Geochronology of Mexican mineral deposits. II: Veta Madre and Sierra epithermal vein systems, Guanajuato district

This paper presents two new high-resolution geochronological determinations for the epithermal deposits in the World-class Guanajuato mining district, in central Mexico. These are a Rb-Sr isochron age in illite at 28.47 ± 0.55 Ma for the Villalpando and San Juan de Dios low sulfidation veins of the Sierra group of veins, and a 40Ar/39Ar plateau age in adularia ("valencianite") at 30.20 ± 0.17 Ma for the La Valenciana ore shoot of the famous Veta Madre intermediate sulfidation vein. These determinations have greater accuracy, precision and trueness than the preexisting K-Ar determinations for similar adularia samples. The accuracy of such determinations supports the idea of a diachronic emplacement of intermediate and low sulfidation deposits in this district, the former being older than the latter, similar to other epithermal deposits in Mexico. Also, the ~2 m.yr. span between the Veta Madre and Sierra groups of epithermal veins is in agreement with other case studies, regardless of the size of the deposit

Petrology, U/Pb dating and (C-O) stable isotope constraints on the source and evolution of the adakite-related Mezcala Fe-Au skarn district, Guerrero, Mexico

The Mezcala gold district, Guerrero, Mexico, is a Cretaceous to early Paleocene oxidized Fe-Au skarn deposit, genetically associated with an adakitic magmatic event of 63±2 Ma, dated by ion-probe U/Pb on zircons. The inner and outer alteration patterns and the mineralogical sequence (high garnet/pyroxene ratio, high ferric/ferrous ratio >1, predominance of Fe-poor garnet and pyroxene, low total sulfides) found in Mezcala are compatible with the description of an oxidized gold skarn type. Carbon and oxygen isotope analyses of the inner calcite zone (–9.98 to –11.64 and +13.21 to +14.59, respectively) unequivocally support a magmatic signature of the hydrothermal fluid. Carbon and oxygen isotopes from the outer calcite zone (–8.81 to +3.45 and +12.95 to +22.77, respectively) suggest a complex mechanism of degassing and subsequent cooling/dilution of the resulting magmatic brine with the meteoric water. Gold appears to be closely associated with the adakite stock, whereas its transport is related with the outflow of highly oxidized magmatic brines from the intrusion. The gold precipitation is triggered by cooling/dilution of the degassed magmatic brine by the meteoric fluid