Mineralizing fluids of the shallow epithermal Au–Ag deposits of the El Barqueño district, Jalisco, Mexico

The El Barqueño district, Jalisco, consists of several polymetallic (Au–Ag–Cu–Zn–Pb) epithermal veins hosted by a volcanoplutonic assemblage of andesitic rocks, granites and granodiorites that belong to the Lower Volcanic Supergroup of the Sierra Madre Occidental. The paragenetic sequence indicates two main mineralization stages. Stage I occurs as breccias with base-metal sulfides and virtually no gold and silver. Stage II is made up of several crustiform quartz bands that form symmetrical vein fillings and contain the precious-metal mineralization. Rhombohedral adularia crystals and quartz pseudomorphs of bladed calcite are widespread in stage II, providing evidence for boiling. Au- and Ag-bearing metallic minerals locally precipitated directly on adularia crystals, an association indicating boiling as the main mechanism of metal deposition. Temperature and salinity of mineralizing fluids range from 101 to 252 °C and 0.2 to 19.1 wt.% NaCl equiv., respectively. While temperatures cluster in the range from 120 to 160 °C, salinities have a bimodal distribution, with most of them falling either below 3 wt.% NaCl equiv. or higher than 10 wt.% NaCl equiv. These two arrays of salinity values describe mixing trends only locally, and may reflect different sources of mineralizing fluids, possibly dominated by meteoric and magmatic sources, respectively. An adularia separate from stage II was dated by 40Ar / 39Ar geochronology at 57.91 ± 0.44 Ma (isochron age). Thus, El Barqueño is the oldest known epithermal deposit in Mexico, and suggests the existence of a previously unrecognized Paleocene metallogenic event in southwestern Mexico.