Geochronology of Mexican mineral deposits. I: the San Martín polymetallic skarn, Zacatecas

Geocronología de depósitos minerales mexicanos. I: el skarn polimetálico de San Martín, Zacatecas

Antoni Camprubí^1,*, Eduardo González-Partida², Victor A. Valencia³, Fernando Barra⁴

¹ Instituto de Geología, Universidad Nacional Autónoma de México. Ciudad Universitaria, 04510 México, D.F., Mexico.^* Corresponding author: camprubitaga@gmail.com

³ School of the Environment, Washington State University. Pullman, WA 99164-2812, U.S.A.

⁴ Departamento de Geología, Universidad de Chile. Plaza Ercilla #803, Casilla 13518 Correo 21, Santiago, Chile.

Manuscript received: Febraury 24, 2015
Corrected manuscript received: March 23, 2015
Manuscript accepted: March 25, 2015

Abstract

The Zn-Pb-Cu(-Ag±Au) San Martín deposit in northwestern Zacatecas is one of the most economically important and biggest skarns in Mexico. Mineral associations in this deposit belong to the sulfide skarn type (with rather "classical" prograde and retrograde zones) and contain peripheral subepithermal to epithermal veins. Re-Os ages were obtained for two molybdenite samples from deep Cu-Zn-Pb±Mo±Bi±Ag retrograde mineralization in these deposits, of 43.7 ± 0.3 and 44.0 ± 0.2 Ma, which correspond essentially to the same age. These are 1 to 3 Myr younger than preexisting K-Ar ages for the granitic stock that generated these mineral deposits. Prograde mineral associations are hereby interpreted to have been directly associated with this intrusion. Therefore, we may calculate a simple decrease rate in temperatures of hydrothermal fluids between ~100º and > 300 ºC per million years from prograde to retrograde skarn associations. Subepithermal and epithermal veins, however, remain undated.

Resumen

El depósito de Zn-Pb-Cu(-Ag±Au) de San Martín en el noroeste de Zacatecas es uno de los skarns de mayor importancia económica y tamaño en México. Las asociaciones minerales de este depósito pertenecen a la tipología de los skarns de sulfuros (con zonaciones prógradas y retrógradas "clásicas") y contiene en su periferia vetas subepitermales a epitermales. Se obtuvieron edades Re-Os en este depósito para dos muestras de molibdenita de mineralizaciones retrógradas de Cu-Zn-Pb±Mo±Bi±Ag, en 43.7 ± 0.3 y 44.0 ± 0.2 Ma, que corresponden esencialmente a la misma edad. Éstas son de 1 a 3 millones de años más recientes que las edades K-Ar preexistentes del stock granítico responsable de la formación de estos depósitos. Las asociaciones minerales prógradas se interpretan en este trabajo como directamente asociadas a dicho intrusivo. Por lo tanto, puede calcularse una simple tasa de descenso de temperaturas para los fluidos hidrotermales de entre ~100º y > 300 ºC por millón de años, desde las asociaciones prógradas, de skarn, a las retrógradas. Las vetas subepitermales y epitermales, sin embargo, todavía carecen de determinaciones geocronológicas.

Palabras clave: San Martín, Zacatecas, México, skarn de sulfuros, menas polimetálicas, Re-Os, molibdenita.

1. Introduction

The San Martín mining district (northwestern Zacatecas, central-northern Mexico) is composed of Zn-Pb-Cu(-Ag±Au) sulfide skarn deposits and Ag-rich epithermal to subepithermal deposits. It contains average mill-head grades of 450 ppm Ag and 0.5 ppm Au. At present, large mantos are mined, with average grades of 5% Zn, 1% Cu, 0.5% Pb, and 150 ppm Ag. Total ore reserves, including mined ores, are estimated at over 100 Mt, thus indicating that San Martín is a world-class mining district. This deposit used to be considered as the biggest of its kind in Mexico until the discovery of the giant Peñasquito deposit in the Mazapil district, in northeastern Zacatecas.

