Fluid inclusion and stable isotope study of the Cobre–Babilonia polymetallic epithermal vein system, Taxco district, Guerrero, Mexico

doi:10.1016/j.gexplo.2005.11.011

Journal of Geochemical Exploration

Volume 89, Issues 1–3, April–June 2006, Pages 33-38

https://doi.org/10.1016/j.gexplo.2005.11.011 Get rights and content

Abstract

The Cobre–Babilonia vein system formed during a single major hydrothermal stage and is part of the Taxco district in Guerrero, southern Mexico. Homogenization and ice melting temperatures range from 160 to 290 °C and from − 11.6 to − 0.5 °C, respectively. We determined an approximate thermal gradient of 17 to 20 °C per 100 m using fluid inclusions. A thermal peak marked by the 290 °C isotherm is interpreted as a major feeder channel to the veins. The highest content of Zn + Pb in ore coincides with the 220 and 240 °C isotherms. Salinities of mineralizing fluids range from 0.8 to 15.6 wt.% NaCl equiv, and are distributed in two populations that can be related with barren or ore-bearing vein sections, with 0.8 to 6 wt.% NaCl equiv and 7 to 15.6 wt.% NaCl equiv, respectively. δ¹³C and δ¹⁸O water values from calcite from the Cobre–Babilonia vein system and the Esperanza Vieja and Guadalupe mantos range − 5.4‰ to − 10.4‰ and 9.9‰ to 13.4‰, respectively. δ³⁴S values range from 0‰ to 3.2‰ and − 0.7‰ to − 4.3‰ in sphalerite, − 4‰ to 0.9‰ in pyrite, and − 1.4‰ to − 5.5‰ in galena. Both fluid inclusion and stable isotope data are compatible with magmatic and meteoric sources for mineralizing fluids. Also, sulfur isotope compositions suggest both magmatic and sedimentary sources for sulfur.

Introduction

The Taxco district is found in the northern part of Guerrero state in southern Mexico (Fig. 1), close to the boundaries with Morelos and México states. The district lies in the southeastern part of a broad north–northwest trending belt of polymetallic epithermal deposits (Damon et al., 1981, Clark et al., 1982, Camprubí et al., 2003a), which includes the Guanajuato, Pachuca-Real del Monte, Zacatecas, Sombrerete and Fresnillo districts. In neighboring areas such as Temascaltepec, Sultepec, Amatepec and Miahuatlán-Ixtapan del Oro, other vein deposits are hosted in similar stratigraphic sections. Although Taxco is one of the oldest mining districts in the Americas, it still lacks detailed studies. This district includes Ag–Zn–Pb producing mines (namely the San Antonio, Guerrero, Babilonia, Guadalupe, Golondrina, Pedregal and Hueyapa mines) presently mined or explored by Industrial Minera México SA de CV (IMMSA). The first geologic descriptions of the district by Fowler et al. (1948) and Osborne (1956), and later work by Pérez-Segura (1973), Gómez-Fragoso (1973), and Leija-Vázquez (1973) partially describe some of the mineralized structures. In this paper we provide fluid inclusion and stable isotope data from the Cobre–Babilonia vein system and from the nearby Esperanza Vieja and Guadalupe mantos.

Section snippets

Regional geology

The epithermal deposits in Taxco are related to hydrothermal activity associated with the latest volcanic activity of the Sierra Madre del Sur (SMS). The epithermal veins have not been directly dated, but are attributed an age of 38–36 Ma (Alaniz-Álvarez et al., 2002, Camprubí et al., 2003a). Paleozoic Taxco Schists form the basement, and are unconformably overlain by the propilitized Late Triassic–Early Jurassic Taxco Viejo Greenstone, that consists mainly of propylitized andesitic flows.

Structure and stratigraphy of the veins

There are more than 50 individual veins in the Taxco district, most of them striking NW (30° to 50°), that are associated with large replacement mantos, especially south of the veins. The veins are hosted by all the rocks described above, although the Tertiary pyroclastic rocks show only limited development of the veins, and this is true for the Cobre–Babilonia vein system within a mechanically incompetent rock where veins split in a horse-tail structure. The mantos formed preferentially at the

Fluid inclusions

A fluid inclusion microthermometry study was performed on 130 samples that are distributed in a 3200-m long and 900-m deep section (Fig. 2). The analyzed minerals were sphalerite, quartz and calcite. We analyzed only inclusions hosted by minerals lacking evidence for recrystallization. Primary, secondary, and pseudosecondary inclusions were found, which are liquid-rich or vapor-rich with no daughter minerals. The fluid inclusions are mostly 5 to 15 μm in size, but range up to 50 μm, and are

