Abstract

GONZALEZ-PARTIDA, Eduardo et al. Model for the formation of the Cu stratabound deposits in red beds of Las Vigas (Chihuahua, Mexico). Bol. Soc. Geol. Mex [online]. 2017, vol.69, n.3, pp.611-635. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2017v69n3a6.

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 N 2 , CO, 𝐶𝑂 2 and 𝐶𝐻 4 contents were determined. Δ34SVCDT 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. δ13CVPDB and δ18OVSMOW 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.

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