Previous work in the area comprised mineralogical and geochemical (Aranda-Gómez, 1978; Rubin and Kyle, 1988), structural (Starling et al., 1997), and fluid inclusion studies (González-Partida and Camprubí, 2006). The skarn deposits comprise four prograde zones (Aranda-Gómez, 1978) from the granitic stock outwards: (1) garnet – clinopyroxene zone ± hedenbergite zone, (2) tremolite – garnet – wollastonite zone, (3) tremolite – actinolite – marble zone, and (4) saccharoidal quartz – potassium feldspar and stockwork zones. The retrograde associations include (Aranda-Gómez, 1978): (a) propylitic alteration, with epidote, vesuvianite, chlorite, calcite and minor quartz, (b) silicification adjacent to epithermal veins, and (c) calcite veinlets and pervasive carbonatization, with minor chalcedony, that represents the latest hydrothermal event in the district. The deposition of ore minerals occurred from the beginning of retrogradation and continued with the formation of subepithermal to epithermal veins (e.g., the Noria de San Pantaleón vein).

The sole existing age determination related to the sulfide skarn mineralization is a K-Ar age for biotite from the Cerro de la Gloria granitic stock, at 46.2 ± 1 Ma (Damon et al., 1983). This stock developed endoskarn and exoskarn mineralization, with an external aureole of pervasive carbonatization that extends over 1 km away from the contact with the host Cretaceous carbonate rocks (Cuesta del Cura Formation; Figure 1), which includes peripheral intermediate sulfidation subepithermal to epithermal veins (Camprubí and Albinson, 2006, 2007; González-Partida and Camprubí, 2006). The San Martín sulfide skarn deposit was therefore formed during the metallogenic epoch that is most prospective for skarn deposits in the region, during the Eocene (epoch 4, Figure 13 in Camprubí, 2013). Such a metallogenic feature is due to the eastward progression of magmatism in the Sierra Madre Occidental silicic large igneous province (SLIP) into the carbonate basins in central Mexico and along the long-lived, NW-SE striking San Luis–Tepehuanes Fault Zone, which constitutes most of the southern border of the Mesa Central (or Mexican Altiplano; Nieto-Samaniego et al., 2005, 2007). The stock that generated the skarn deposits is controlled by two WNW-ESE striking sinistral transtensional fault zones in the pre-Laramide basement, located north and south of the intrusion that were reactivated during the Laramide orogeny (Starling et al., 1997). The San Luis-Tepehuanes Fault Zone determined a trend in the distribution of ore deposits that constitutes the strip of land in which these are the most abundant in Mexico, and these formed between the Late Cretaceous and the Oligocene (Camprubí, 2013).

2. Methods and results

Two molybdenite samples for Re-Os dating were obtained from different locations of deep retrograde mineral associations. Both samples were digested using the Carius tube method (Shirey and Walker, 1995), and were analyzed following the procedure described by Barra et al. (2003, 2005). About 50 to 70 mg of pure molybdenite was loaded in the Carius tube with Re and Os spikes, and then dissolved in inverse aqua regia by heating in an oven at 220 °C for ~12 h. After homogenization of the solution, Re and Os were separated using a distillation technique (Nägler and Frei, 1997), in which Os was collected into cold HBr. Later, the dried Os was purified using the microdistillation technique of Birck et al. (1997), while Re was purified using AG1-X8 anion exchange resin. Along with Ba salts to enhance ionization, Re and Os were loaded on Ni and Pt filaments, respectively. Measurements were carried out by negative thermal ion mass spectrometer (NTIMS) at the University of Arizona.

The molybdenite samples thus dated yielded Re-Os ages of 43.7 ± 0.3 and 44.0 ± 0.2 Ma and are presented in Table 1.

3. Discussion and conclusions

Two Re-Os ages were obtained in this study for molybdenite from deep retrograde metallic mineral associations of the San Martín sulfide skarn deposits in Zacatecas, of 43.7 ± 0.3 and 44.0 ± 0.2 Ma. Such ages are, statistically, the same. These, as expected, are younger than the available K-Ar age for the Cerro de la Gloria granitic stock (46.2 ± 1 Ma; Damon et al., 1983), which is associated with these skarn deposits. Then, the time span between the intrusion of the granitic stock —and the subsequent prograde mineralization— and the retrograde mineralization would vary from 1 to 3 Myr, which is reasonable for the thermal peak at similar systems (e.g., Anczkiewicz et al., 2014). This implies a decrease in temperature from 645 ºC, as the maximum temperature in the prograde skarn zone, down to ~300 ºC, as the minimum temperature in the retrograde skarn zone, and even down to ~100 ºC in subepithermal and epithermal veins (after the temperatures of homogenization in fluid inclusions obtained by González-Partida and Camprubí, 2006). Thus, the decrease rate in temperatures of hydrothermal fluids would have varied between ~100º and > 300 ºC per million years (Figure 2).

Acknowledgements

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