Boletín de la Sociedad Geológica Mexicana Volumen 69, núm. 3, 2017, p. 611 ‒ 635

Modelo de formación de los yacimientos estratoligados de Cu en lechos rojos de Las Vigas (Chihuahua, México)

Eduardo González-Partida^1,*, Antoni Camprubí², Jacques Pironon³, Pura Alfonso⁴, Edith Cienfuegos-Alvarado², Pedro A. Morales-Puente², Carles Canet⁵, Luis E. González-Ruiz^3,6, Erik H. Díaz-Carreño⁷

¹ Centro de Geociencias, Universidad Nacional Autónoma de México. Blvd. Juriquilla 3001, 76230 Santiago de Querétaro, Qro., México.
² Instituto de Geología, Universidad Nacional Autónoma de México. Ciudad Universitaria, 04510 Coyoacán, CDMX, México.
³ CNRS, CREGU, GeoRessourceslab, Université de Lorraine. Boulevard des Aiguillettes, F-54506 Vandœuvre-lès-Nancy, Francia.
⁴ Departament d’Enginyeria Minera i Recursos Naturals, Escola Tècnica Superior d’Enginyeria de Mines de Manresa, Universitat Politècnica de Catalunya. Av. de les Bases de Manresa 61-73, 08242 Manresa, Catalunya, España.
⁵ Instituto de Geofísica, Universidad Nacional Autónoma de México. Ciudad Universitaria, 04510 Coyoacán, CDMX, México.
⁶ Geologia Minería y Consultoría, S.A. de C.V. Ciprés 104, Frac. Jurica, 76100Santiago de Querétaro, Qro., México.
⁷ Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México. Blvd. Juriquilla 3001, 76230 Santiago de Querétaro, Qro., México.

 *This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

In the tract of the Chihuahua and Sabina intracratonic basins, stratabound copper deposits are systematically hosted by siliciclastic rocks of the Las Vigas, San Marcos and Huizachal formations. The deposits in the Las Vigas area, Chihuahua, occur in the upper part of the Valanginian-Hauterivian Las Vigas Formation, which is conformably overlain by gypsum and limestones of the Barremian La Virgen Formation. Other local evaporites are those found in the Jurassic La Casita Formation.

The Las Vigas stratabound copper deposit was mined with grades between 2 and 4 % Cu and occasional grades of 1 g/t Au and 80 to 100 g/t Ag in four 1 to 4 m-thick mantos. The hypogene metallic minerals in this deposit are chalcopyrite, bornite, pyrite, sphalerite and galena, along with quartz and calcite, mostly as disseminations. Notably, part of these ores occurs as replacements of fossil tree logs (m-sized) that contain the same minerals as mantos. The close examination of fossil tree logs emphasizes the epigenetic and stratabound character of copper ores. Representative fluid inclusion associations were studied in calcite and quartz from mantos, lenses and fossil logs. Fluid inclusions contain halite, calcite and chalcopyrite daughter crystals, as determined by means of Raman microspectroscopy. The calculated salinity of mineralizing fluids ranges between 16.6 and 32.0 wt. % NaCl equiv., temperatures of homogenization range between 104º and 205 ºC, and relevant N₂, CO, CO₂ and CH₄ contents were determined. δ³⁴S_VCDT values in chalcopyrite range between -12.0 and 13.7 ‰. These values are compatible with sedimentary or metasedimentary sources for sulfur, significant amounts of sulfates that were leached from evaporites, besides the occurrence of bacteriogenic sulfate reduction processes orthe interaction of mineralizing fluids with organic matter. δ¹³C_VPDB and δ¹⁸O_VSMOW values in hydrothermal calcite range between -7.96 and -0.71 ‰ and between 22.38 and 26.39 ‰, respectively; these are compatible with a trend in diagenesis due to burial of marine sediments, much alike MVT deposits in Coahuila and Texas.

The above data indicate that mineralizing fluids in Las Vigas are basinal brines with minor meteoric inputs, which underwent convection within the siliciclastic series by means of differences in density as they dissolved evaporites of the overlying La Virgen Formation. Abundant organic matter in the Las Vigas Formation would have enabled mineral precipitation by means of sulfate reduction —such interpretation is supported by the occurrence of Cu ores along with silicification in fossil tree logs. As a consequence, the Las Vigas deposit is generically ascribed to the group of epigenetic copper deposits hosted by red-bed series that rim marine basins, or deposits of the Kupferschiefer type.

Keywords: Kupferschiefer, stratabound, epigenetic, fluid inclusions, stable isotopes, basinal brines, redox.