Boletín de la Sociedad Geológica Mexicana Volumen 67, núm. 2, 2015, p. 357-366 http://dx.doi.org/10.18268/BSGM2015v67n2a16

Short Note

Geochronology of Mexican mineral deposits. III: the Taxco epithermal deposits, Guerrero

José L. Farfán-Panamá^1,2, Antoni Camprubí^3,*, Eduardo González-Partida⁴, Alexander Iriondo⁴, Enrique Gonzalez-Torres^3,5

¹ Unidad Académica de Ciencias de la Tierra, Universidad Autónoma de Guerrero, Ex-Hacienda de San Juan Bautista, 40323 Taxco el Viejo, Gro., México.
²Programa de Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, 76230 Querétaro, Qro., México.
³ Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, D.F., Mexico.
⁴Centro de Geociencias, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, 76230 Querétaro, Qro., Mexico.
⁵Facultad de Ingeniería, Universidad Nacional Autónoma de México. Ciudad Universitaria, 04510, D.F., Mexico.

^* This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

New ⁴⁰Ar/³⁹Ar (34.96 ± 0.19 Ma) and U-Pb ages (35.44 ± 0.24 and 34.95 ± 0.37 Ma) obtained in this study for the Calavera group of dikes, which predate intermediate sulfidation epithermal mineralization in the Taxco mining district, constrain the formation of such deposits to less than 34.96 Ma (latest Eocene). These deposits might arguably have formed at ~ 33 Ma, thus coinciding in age with the La Azul fluorite deposits, within the same district. Although this age is significantly younger than previously existing estimations, the deposits at Taxco consistently cluster into a Late Eocene to Oligocene metallogenic event. Such event was closely associated with the volcanism in the northern part of the Sierra Madre del Sur, specifically to the most prominent flare-up of subduction-derived volcanism before it ceased in the region and refashioned into the Trans-Mexican Volcanic Belt.

Keywords: Taxco, Mexico, epithermal deposits, intermediate sulfidation, ⁴⁰Ar/³⁹Ar ages, U-Pb ages, zircon.

Resumen

Las nuevas edades ⁴⁰Ar/³⁹Ar (34.96 ± 0.19 Ma) y U-Pb (35.44 ± 0.24 y 34.95 ± 0.37 Ma) obtenidas en este estudio para el conjunto de diques Calavera, cuyo emplazamiento precedió al de las mineralizaciones epitermales de sulfuración intermedia del distrito minero de Taxco, constriñen la formación de dichos depósitos a menos de 34.96 Ma (Eoceno tardío). Estos depósitos pudieran haberse formado razonablemente a ~ 33 Ma, coincidiendo en edad con los depósitos de fluorita de La Azul, en el mismo distrito. Aunque esta edad es significativamente menor que las estimaciones preexistentes, los depósitos de Taxco pertenecen de forma consistente al episodio metalogenético del Eoceno tardío al Oligoceno. Dicho episodio estuvo ligado cercanamente al emplazamiento del volcanismo de la porción norte de la Sierra Madre del Sur y, específicamente, al evento de mayor envergadura del volcanismo de subducción previo al cese del volcanismo en esta región y a su reconfiguración en la Faja Volcánica Mexicana.

Palabras clave: Taxco, México, depósitos epitermales, sulfuración intermedia, edades ⁴⁰Ar/³⁹Ar, edades U-Pb, circón.

1. Introduction

The Taxco district (Figure 1) is located in the northern part of the state of Guerrero and consists dominantly of polymetallic intermediate sulfidation deposits (Camprubí and Albinson, 2006, 2007) as veins and stockworks, plus replacement mantos of possible skarn genetic affinity. Only a few Au-rich veins in this district can be ascribed to the low sulfidation subtype of epithermal deposits. This district is one of the ‘classical’ silver mining districts in Mexico that has been extensively mined since the 16^th century, although the Aztecs initiated mining in the region during the 15^th century. The present mineral reserves in the Taxco district exceed 7 Mt at 91 g/t Ag, 6.83 % Zn and 1.05 % Pb (Servicio Geológico Mexicano, 2004), although its estimated historical production exceeds 30 Mt (Albinson et al., 2001), and includes Ag-Zn-Pb producing mines (namely the San Antonio, Guerrero, Babilonia, Guadalupe, Golondrina, Pedregal and Hueyapa mines). The formation of these deposits is related to the hydrothermal activity associated with the magmatism of the Sierra Madre del Sur (Camprubí et al., 2006; Camprubí, 2013). For succinct descriptions of the local geology, see Alaniz-Álvarez et al. (2002), Servicio Geológico Mexicano (2004), and Camprubí et al. (2006).

