1. Introduction

The Zn-Pb-Ag(-W) skarn deposits at Zacatepec, also named Mɨɨygɨxy (in Mixe, or Ayüük, language), are located between the Zacatepec and Jayacaxtepec villages in the Sierra de Juárez, about 80 km east-northeast of Oaxaca city in southern Mexico. Part of the mineralization is known as the Niño Perdido prospect, and no mining works exist other than small prospecting galleries in these deposits. Geological studies in this region are scarce due to accessibility problems (both natural and social; see ^{Canet et al., 2011}), therefore the lithological units are poorly constrained in age. The studied deposits are located in the highlands of the traditional territory of the Mixe or Ayüükj’ä’äy indigenous group, the so-called Mixe District. This locality is found within the easternmost regions of the Sierra Madre del Sur, in central-northeastern Oaxaca, which is a region that is dominantly occupied by middle Miocene volcanic rocks (Figure 1; see also Figure 3 in ^{Martínez-Serrano et al., 2008}). The middle Miocene volcanic event represents the last major volcanic activity in the Sierra Madre del Sur (^{Morán-Zenteno et al., 2005}, ²⁰⁰⁷; ^{Nieto-Samaniegoet al., 2006}) although there is synchronicity at some extent between this volcanic event and the earliest stages of the Trans-Mexican Volcanic Belt (^{Martínez-Serrano et al., 2008}).

The polymetallic skarns in this area formed between Lower Cretaceous carbonate rocks (probably of the Sierra Madre Formation) and Cenozoic monzonitic to granitic rocks (^{Canet et al., 2011}). The latter are plutonic or hypabyssal intrusive rocks, but undertaking any detailed mapping has not been possible; therefore, the exact dimensions and shape of such intrusive rocks remain unknown. Ore-bearing associations due to skarns in the Zacatepec area occur as irregular masses along the contact between the Cretaceous limestones and Cenozoic porphyritic dacites or in their vicinities (see Figure 1 in ^{Canet et al., 2011}).

Figure 1 Synthetic geological map of the Sierra Madre del Sur with the distribution of Cenozoic magmatic rocks (simplified from ^{Morán-Zenteno et al., 2005}, ²⁰⁰⁷). Key: TMVB = Trans-Mexican Volcanic Belt. 

The data below summarize the metallogenic characteristics of these deposits, as indicated by ^{Canet et al. (2011)}. For the most part, the prograde mineralization consists in skarns of calcic garnet with disseminated hematite, whereas skarns rich in calcic clinopyroxenes locally occur, farther from the contact between the porphyry and the limestones. As a result of a retrograde event, ore mineral assemblages containing sphalerite, galena, chalcopyrite, besides minor scheelite, vikingite and an unknown Ag₃BiS₃phase (probably a mineral of the aikinite group) were formed. Locally, magnesian skarns yield the highest metal grades. Microthermometric studies of fluid inclusions allowed to determine temperatures of homogenization between 160° and 470 °C, and salinities between 10.5 and 19.8 wt.% NaCl equiv. The temperature of formation of the prograde skarn, calculated from microthermometric data with an assumed pressure of 500 bar, ranges between 470° and 510 °C. The formation of the retrograde assemblage implies a decrease in temperature and, initially, in salinity, coupled with a great increase in ƒO₂. These changes can be explained by mixing with cooler, oxidizing and dilute meteoric water. Subsequently, a progressive increase in fluid salinity as temperatures of homogenization dropped from 300°C suggests that boiling took place triggering sulfide precipitation.

The formation of polymetallic skarn deposits in the Zacatepec area was attributed by ^{Canet et al. (2011)} to be associated with the magmatism of the Sierra Madre del Sur. The present paper aims to better constrain the age of these deposits and the magmatic rocks with which they are genetically associated, as both Paleogene (Eocene-Oligocene) and Miocene magmatic rocks are present in the study region (see Figure 3 in^{Martínez-Serranoet al., 2008}). Subsequently, this paper also aims to raise awareness on the latest metallogenic stages of the Sierra Madre del Sur, an issue that is formally addressed for the first time.

This paper also contributes to a dating program for Mexican mineral deposits that includes several types (^{Camprubí et al., 2015}, ^2016a, ^2016b, ²⁰¹⁸; ^{Farfán-Panamá et al., 2015}; ^{Martínez-Reyes et al., 2015}; ^{Enríquez et al., 2018}).

2. Methods and results

The⁴⁰Ar/³⁹Ar analyses were performed at the Geochronology Laboratory of the Departmento de Geología, Centro de Investigación Científica y Educación Superior de Ensenada (CICESE, Baja California, Mexico). The argon isotope experiments were conducted on hornblende and biotite fragments separated from sample SP-120 (granitoid). The mineral grains were heated with a Coherent Ar-ion Innova 370 laser.

The extraction system is online with a VG5400 mass spectrometer. The sample and irradiation monitors, were irradiated in the U-enriched research reactor of University of McMaster in Hamilton, Canada, at position 5C. To block thermal neutrons, the capsule was covered with a cadmium liner during irradiation. To determine the neutron flux variations, aliquots of the irradiation monitor FCT sanidine (28.201 ± 0.046 Ma; ^{Kuiperet al., 2008}) were irradiated alongside sample SP-120. Upon irradiation the monitors were fused in one step while the fuchsite sample SP-120 was step-heated. The argon isotopes were corrected for blank, mass discrimination, radioactive decay of³⁷Ar and³⁹Ar and atmospheric contamination. For the Ca neutron interference reactions, the factors given by ^{Masliwec (1984)} were used. The decay constants recommended by ^{Steiger and Jäger (1977)} were applied in the data processing. The equations reported by ^{York et al. (2004)} were used in all the straight line fitting routines of the argon data reduction. The relevant⁴⁰Ar/³⁹Ar data are presented in Table 1, which includes the results of the individual steps, and the integrated, plateau and isochron ages. The analytical precision is reported as one standard deviation (1 σ). The error in the integrated, plateau and isochron ages includes the scatter in the irradiation monitors.

