Geochronology of Mexican mineral deposits. II: Veta Madre and Sierra epithermal vein systems, Guanajuato district

Geocronología de depósitos minerales mexicanos. II: los sistemas de vetas epitermales de Veta Madre y de la Sierra, distrito de Guanajuato

Juan José Martínez-Reyes^1,2, Antoni Camprubí^3,*, I. Tonguç Uysal⁴, Alexander Iriondo⁵, Eduardo González-Partida⁵

¹ Departamento de Ingeniería en Minas, Metalurgia y Geología, Universidad de Guanajuato. Ex-Hacienda de San Matías s/n, Fracc. San Javier, 36025 Guanajuato, Gto., Mexico.

³ Instituto de Geología, Universidad Nacional Autónoma de México. Ciudad Universitaria, 04510 México, D.F., Mexico. ^* camprubitaga@gmail.com

⁴ Geothermal Energy Centre of Excellence, University of Queensland. Brisbane St Lucia, QLD 4072, Australia.

⁵ Centro de Geociencias, Universidad Nacional Autónoma de México. Boulevard Juriquilla 3001, 76230 Querétaro, Qro., Mexico.

Manuscript received: February 16, 2015.
Corrected manuscript received: March, 23, 2015.
Manuscript accepted: March 25, 2015.

Abstract

Keywords: Guanajuato, Mexico, epithermal deposits, intermediate sulfidation, low sulfidation, Rb-Sr ages, illite, Ar/Ar ages, adularia.

Resumen

En este trabajo se presentan dos nuevas determinaciones geocronológicas de alta resolución para los depósitos epitermales del distrito minero de clase mundial de Guanajuato, en México central. Éstas son de una edad de isocrona Rb-Sr en illita de 28.47 ± 0.55 Ma de las vetas de baja sulfuración Villalpando y San Juan de Dios del sistema de la Sierra, y una edad de meseta ⁴⁰Ar/³⁹Ar en adularia ("valencianita") de 30.20 ± 0.17 Ma de la zona mineralizada de La Valenciana en la famosa Veta Madre, de sulfuración intermedia. Estas determinaciones tienen mayor exactitud, precisión y fidelidad que las determinaciones K-Ar preexistentes para muestras de adularia similares. La exactitud de las determinaciones en este trabajo apoya la idea de un emplazamiento diacrónico entre los depósitos de sulfuración intermedia y baja de este distrito, siendo los primeros de ellos más antiguos que los segundos, de forma similar a otros depósitos epitermales en México. Adicionalmente, el rango de ~2 m.a. entre la formación de la Veta Madre y el grupo de vetas epitermales de la Sierra es congruente con otros casos de estudio, independientemente del tamaño de los depósitos.

Palabras clave: Guanajuato, México, depósitos epitermales, sulfuración intermedia, sulfuración baja, edades Rb-Sr, ilita, edades Ar/Ar, adularia.

1. Introduction

The Guanajuato mining district (homonymous state, central Mexico) has been historically one of the largest silver producers in Mexico. Such endowment comes from polymetallic or Au-Ag epithermal deposits that have been extensively mined since the 16^th century. The mineral wealth from these deposits was estimated in 40 Mt at 850 g/t Ag and 4 g/t Au, plus significant concentrations in Pb and Zn (Albinson et al., 2001). These deposits are basically intermediate to low sulfidation epithermal veins and stockworks, in which intermediate sulfidation mineral assemblages, when present, occupy the deepest portions of mineralized structures (or "type B", according to Camprubí and Albinson, 2006, 2007). In fact, the different groups of mineralized structures cluster into dominantly intermediate or low sulfidation veins. Such clusters are named, west to east, as the La Luz system, the Veta Madre (which can be translated as "Mother Lode"), and the Sierra system (Figure 1), also known as El Cubo system. The large Veta Madre vein is dominantly an intermediate sulfidation vein (Camprubí and Albinson, 2006, 2007), whereas the La Luz and Sierra systems are, at their presently known exposures, low sulfidation sets of veins (Abeyta, 2003; Devlin and Hansen, 2009).

