1. Introduction
The Peñón Blanco district in eastern-central Durango state (north-central Mexico) consists of several mineralized areas that contain epithermal deposits, named Yerbanís, Cerro Blanco, and San Juan de Mogotes (also known as La Esperanza) (Servicio Geológico Mexicano, 2003, 2004, 2005). The San Juan de Mogotes mineralized area contains silica sinter deposits that are associated with underlying kaolinized and alunitized rocks (Servicio Geológico Mexicano, 2003) that constitute an advanced argillic alteration assemblage, possibly as a result of steam-heated grounds. Such evidence was interpreted to be associated with low sulfidation epithermal deposits (Servicio Geológico Mexicano, 2003), which include the Lorena, Ángeles, Providencia and Guadalupe veins. Also, the San Juan de Mogotes area includes Sn and Hg showings (Servicio Geológico Mexicano, 2003), and exploration surveys conducted by Exploraciones Mineras Parreúa S.A. de C.V. revealed significant Au and Ag concentrations in association with the aforementioned veins (Santiago Olavide, written communication, 2007). This area lies in the vicinity of the Avino-Zaragoza district, which contains tin veins associated with fluorine-rich rhyolites dated at 29.6 ± 0.1 Ma (Rb-Sr; Huspeni et al., 1984), as well as Au-Ag, polymetallic (Ag-Au-Pb-Zn-Cu) and fluorite epithermal veins (Servicio Geológico Mexicano, 2003, 2013), the dominant type of deposits in this district. Au-Ag and polymetallic+fluorite epithermal deposits in Mexico tend to belong to the low and intermediate sulfidation subtypes, respectively (Camprubí and Albinson, 2006, 2007).
The study region, near the central part of the western half of the Mesa Central, as part of the most productive metallogenic epoch (Albinson et al., 2001; Camprubí et al., 2003; Camprubí, 2013), is particularly rich in Oligocene mineral deposits (Figure 1). Metallogenesis during the Oligocene was associated with the most prominent ignimbrite flare-up in the Sierra Madre Occidental silicic large igneous province (Camprubí et al., 2003; Ferrari et al., 2005, 2007). This region is found around the confluence between the westernmost Parras transversal zone and the northwestern ending of the large San Luis-;Tepehuanes fault zone (Nieto-Samaniego et al., 2005, 2007; Camprubí, 2013). Since the Late Cretaceous, many mineral deposits formed around this confluence, including an uncanny variety and quantity during a short period in the Oligocene (see Figures 8 and 13 in Camprubí, 2013).
In this paper, we present the first 40Ar/39Ar ages for low sulfidation epithermal deposits at the Peñón Blanco district, with the aim to better understand the metallogenic evolution of the heavily mineralized region of the northwestern part of the San Luis-;Tepehuanes fault zone, in eastern Durango.
2. Methods and results
A pure mineral separate of adularia from epithermal vein material from the San Juan de Mogotes mineralized area in the Peñón Blanco district was dated by 40Ar/39Ar geochronology (Figure 2 and Table 1). The analyzed sample (SJ-4) corresponds to adularia crystals within crustiform quartz bands in low sulfidation mineralization from the Lorena vein. The vein material was ground down to particles that ranged in size from 250 to 180 μm and 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.
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.
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 KD53 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, 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 a plateau age at 31.29 ± 0.08 Ma and it is supported, within analytical error, by the less precise isochron age at 31.30 ± 0.20 Ma.
3. Discussion and conclusions
The age for adularia sample SJ-4 from vein material of the epithermal deposits at the San Juan de Mogotes mineralized area in the Peñón Blanco district is 31.29 ± 0.08 Ma (early Oligocene). This is the first age determination on mineralization material from this region and time span (Table 2), and corresponds to the range of ages that is most characteristic for Cenozoic ore deposits in Mexico. Not only does this deposit belong to the most metallogenetically productive epoch, but also to the region that contains the highest concentration in magmatic-hydrothermal ore deposits in Mexico; that is, the western portion of the Mesa Central, particularly in the vicinities of the San Luis-;Tepehuanes fault zone, during the Oligocene (Camprubí, 2013). The metallogenic importance of this time and space frame is illustrated by the occurrence of what is currently the largest silver deposit in the world (Fresnillo, in Zacatecas).
