MINERALOGY, FLUID CHARACTERISTICS, AND DEPOSITIONAL ENVIRONMENT OF THE PALEOCENE EPITHERMAL Au-Ag DEPOSITS OF THE EL BARQUENO DISTRICT, JALISCO, MEXICO

Abstract

The El Barqueno epithermal vein district is located in the southwestern part of the Sierra Madre Occidental of Mexico. It is made up of several polymetallic (Au-Ag-Cu-Zn-Pb) epithermal veins hosted by a volcanoplutonic assemblage of andesitic rocks, granites, and granodiorites, which belong to the Lower Volcanic Supergroup of the Sierra Madre Occidental, within the Jalisco block. The veins trend from 45° to 100°, dipping to the southeast and south, and steepening with depth. The maximum strike length of the veins at El Barqueno is about 5 km, and the maximum vein thickness is about 10 m (avg 2–3 m), narrowing with depth. The paragenetic sequence indicates two main mineralization stages. Stage I occurs as breccias with base metal sulfides and virtually no gold or silver. Stage II consists of crustiform quartz bands that form symmetrical vein fillings and contain the precious metal mineralization. Rhombohedral adularia crystals and quartz pseudomorphs after bladed calcite are widespread in stage II, providing evidence for boiling. Gold- and silver-bearing metallic minerals locally precipitated directly on adularia crystals, an association indicating boiling as the main mechanism of metal deposition. Conductive cooling and mixing also may have been important. The temperature and salinity of the mineralizing fluids ranged from 101° to 252°C and 0.2 to 19.1 wt percent NaCl equiv, respectively. Whereas temperatures cluster in the range of 120° to 160°C, salinities have a bimodal distribution, most of them falling below 3 or higher than 10 wt percent NaCl equiv. These two arrays of salinity values only locally reflect mixing and may represent different sources of mineralizing fluids, possibly dominated by meteoric and magmatic sources, respectively. An adularia separate from crustiform quartz bands of stage II was dated by 40 Ar/ 39 Ar geochronology at 57.91 ± 0.44 Ma (isochron age). Thus, El Barqueno has the oldest absolute age of any epithermal deposit in Mexico and suggests the existence of a previously unrecognized Paleocene metallogenic event in southwestern Mexico associated with several types of ore deposits, including iron oxide-copper-gold, porphyry-type skarn and epithermal deposits. This further suggests that older rocks beneath the Upper Volcanic Supergroup of the Sierra Madre Occidental may be prospects for other epithermal veins and that exploration for epithermal deposits and related mineral deposit types should be focused in the oldest outcrop portions of the Sierra Madre Occidental.