Stable isotopes

δ¹⁸O and δ¹³C values in calcite were measured using the CO₂ obtained using the conventional method of McCrea (1950). The analyses were carried out with a Finnigan Delta Plus XL mass spectrometer at the Instituto de Geología of the UNAM. Oxygen and carbon isotopes are reported in using the delta permil notation with respect to the V-SMOW and V-PDB standards, respectively. Calcite samples show δ¹³C of − 10.4 for the Guadalupe vein, − 7.5 for the Esperanza Vieja manto, and between − 5.4‰ and − 9‰ for

Discussion and conclusions

The mineralogy of veins and mantos, the dominance of the Zn–Pb–Cu associations over Ag–Au, plus the characteristics of associated mineralizing fluids and stable isotopes, indicate that the Cobre–Babilonia vein system is an epithermal deposit that belongs to the intermediate sulfidation type, though mineralizing fluids are less saline in late mineral associations and thus suggest an evolution towards the low sulfidation field (see Sillitoe and Hedenquist, 2003, Camprubí and Albinson, in press).

Acknowledgements

This work was partially funded with the Conacyt research projects 46473 and 42642. Special thanks are devoted to IMMSA and the staff at Taxco for their assistance. The stable isotope analyses were carried out by Edith Cienfuegos at the Instituto de Geología of the UNAM and Clemente Recio at the Universidad de Salamanca. We also thank Caridad Hernández, Anastasio Lozano, and Francisco Otero for their assistance in isotopic analyses. We also gratefully acknowledge the critical reviews of G.