Previous studies in this area that are related at some extent with the formation of epithermal deposits deal with regional geological or structural aspects (Gross, 1975; Lapierre et al., 1992; Randall et al., 1994; Loriga, 1999; Aranda-Gómez et al., 2003) or with the mineralogy, the characteristics of mineralizing fluids and depositional environment of such deposits (Petruk and Owens, 1974; Gross, 1975; Buchanan, 1981; Mango et al., 1991, 2014; Abeyta, 2003; Orozco-Villaseñor, 2010; Moncada and Bodnar, 2012; Moncada et al., 2012). For a comprehensive succinct review of the geology of the Guanajuato district, see Moncada et al. (2012). The felsic to intermediate magmatism and structural features that occurred between 37 and 27 Ma are commonly invoked as the likeliest setting that allowed the epithermal deposits in Guanajuato to form (Godchaux et al., 2003). However, the window for the formation of such deposits appears to be narrower than the time span mentioned above in association with hypabissal magmatic activity of the Sierra Madre Occidental silicic large igneous province (SLIP) in this region as volcanism ceased. Thus, Taylor (1971, in Randall et al., 1994), Gross (1975), and Saldaña-Alba (1991) reported K-Ar ages in adularia for the epithermal mineralization in the Veta Madre and the Sierra groups of veins between 30.7 and 27.0 Ma (Oligocene). Such ages correspond to the seemingly most productive metallogenic period in Mexico, especially with regard to epithermal deposits (see figures 8 and 13 in Camprubí, 2013). Such period followed the climax of ignimbrite volcanism in the Sierra Madre Occidental silicic large igneous province (SLIP) before its magmatism waned and migrated southwards into the Trans-Mexican Volcanic Belt, and reshaping the regional distribution of epithermal deposits (Camprubí et al., 2003; Camprubí, 2013).

2. Material and Methods

2.1. Rb-Sr dating of illite from the Villalpando and San Juan de Dios veins (Sierra system)

Clay mineral analyses were conducted by XRD on clay separates (< 2 µm). The XRD analyses were carried out on a Bruker Advance MK III X-Ray diffractometer with Bragg-Brentano geometry and CuKα radiation, operated at 40 kV and 30 mA at a scanning rate of 1 °2θ/min and 0.05 °/step. Samples were prepared for clay-fraction separation by gently hand-crushing the rocks to sand size to avoid artificially reducing grain size of detrital/primary mineral components. Samples were then disaggregated in distilled water using an ultrasonic bath. Different clay size fractions (2 – 1, < 1, 2 – 0.5 and < 0.5 µm) were obtained by centrifugation, and the decanted suspensions were placed on a glass slide. To ensure no detrital contamination, samples were centrifugally separated and rigorously analyzed with XRD. Samples showing contamination with detrital K-feldspar were discarded from analysis. Following XRD analysis of air-dried samples, the oriented clay-aggregate mounts were placed in an ethylene–glycol atmosphere at 30 – 40 °C overnight prior to additional XRD analyses. However, the XRD 001 peak position of the illite does not change after ethylene glycol treatment, which is indicative of the absence of smectite clays.

For the Rb–Sr dating, illitic clay separates were leached for 15 minutes at room temperature in 1 N distilled HCl (Clauer et al., 1993). Leachate and residue were separated by centrifuging. The residue was rinsed repeatedly with milli-Q water, dried and reweighed. Clay separates were analyzed in two separate batches. Leachate, residue, and untreated samples were dissolved in a mixture of distilled HF and HNO₃ and measured by Thermo X-series 1 quadrupole ICP–MS with precision better than 0.5 % (1σ). The Sr-enriched fraction was separated using cation exchange resins. Sr isotopic ratios were measured on a VG Sector-54 thermal ionization mass spectrometer (TIMS) in the Radiogenic Isotope Laboratory at the University of Queensland (Australia). Sr was loaded in TaF5 and 0.1 N H₃PO₄ on a tantalum or tungsten single filament. Sr isotopic ratios were corrected for mass discrimination using ⁸⁶Sr/⁸⁸Sr = 0.1194. Long-term (6 years) reproducibility of statically measured NBS SRM 987 (2σ; n = 442) is 0.710249 ± 28. More recent dynamically measured SRM 987 had ⁸⁶Sr/⁸⁸Sr ratios of 0.710222 ± 20 (2σ; n = 140). Rb–Sr isochron ages were calculated using the ISOPLOT program (Ludwig, 2003).

For this study, we selected samples of pervasively altered rhyolitic rocks (Figure 2): two of them came from fresh outcrops adjacent to the Villalpando vein, and one sample came from underground exposures of the San Juan de Dios vein. In both cases, they were obtained from phyllic alteration assemblages. The samples of the Villalpando vein consist of illite and smectite (not mixed-layered illite-smectite), whereas the sample from the San Juan de Dios vein consists of pure illite. The samples thus used for Rb–Sr geochronology yielded an isochron age at 28.47 ± 0.55 Ma. These results are presented in Table 1 and Figure 3.