Sample | Mineral District | Coordinates | Type of deposit | Additional information | Age (Ma) | Method | Mineral | Source |
---|---|---|---|---|---|---|---|---|
Indé | Epithermal fluorite and polymetallic deposits (IS?) | 38.39 ± 1.84 | Ar/Ar* (plateau) | Biotite | Tuta et al. (1988) | |||
La Ciénega | Epithermal polymetallic deposits (IS) | <30.0? | K-Ar* | w.r. | Labarthe (1996), in de la Garza et al. (2001) | |||
Río Verde | Sn veins associated with F-rich rhyolites | 32.3 ± 0.3 | Rb-Sr* | w.r. | Huspeni et al. (1984) | |||
SJ-4 | Peñón Blanco | 24º40'06.93" N, 104º06'21.44" W | LS Au-Ag epithermal veins | Lorena vein, San Juan de Mogotes area. The district contains scant Sn veins | 31.29 ± 0.08 | Ar/Ar** (plateau) | Adularia | This study |
El Tovar | Epithermal polymetallic deposits (IS?) | 31.3 | K-Ar* | w.r. | Clark et al. (1979) | |||
La Ochoa | Sn veins associated with F-rich rhyolites | 31.04 ± 0.36, 31.08 ± 0.35, 31.0 ± 0.4, 31.1 ± 0.3, 29.6 ± 0.4 | K-Ar* | w.r. | Huspeni et al. (1984), Tuta et al. (1988) | |||
Cerro de los Remedios | Sn veins associated with F-rich rhyolites | 31.0 ± 0.4, 28.3 ± 0.3 | K-Ar* | w.r. | Tuta et al. (1988) | |||
América-Sapiorís | Sn veins associated with F-rich rhyolites | 30.3 ± 0.7 | Rb-Sr* | w.r. | Huspeni et al. (1984) | |||
Avino-Zaragoza | Sn veins associated with F-rich rhyolites | The district consists dominantly of polymetallic IS epithermal deposits | 29.6 ± 0.1 | Rb-Sr* | w.r. | Huspeni et al. (1984) | ||
Bacís | LS-IS polymetallic epithermal deposit | 27 | K-Ar* | ? | Smith (1995) | |||
Coneto de Comonfort | Sn veins associated with F-rich rhyolites | The district consists dominantly of Ag-Au LS epithermal deposits | Oligocene to MiocenePonce | Ponce-Sibaja & Gutiérrez-Tapia (1978) |
Key: IS = Intermediate sulfidation; LS = low sulfidation; w.r. = whole rock. Asterisks (*) denote that the analyzed samples correspond to host rocks, whereas double asterisks (**) denote that the analyzed samples correspond to hydrothermal minerals.
The dominant types of deposits in the western half of the Mesa Central during the Oligocene are epithermal deposits and tin veins in association with fluorine-rich rhyolites (Figure 1). The possible genetic link between Au-Ag or polymetallic epithermal deposits with tin veins associated with fluorine-rich rhyolites (including Sn-Hg-F-Sb deposits), and the associated fluorite hydrothermal veins, remains to be characterized. Such is also the case of the southeastern ending of the Mesa Central in San Luis Potosí (Camprubí, 2013), in which topaz rhyolites are conspicuous (Leroy et al., 2002; Rodríguez-Ríos et al., 2007, 2013). Fluorite is a common mineral within or in association with epithermal deposits in the study region, either forming individual veins (e.g., Clark et al., 1977) or as an accessory mineral (Servicio Geológico Mexicano, 2013). No causality between the occurrence of highly differentiated fluorine-rich rhyolites (Leroy et al., 2002; Rodríguez-Ríos et al., 2007, 2013) and fluorite-rich epithermal deposits has been soundly interpreted to date, but it is a likely hypothesis for future studies. Central and eastern Durango poses as an ideal region for research about possible genetic links between epithermal deposits and tin veins associated with fluorine-rich rhyolites due to their close time and space distribution (Clark et al., 1977; Huspeni et al., 1984; Tuta et al., 1988; Camprubí, 2013). This is also the case for the Sombrerete and Sierra de Chapultepec deposits in Zacatecas (Albinson, 1988), which are epithermal and tin vein deposits, respectively, also associated with the San Luis-;Tepehuanes fault zone (Figure 1; Camprubí, 2013). Such correspondences occur even at the district scale, which is the case of the grouping of deposits at Avino and Coneto de Comonfort, both dominantly epithermal deposits (Ponce-Sibaja and Gutiérrez-Tapia, 1978; Servicio Geológico Mexicano, 2003, 2013), and scant tin veins are also found in the Peñón Blanco district itself (Servicio Geológico Mexicano, 2003). The formation of tin veins associated with fluorine-rich rhyolites continued briefly into the Miocene as the volcanic activity of the Sierra Madre Occidental silicic large igneous province shrunk dramatically southwards prior to the opening of the Gulf of California and the rearrangement of volcanism into the Trans-Mexican Volcanic Belt (Camprubí, 2013). Tin vein deposits were abundantly accompanied by epithermal deposits in that case as well (see Figures 9 and 10 in Camprubí, 2013).