Fluid geochemistry of the epithermal deposits in the southeastern part of the Temascaltepec District, Mexico State, Mexico

Abstract

This study is focused on the genetic relationship between different zones containing epithermal veins in the Temascaltepec mining district. For this purpose, the characteristics of their respective mineralizing  fluids  contained  in  fluid  inclusions,  stable  isotope  geochemistry,  and  spectrometric volatile analyses of fluid inclusions were used. The Temascaltepec district is located 150 km south- east of Mexico City, in Mexico state, and is constituted by several Ag–Au bearing low–sulfidation epithermal veins. In this area, three main vein sets occur from NW to SE: (1) El Coloso, (2) La Guitarra, which laterally connects to El Coloso, and (3) Mina de Agua, comprising over 50 known veins with dominant NW–SE orientation. These three vein sets are hosted by three different lithologic units: Eocene volcanic rocks, Paleocene monzogranites, and pre–Albian metamorphic rocks, respectively.

The epithermal veins from the Temascaltepec district were mineralized during hydrothermal activity associated with the thermal manifestations of late volcanism and sub–volcanic intrusions of the Sierra Madre del Sur. The Temascaltepec River normal fault divided the ensemble of epithermal veins into two vertically displaced blocks, although it is assumed an original lateral continuity of the veins from the El Coloso zone until the Mina de Agua area. The latter represents the uplifted block, formed at a greater depth than the rest of epithermal veins in the district. The veins of the Mina de Agua sector are mostly hosted in the Taxco Schists unit. The fluid inclusion microthermometric analyses were done on quartz samples from the main mineralization events, and calcite samples from the last mineralization stage of  Socorro and San Bernabé veins. Temperature and salinity of mineralizing  fluids  range  from  60  to  271°C  and  2.0  y  5.5  wt.%  NaCl  equiv  respectively.  The temperature variation found in the Mina de Agua sector is similar to that found in the La Guitarra deposit, although the mean temperature values for each sample from the Mina de Agua sector are significantly  higher.  No  differences  on  physico–chemical  characteristics  are  found  between Ag–bearing and barren segments of the veins. d¹⁸Owater, values are –0.6‰ in quartz and  –1.6‰in calcite; and dD_water  values are –48 and –53‰ in quartz, and –9.7‰ in calcite. These values are similar to those obtained in the La Guitarra–El Coloso ensemble and suggest that the meteoric water content in the main mineralization events is greater in the Mina de Agua deposits than in the La Guitarra deposit but magmatic fluids may also be present. The  ⁸⁷Sr/⁸⁶Sr value (0.70677) obtained from a calcite sample from the last mineralization event indicates a mantle influence that can be possibly derived from leaching of the magmatic host rocks by meteoric waters that had some degree of deep circulation. The spectrometric analyses of volatiles contained in fluid inclusions shows N₂/Ar ratios up to 739, and N₂–He–Ar and N₂–CH₄–Ar relations that indicate the occurrence of magmatic fluids, deep–circulated meteoric fluids and fresh meteoric waters as main mineralizing flu ids. This conclusion is equivalent to that obtained through the stable isotope study. The distinct ratios between Ag, Au and base metals found in the veins from the La Guitarra–El Coloso ensemble, and from the Mina de Agua sector (higher in Au and base metals, respectively), the difference in topographic level of  both  areas,  the  occurrence  of  the  Temascaltepec  River  normal  fault  between them, and the physico–chemical characteristics of mineralizing fluids, are compatible with a deeper formation of the veins in the Mina de Agua area than the rest of epithermal veins of the district. This in turn implies that epithermal veins from both the Mina de Agua sector and the La Guitarra–El Coloso ensemble were likely formed at the same time as a result of a single hydrothermal system.