2.2. ⁴⁰Ar/³⁹Ar dating of adularia from the La Valenciana shoot (Veta Madre vein)

A pure mineral separate of adularia (Figure 2) from crustiform vein material (mostly quartz) from the La Valenciana ore shoot in the Veta Madre vein (central part of the Guanajuato district) was dated by ⁴⁰Ar/³⁹Ar geochronology (Figure 4 and Table 2). A large adularia crystal was torn to pieces that ranged in size from 250 to 180 µm and then were separated using heavy liquids and hand picking to a purity of > 99 %. The resulting 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 adularia sample (~20 mg) were packaged in copper capsules and sealed under vacuum in quartz tubes. The sample aliquots were then irradiated in package number KD52 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 adularia sample was analyzed at the U.S. Geological Survey Thermochronology lab in Denver (Colorado, USA), using the ⁴⁰Ar/³⁹Ar step-heating method (from 600° to 1500 °C; Table 2) 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 corresponds to a "classical" collector's specimen of large and saddle-shaped drusy adularia crystals –or "valencianite"– that are characteristic of this specific location. This sample yielded a ⁴⁰Ar/³⁹Ar plateau age at 30.20 ± 0.17 Ma.

3. Discussion and conclusions

Two ages were obtained in this study for epithermal deposits at the Guanajuato district: (1) Rb-Sr isochron age (Figure 3) at 28.47 ± 0.55 Ma for the Villalpando and San Juan de Dios low sulfidation veins of the Sierra system (eastern part of the district), and (2) an ⁴⁰Ar/³⁹Ar plateau age (Figure 4) at 30.20 ± 0.17 Ma for the La Valenciana ore shoot of the Veta Madre intermediate sulfidation vein (central part of the district). Such ages fall within the range of K-Ar ages reported by Gross (1975), Saldaña-Alba (1991) and Randall et al. (1994) for the Veta Madre vein (between 30.7 and 27.0 Ma; Table 3). These ages were obtained in the same type of adularia samples as the one used in this study, from the same location ("valencianite" from the La Valenciana ore shoot). Therefore, the ⁴⁰Ar/³⁹Ar plateau age in this study has higher accuracy, precision and trueness than preexisting K-Ar determinations. Similar characteristics can be assumed for the Rb-Sr isochron age in illite for the veins at the Sierra system, as the reliability of this method has been consistently validated (e.g., Middleton et al., 2015). Differences in age of ~2 m.yr. from various hydrothermal mineral assemblages of other Mexican epithermal deposits have also been found through high-resolution dating techniques. Such feature occurs regardless of the size of the deposits, whether they are relatively small (Temascaltepec, State of México; Camprubí et al., 2003) or giant deposits (Fresnillo, Zacatecas; Velador et al., 2010). As it is the case of the Guanajuato district, the Fresnillo and Temascaltepec deposits contain both intermediate and low sulfidation mineralization, although the former is dominantly an intermediate sulfidation deposit and the latter is dominantly a low sulfidation deposit (see Figure 14 in Camprubí and Albinson, 2007). Also, the difference in age between epithermal deposits and the Chichíndaro rhyolitic dome (dated at 32.0 ± 1.0 Ma; Gross, 1975) –which is the youngest volcanic rock prior to the emplacement of epithermal deposits– spans 1 to 3 m.yr. The latter is comparable to similar age gaps in other intermediate to low sulfidation epithermal deposits in Mexico (namely, Fresnillo, Pachuca-Real del Monte, Tayoltita and Temascaltepec; Lang et al., 1988; McKee et al., 1992; Enríquez and Rivera, 2001; Camprubí et al., 2003; Camprubí and Albinson, 2007; Velador et al., 2010), whereas such gaps are significantly shorter for high sulfidation deposits (La Caridad Antigua; Valencia et al., 2005, 2008).

The precision of both sets of ages in this study suggests that the sets of veins whence these were obtained are actually diachronic. This could imply that intermediate sulfidation veins of the Veta Madre system are slightly, albeit significantly, older than low sulfidation veins of the Sierra system. Such feature is in accordance with the common observation in Mexican epithermal deposits in which, when occurring in the same deposit, intermediate sulfidation mineralization normally predates low sulfidation mineralization. Such systematic behavior can be observed both at the scale of a single mineralized structure and at a district scale, as it is also the case of the Zacatecas district (see Camprubí and Albinson, 2007). Further validation for a systematic chronology of low and intermediate sulfidation ores may have relevant consequences in the exploration for epithermal deposits in which the presence of both low and intermediate sulfidation mineralization is plausible. For instance, although it will remain as a matter for speculation, it might be possible that the La Luz or Sierra systems contain significant intermediate sulfidation ores below the known extent of their low sulfidation ores, whether these occur with some spatial continuity or not (that is, "stacked" in the sense employed by Camprubí and Albinson, 2007).

Acknowledgments

This study was financed by means of the CONACYT grant number 155662. We thank Yeu-xing Feng and Ai Duc Nguyen for their help with analytical work and technical assistance to perform Rb-Sr and trace element analyses. Special thanks go to Turgay Demir for his assistance during sample preparation. The authors also 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. The age determinations in this study were first mentioned by Camprubí (2013). Formal reviews were conducted by José María González-Jiménez and Enrique Merino Martínez, whose comments helped to improve this paper.

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