In this region of the Sierra Madre del Sur, Alaniz-Álvarez et al. (2002) and Morán-Zenteno et al. (2004, 2005, 2007) described a NW-SE striking tectonomagmatic alignment of volcanic centers —parallel to the present-day Pacific margin— that stretches ~ 200 km between the Cerro Purungueo intrusive and the Huautla volcanic field. This arrangement is associated with regional sinistral strike-slip and transtensive fault systems, and was postulated as a major crustal-scale discontinuity (Alaniz-Álvarez et al., 2002; Morán-Zenteno et al., 2004). The volcanic centers that constitute this alignment are the Cerro Purungueo intrusive (Ferrari et al., 2004), the Nanchititla (Chávez-Álvarez et al., 2012), Sultepec–La Goleta (Díaz-Bravo and Morán-Zenteno, 2011) and Taxco volcanic centers (Alaniz-Álvarez et al., 2002), the Buenavista–Tilzapotla caldera (Morán-Zenteno et al., 2004), and the Huautla volcanic field (González-Torres et al., 2013). Most of these volcanic centers have associated epithermal (e.g., Sultepec, Taxco, Huitzuco and Huautla) or skarn deposits (Buenavista de Cuéllar; see Camprubí, 2013). Alaniz-Álvarez et al. (2002) and Morán-Zenteno et al. (2004) reported late Eocene ages for the sinistral strike-slip faulting in the Taxco and Buenavista–Tilzapotla volcanic centers. Camprubí et al. (2003) attributed an age of 38 to 36 Ma to the intermediate sulfidation epithermal deposits at Taxco by using data from Alaniz-Álvarez et al. (2002). Pi et al. (2005) dated the La Azul fluorite deposit near the Acamixtla village between 33.0 and 30.0 Ma ([U-Th]/He in fluorite) and advocated for an epithermal model for their formation. This deposit is the only one within the Taxco mining district for which radiometric ages are available. The plausibility of the La Azul fluorite deposit as part of the epithermal type, as opposed to a Mississippi Valley Type model, was further discussed by Pi et al. (2006) and Tritlla and Levresse (2006).

This paper presents the first ⁴⁰Ar/³⁹Ar and U-Pb age determinations for the Calavera group of dikes, which predated the intermediate sulfidation epithermal deposits at the Taxco district (Figure 2; also see Figure 2 in Camprubí et al., 2006), in order to better constrain their age.

Figure 1. Location and geological map of the Taxco district, Northern Guerrero state, Mexico, modified from De Cserna and Fries (1980) and Camprubí et al. (2006). Valanginian ages (137.1 ± .9 Ma) for the Taxco Viejo Schist were obtained by Campa-Uranga et al. (2012). See A-A’ cross section in Figure 2. Key: SMO = Sierra Madre Occidental, SMS = Sierra Madre del Sur, TMVB = Trans-Mexican Volcanic Belt.

Figure 2. Representative cross section for the spatial relationship between the Calavera dike swarm and the epithermal veins that postdate it. Same legend as in Figure 1.

2. Methods and results

2.1. ⁴⁰Ar/³⁹Ar analytical procedure

A pure mineral separate of potassium feldspar from a mafic dike of the Calavera group of dikes in the wallrock assemblage within the El Cobre–Babilonia vein tract (Mi Carmen ore shoot) of the Taxco district was dated by ⁴⁰Ar/³⁹Ar geochronology (Figure 3 and Table 1). Potassium feldspar crystals that ranged in size from 250 to 180 µm were separated using heavy liquids and hand picking to a purity of > 99 %. The sample was washed in acetone, alcohol, and deionized water in an ultrasonic cleaner to remove dust and then re-sieved by hand using a 180-µm sieve.

Aliquots of the potassium feldspar sample (~ 20 mg) were packaged in copper capsules and vacuum sealed into quartz tubes. The sample aliquots were then irradiated in package number KD29 for 20 hours in the central thimble facility at the TRIGA reactor (GSTR) at the U.S. Geological Survey in Denver, Colorado. The monitor mineral used in the package was Fish Canyon Tuff sanidine (FCT-3) with an age of 27.79 Ma (Kunk et al., 1985; Cebula et al., 1986) relative to MMhb-1 with an age of 519.4 ± 2.5 Ma (Alexander et al., 1978; Dalrymple et al., 1981). The type of container and the geometry of the sample and standards were similar to that described by Snee et al.(1988).