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Citation Excerpt :
All the above units are covered in discordance by the Red Beds of the Balsas Formation of early Paleogene age, and finally, by acidic volcanics of the Tilzapotla Formation of the Oligocene age (Fig. 1). Mineralization in Taxco occurs in hydrothermal veins, replacement mantos, and stockworks hosted in schist, limestone, sandstone and shale of the Taxco, Morelos, and Mexcala Formations, even though scarce narrow veins have been reported in the lowest levels of the Tilzapotla Formation (Camprubí et al., 2006; Torres-Tafoya and Gonzalez-Niño, 2009). Up to 50 important veins and mantos have been identified from 500 to 2000 m in length and from 1 to 3 m to 7–10 m in width.
This study was aimed to identify the main processes involved in the release of metals using sulfur isotopes and those acting on the fractionation of copper isotopes. We determined sulfur and copper isotopic ratios in rocks, tailings, precipitates and soils samples from the historical mining district of Taxco, Southern Mexico. Ratios of sulfur isotopes are reported in notation of δ per mil, and standardization was based on international standards OGS-1 and NBS123. Only copper ratio isotopes were studied in pyrite (Py), sphalerite (Sph) and galena. Results are reported as δ⁶⁵Cu = ((⁶⁵Cu/⁶³Cu_sample/⁶⁵Cu/⁶³Cu_{NIST976standar}) − 1) ∗ 1000. Error for all the analyzed samples was 0.01‰ and any mass bias was corrected trough bracketing method. The sulfur isotopic value of a sample from the Mexcala shale Formation was − 7.7‰. The δ³⁴S values in tailing ponds ranged from − 2.6 to 1.2‰, in precipitates from − 7.2 to − 3.2‰, and in soils from − 0.8 to 4.5‰. Isotopic δ³⁴S values in rocks evidenced a magmatic influence of sulfides present in the Mexcala Formation. Oxidation of sulfides contained in tailings and precipitates was observed related with their low δ³⁴S values close to zero, oxidation followed the order: sphalerite > pyrite > pyrite rich in copper > chalcopyrite > galena. In soils, oxidation is not the main process as supported by their positive δ³⁴S values and low sulfate content. Results of δ⁶⁵Cu values in rocks were − 0.09 to 0.54‰; in sulfides values were δ⁶⁵Cu = 0.18‰ (Py) and δ⁶⁵Cu = − 0.20‰ (Sph), concentration of copper in galena was not enough to get a reliable isotopic value. In tailings δ⁶⁵Cu ranged from − 1.19 to 0.91‰ and in precipitates from − 0.36 to 0.92‰; in soils values were between 0.26 and 1.72‰. Results in rocks indicate an igneous influence on copper provenance. The δ⁶⁵Cu values in Py and Sph were within the range of terrestrial primary sulfides (from − 1 to 1‰) confirming its igneous provenance. Processes related with fractionation of copper isotopes were oxidation of tailings and reprecipitation of ⁶⁵Cu in tailings with a high content of Fe oxyhydroxides. Enrichment in ⁶⁵Cu in precipitates and soils is favored by organic matter presence that retains ⁶³Cu. Erosion of rocks and tailings is the main source of copper being rocks the initial and tailings the end members. The isotopic studies on environmental research provide a useful tool on: 1) The behavior of copper and sulfur isotopes in environments where both, natural and anthropogenic activities, are present. 2) Understanding surficial processes of dispersion of metals and identifying their source in mining areas surrounded by villages and agricultural soils.
<sup>40</sup>Ar/<sup>39</sup>Ar geochronology and revised stratigraphy of the late Eocene Taxco volcanic field, southern Mexico
2017, Journal of South American Earth Sciences
Citation Excerpt :
Farfán-Panamá et al. (2013, 2015) obtained 40Ar/39Ar and U-Pb determinations for dikes that pre-date mineralized structures, which indicate that the Taxco epithermal Ag-Pb-Zn deposits are younger than 34.96 Ma (see further details in section 4.2.1). The formation of these epithermal deposits is considered to be closely associated with late Eocene-early Oligocene silicic magmatism in the north-central part of the SMS, not only in Taxco but also in other nearby regions (Camprubí et al., 2006; Camprubí, 2013; Farfán-Panamá et al., 2013, 2015). In other epithermal deposits in Mexico, a ∼2 Myr gap has been observed between the end of volcanic and hypabyssal activity that is genetically associated with epithermal mineralization and the mineralization event itself, and it has been suggested that this is also the case in the Taxco mining district (Camprubí et al., 2003; Farfán-Panamá et al., 2015).
The late Eocene Taxco volcanic succession constitutes an important record of explosive silicic volcanism in the north-central Sierra Madre del Sur of southern Mexico. Detailed stratigraphic studies with age dating of individual units in this area were lacking in spite of it being an important mining district. Re-examination of the stratigraphy together with geochronologic studies were used to define the eruptive style and constrain the age of the main episode of silicic activity. Major element data show a rhyolitic composition for all the units analyzed. The volcanic succession records silicic explosive eruptions during which ash-flows, surge and ash-fall deposits, lava flows and domes were emplaced. Earliest activity was contemporary with the end of sediment accumulation of the continental Balsas Group. The first main episode of volcanic activity produced massive, moderately welded, crystal-poor lithic ignimbrites and ash-fall deposits, which evolved into crystal-rich, densely welded ignimbrites with flattened pumice clasts (San Gregorio ignimbrite) thought to represent erupted mush related to caldera collapse during emptying of the magma chamber. A second episode of non-welded, vapor-phase, indurated ignimbrites and surge deposits (Sombrerito ignimbrite), which are rich in pumice but crystal-poor, suggest volatile-rich magmatism and replenishment of the magma chamber by evolved magmas. Rhyolite emplaced in volcanic units of both episodes point to volatile-poor magmas during which rhyolite flows and domes formed.
⁴⁰Ar/³⁹Ar dating of the units, from the crystal-rich San Gregorio ignimbrite near the base of the succession to rhyolites at the top, has constrained the age of the Taxco volcanic field to a relatively short period between ∼36 and 34.5 Ma. Most of the sanidine ages obtained are indistinguishable within error: the densely welded San Gregorio ignimbrite (34.75 ± 0.26, 35.22 ± 0.26 Ma) and the overlying Peral ignimbrite (35.29 ± 0.41, 35.68 ± 0.20). Plagioclase of the Tenería Formation rhyolites gave the same age within error (34.45 ± 1.10, 34.94 ± 0.34 Ma), documenting the end of the main volcanic episodes. A younger plagioclase age of 30.89 ± 1.45 Ma of a dome near Tetipac gives evidence of later silicic magmatism in the area.
The source of the ignimbrites of the Taxco region is inferred to be located in the Tenería area, where abundant rhyolite dome structures seem to obliterate the main caldera structure. Differences in age, mineralogy and geochemistry between the Taxco succession and nearby volcanic zones where caldera structures have been identified preclude sources in these areas.

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Fluid inclusion and stable isotope study of the Cobre–Babilonia polymetallic epithermal vein system, Taxco district, Guerrero, Mexico

Abstract

Introduction

Section snippets

Regional geology

Structure and stratigraphy of the veins

Fluid inclusions

Stable isotopes

Discussion and conclusions

Acknowledgements

Journal of Volcanology and Geothermal Research

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Controls on formation of low-sulfidation epithermal deposits in Mexico: constraints from fluid inclusion and stable isotope data

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The La Guitarra Ag–Au low sulfidation epithermal system, Temascaltepec district, Mexico: fluid inclusion and stable isotope data

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Ages of epithermal deposits in Mexico: regional significance and links with the evolution of Tertiary volcanism

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Cenozoic mineral deposits and subduction-related magmatic arcs in Mexico

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Evolución de los arcos magmáticos en México y su relación con la metalogénesis

Revista del Instituto de Geología, Universidad Nacional Autónoma de México