The dated sample (SP-120) comes from a granitoid body that is associated with prograde skarn mineralization in the area of Zacatepec, northeastern Oaxaca. The⁴⁰Ar/³⁹Ar results are listed in Table 1 and presented in Figure 2. The analyzed sample yielded an isochron age at 17.52 ± 0.14 Ma for biotite and an isochron age at 17.33 ± 0.40 Ma for hornblende (early Miocene).

Figure 2 ⁴⁰Ar/³⁹Ar age spectra and isochrons for the SP-120 sample from a granitic rock in association with the prograde associations in the polymetallic skarns in the Zacatepec area, Oaxaca. 

3. Discussion

The⁴⁰Ar/³⁹Ar ages obtained in this study for a prograde skarn-related granitoid in the Zacatepec-Jayacaxtepec area (17.52 ± 0.14 and 17.33 ± 0.40 Ma), correspond to the early Miocene. Rhyolitic tuffs in the area yielded⁴⁰Ar/³⁹Ar ages at 15.48 ± 0.02 Ma (^{Iriondoet al., 2004}) that may mark the minimum age for such skarn-type mineralization. Ore deposits of this age have been traditionally bypassed for the easternmost regions of the Sierra Madre del Sur (notice their scarcity in the review by ^{Camprubí, 2013}). This emphasizes the need for further metallogenetic studies in Oaxaca. Such ages are in accordance with the last volcanic event of the Sierra Madre del Sur before its activity waned (^{Morán-Zenteno et al., 2005}, ²⁰⁰⁷; ^{Nieto-Samaniegoet al., 2006}; ^{Martínez-Serrano et al., 2008}). Intrusive rocks in the study area, within the Sierra de Juárez region, have been generally attributed Paleogene ages (see Figure 3 in ^{Martínez-Serrano et al., 2008}). Miocene intrusive rocks in the Sierra Madre del Sur are generally to be found in the coastal batholiths of southeastern Oaxaca, and Miocene volcanic rocks are widespread inland in central Oaxaca (Figure 1). However, other mineralized areas in central-eastern Oaxaca may have similar ages to those in the Zacatepec-Jayacaxtepec region. Such areas are Natividad in Ixtlán de Juárez, Cobre Grande in San Baltazar Guelavila, Altagracia-Águila-Arista near San José de Gracia, Taviche and Lachigalla near Ejutla de Crespo, Santa Margarita-Azucena near Nejapa, Aurena near Santa Cruz Bamba, and Guielavazar in Santa María Guienagati (Table 2 and Figure 3).

Figure 3 Geological map that shows the distribution of Miocene rocks of the Sierra Madre del Sur igneous province in central Oaxaca state, with the distribution of known ore deposits in the region, and known ages of hypabyssal or volcanic rocks within it. The geology of the region and the distribution of ore deposits has been adapted from ^{Aguilera-Martínez et al. (2000)}, ^{González-Ramos et al. (2000)}, ^{Martínez-Amador et al. (2000)}, ^{Sánchez-Rojas et al. (2000)}, and ^{Motolinía-García et al. (2002)}. The sources for ages are ^{Ferrusquía-Villafranca et al. (1974)}, ^{Murillo-Muñetón and Torres-Vargas (1987)}, ^{Ferrusquía-Villafranca and McDowell (1991)}, ^{Ferrusquía-Villafranca (1999)}, ^{Iriondo et al. (2004)}, ^{Solé et al. (2007)}, ^{Martínez-Serrano et al. (2008)}, ^{Poliquin (2009)}, ^{Pérez-Gutiérrez (2010)}, ^{Keppie et al. (2012)}, and Solís-Pichardo (written communication). 

All ages for the deposits above are estimated in relation to the ages of their host rocks, as follows below. These are directly equivalent to ages of part of the ore deposits only when the dated hypabyssal rocks are responsible for prograde mineralization in skarns-the remaining ages can only be considered as maximum ages for the emplacement of the ore deposits hosted by them:

In summary, these magmatic-hydrothermal ore deposits-along with those in the study area-may collectively bracket a highly productive early to middle Miocene metallogenic period (between <23.8 and <13.01 Ma) for the easternmost regions of the Sierra Madre del Sur.

Earlier metallogenic activity in the Sierra Madre del Sur (especially during the Eocene and Oligocene) is well documented, particularly due to the occurrence of large ore deposits of historical economic importance such as those in the Taxco or Mezcala districts. However, the potential of Miocene metallogeny in this igneous province has received very little attention. Miocene metallogenic provinces in Mexico have been identified to be associated with (1) the terminal volcanism of the Sierra Madre Occidental during the early Miocene, (2) the inception of volcanism of the Trans-Mexican Volcanic Belt during the middle Miocene, (3) the opening of the Gulf of California and the rifting-off of the Baja California peninsula during the middle Miocene, (4) the installation of alkaline magmatism in Chiapas between the middle Miocene and the Pliocene and, only timidly formulated, and (5) the terminal volcanism of the Sierra Madre del Sur (^{Camprubí, 2013}). Therefore, this paper provides sound evidence (and makes a case) for the metallogenic relevance of the early to middle Miocene metallogenic province of the Sierra Madre del Sur in the central part of the Oaxaca state. The central region of Oaxaca contains many hydrothermal ore deposits (Figure 3) whose origin, however uncharacterized, is likely to have a similar “blood relationship” with the Sierra Madre del Sur volcanism as the deposits listed above.

4. Conclusions