The potassium feldspar sample (GP-B-48) was analyzed at the U.S. Geological Survey Thermochronology lab in Denver, Colorado, using the ⁴⁰Ar/³⁹Ar step-heating method and a VG Isotopes 1200B mass spectrometer fitted with an electron multiplier. For additional information on the analytical procedure see Kunk et al.(2001). The analyzed sample yielded an isochron age at 34.90 ± 0.2 Ma and an average age at 34.96 ± 0.19 Ma that is hereby interpreted as the age of crystallization of the Calavera group of dikes. These analyses are displayed in Table 1 and Figure 3.

Figure 3. ⁴⁰Ar/³⁹Ar age spectrum and isochron for the GP-B-48 potassium feldspar sample from the Calavera group of dikes in the Mi Carmen ore shoot of the Taxco mining district.

Table 1. ⁴⁰Ar/³⁹Ar step-heating data for a potassium feldspar separate of the Calavera dikes from Taxco.

Ages calculated assuming an initial ⁴⁰Ar/³⁶Ar = 295.5 ± 0.
All precision estimates are at the one sigma level of precision.
Ages of individual steps do not include error in the irradiation parameter J.
No error is calculated for the total gas age.

2.2. U-Pb analytical procedure

Two samples were selected for U-Pb dating in zircon separates from intrusive bodies of the Calavera dike set in the southwestern part of the Taxco district; in both cases, the samples came from dikes that formed just before epithermal mineralization. The U-Pb zircon analyses were performed at the Isotopic Studies Laboratory (LEI) at the Centro de Geociencias of the Universidad Nacional Autónoma de México. An excimer (193 nm) laser ablation system by Resonetics was attached to a quadruple Thermo-X series ICP-MS spectrometer to carry out the analyses. The system has been described by Solari et al.(2010) and all data have been reduced by in-house software “UPb.age” (Solari and Tanner, 2011) and plotted with the computational software “Isoplot 3.0” (Ludwig, 2003).

The analyzed samples yielded ages at 35.44 ± 0.24 (sample C-3) and 34.95 ± 0.37 Ma (sample C-5). These analyses are displayed in Table 2 and Figure 4.

Figure 4. Tera-Wasserburg U-Pb concordia plots (a, b, d and e) and plots of weighted averages of individual ²⁰⁶Pb/²³⁸U ages (c and f) of analyzed zircons from two samples of the pre-epithermal mineralization Calavera group of dikes from the Taxco district. Solid-line ellipses, with black square centers, are data used for age calculations; gray-line ellipses are data excluded from age calculations due to different degrees of Pb-loss and/or zircon inheritance. All U-Pb data are plotted with 2-sigma errors and all calculated weighted mean ages are also listed at the 2-sigma level. Original U(Th)-Pb data can be found for inspection in Table 2.

Table 2. U-Pb determinations in zircon from the Calavera dikes from Taxco.

Table 2. (Continued) U-Pb determinations in zircon from the Calavera dikes from Taxco.

3. Discussion and conclusions

The ⁴⁰Ar/³⁹Ar (34.96 ± 0.19 Ma) and U-Pb ages (35.44 ± 0.24 and 34.95 ± 0.37 Ma) obtained in this study for the Calavera set of dikes that predate epithermal mineralization in the Taxco mining district, given the different closure temperatures of the dated minerals (potassium feldspar and zircon) with respect to each dating method (e.g., Figure 1 in Chiaradia et al., 2013), are congruent with the rapid cooling expected for a dike swarm. Also, these ages are similar to that obtained for the Acamixtla ignimbrite (35.77 ± 0.42 Ma), which belongs to the Taxco volcanic field (González-Torres et al., 2013), and are younger than those obtained by Alaniz-Álvarez et al. (2002) for similar rocks. Therefore, epithermal deposits must be younger than 34.90 Ma. Still, the ages in this study would cluster into a Late Eocene metallogenic event in the Sierra Madre del Sur, along with the Placeres del Oro, Pinzán Morado, Las Fraguas and Huautla epithermal deposits, and the Piedra Imán and Buenavista de Cuéllar IOCG ‘clan’ deposits (Table 3; see also Table 1 and Figure 7 in Camprubí, 2013), all of them located in the northern Guerrero state or its vicinities. Such ages also occur between two of the volcanic episodes in the Sierra Madre del Sur (between ~ 36.5 and ~ 34.5 Ma; González-Torres et al., 2013) that constitute the last relevant flare-up episode before the extinction of its subduction-derived volcanism and the rearrangement of such activity into the Trans-Mexican Volcanic Belt during the Miocene. In spite of being relatively restricted in space, especially when compared to the Oligocene flare-up of the Sierra Madre Occidental and the massive formation of associated ore deposits (see Camprubí, 2013, and references therein), this volcanic episode in the Sierra Madre del Sur makes of this region a highly prospective one for epithermal and skarn deposits (either sulfide or iron oxide skarns of the IOCG ‘clan’) during the Late Eocene.

