ignimbrites at the top. The ridge beyond the La Loca ridge is the La Leona Ridge, whose summit is capped by a thick section of the Bufa Ignim-brite. The Bufa is in tectonic contact with the Calderones along the La Leona normal fault (Fig. 6), whose trace follows the base of the dip slope of the ridge. Unlike most of the normal faults in the district, it dips to the NE, toward us; the low hills and ridges on the downthrown block are underlain by the Calderones and Cedro Formations. Even further in the distance we can see a series of small tilted mesas on the other side of the La Leona ridge. These mesas contain the distal facies of the Calderones, near Las Torres mine.
Guided by the mine geologist, we will go from here to the San Lorenzo de Villalpando shaft.
Geology and tectonics of the southeastern portion of the Sierra de Guanajuato 161
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Stop 3.4. Tiro de San Lorenzo de Villalpando: Collapse Megabreccia(UTM 14Q273282, 2326099)
At this stop there are outcrops in a recent excavation made by the mining company that we interpret as the intracaldera collapse megabreccia related to the paroxysmal eruption of the Calderones sequence. The breccia includes fragments of various sizes, from at least 10 m down to a few centimeters. In the area some blocks make up entire outcrops. By far, the most common lithology at all fragment sizes is black phyllite of the Esperanza Formation. The matrix is diffi cult to see, because the deposit is deeply altered; the fi ne-grained fraction of the breccia has mostly been converted to a yellowish brown to reddish brown clay. Upon careful inspection, however, one can observe that this soil con-sists of materials of the same type as the larger blocks, pulverized to the size of sandy grit or fi ne gravel. The deposit as a whole is quite altered by the action of hydrothermal solutions that perme-ated it, producing abundant cross-cutting tiny veinlets of quartz and rendering it susceptible to intense weathering. All of the larger blocks are pervasively fractured. Blocks of the Esperanza Formation here have a somewhat better developed schistosity than that observed at Stop 3.3. Megablocks of fi nely fl ow-banded dacitic Peregrina dome rock are also found in the area.
From this stop we leave the area of the Peregrina Mine, go back through Guanajuato City and drive back to Querétaro along road Mex 110D.
ACKNOWLEDGMENTS
Financial support for our work was provided by Conacyt grants 47041 and 129550 to J. Aranda. We thank Javier Antonio Báez López and Angel Francisco Nieto Samaniego who kindly pro-vided unpublished ages of several volcanic units that crop out in the mining district. Likewise, Michelangelo Martini gave us a copy of his paper prior to formal publication. Juan Tomás Vazquez prepared a large number of thin sections for us. We also thank to Lyle Pritchard, Paul Arscott, and Antonio Raya from Compañía Minera El Cubo (Aurico Gold Corporation) for allow-ing us to visit the megabreccia exposures inside the mining unit.
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