The volcanic centers of the previously described magmatic lineament have been interpreted as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. According to this idea, widespread Eocene magmatism and injection of mantle-derived melts into the crust promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system; within this context, intermediate-siliceous compositions were produced by low-pressure fractional crystallization, crustal contamination, and anatexis (Mori et al., 2012; González-Torres, 2013).

We may also examine the plausibility of the two proposed genetic affinities for the La Azul fluorite deposits in the Taxco district, as Pi et al. (2005, 2006) advocate for an epithermal model, whereas Tritlla and Levresse (2006) favor a Mississippi Valley Type (MVT) model instead. Firstly, fluorite is a common mineral in intermediate sulfidation epithermal deposits (Lyons, 1988; Ponce and Clark, 1988; Albinson and Rubio, 2001; Albinson et al., 2001; Camprubí et al., 2001; Camprubí and Albinson, 2006, 2007), including those in the Taxco district (Camprubí et al., 2006). Such characteristic in deposits of different ages and localities implies that F- would have been a major ion in ore-forming solutions associated with intermediate sulfidation epithermal environments. Secondly, the ages in this paper for the Calavera group of dikes indicate that epithermal mineralization would be younger than ~ 34.96 Ma. The common knowledge indicates that the time span between the youngest volcanic or hypabyssal rocks that predate genetically linked epithermal mineralization —regardless of their state of sulfidation, or the size of the deposits— and epithermal mineralization itself is ~ 2 m.yr. in Mexican deposits (as determined in the Fresnillo, Guanajuato, Pachuca-Real del Monte, Tayoltita, and Temascaltepec districts; see Lang et al., 1988; McKee et al., 1992; Enríquez and Rivera, 2001; Camprubí et al., 2003; Camprubí and Albinson, 2007; Velador et al., 2010; Martínez-Reyes et al., 2015, and references therein), whereas such gaps are significantly shorter for high sulfidation deposits (La Caridad Antigua; Valencia et al., 2005, 2008). Assuming that this were the case, it would be reasonable to expect that the earliest epithermal deposits of Taxco formed at ~ 33 Ma, which coincides with the range of ages between 33.0 and 30.0 Ma determined by Pi et al. (2005) for the La Azul fluorite deposit. Notably, the Huautla Formation of the neighboring Huautla mining district, a heterogeneous volcanic succession that hosts hydrothermal alteration zones and epithermal veins, has a similar U-Pb age at 32.9 ± 0.6 Ma (González-Torres et al., 2013). That would then imply (1) that the ‘classical’ polymetallic intermediate sulfidation deposits at Taxco and the small fluorite deposits nearby formed at the same time, and (2) that their formation by means of very different fluids and mineralizing processes (as of magmatic-hydrothermal and epithermal model vs. basinal brines and MVT model; see Table 3) would have been highly implausible. Therefore, from this point of view, it is likely to ascribe the La Azul fluorite deposit to the epithermal type, as postulated by Pi et al.(2005, 2006).

Table 3. Age determinations for the Calavera group of dikes that predate the Taxco epithermal deposits, in comparison with ages of ore deposits in the northern part of the Sierra Madre del Sur, Mexico.

Acknowledgements

This study was financed by means of the CONACYT grants 58825Y and 155662, and the PAPIIT-UNAM grant number IN101510. The authors wish to thank Michael Kunk for providing access and guidance to perform the 40Ar/39Ar geochronology studies at the U.S. Geological Survey Thermochronology Lab in Denver, Colorado, and to Carlos Ortega for his help during U/Pb analysis at the Centro de Geociencias (UNAM). Formal reviews were conducted by José María González-Jiménez and an anonymous referee, whose comments helped to improve this paper.

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Manuscript received: March 17, 2015
Corrected manuscript received: June 30, 2015
Manuscript accepted: July 8, 2015