Genesis of the telescoped Eocene silver and Oligocene gold San Dimas deposits, Sierra Madre Occidental, Mexico: Constraints from fluid inclusions, oxygen - deuterium and noble gases isotopes

Abstract

The San Dimas district is a world-class Ag/Au deposit, developed as a telescoped Eocene-Oligocene Ag/Au mineralization located in the Sierra Madre Occidental (SMO) of western Mexico. San Dimas exhibits multiple mineralization events during different magmatic and tectonic episodes from Late Cretaceous to early Oligocene. The well-preserved magmatic-hydrothermal system provides an excellent opportunity to determine the source of silver and gold, the evolution of the hydrothermal fluids, and the controls on the mineralization precipitation. Mineralogical, fluid inclusions (FI), stable and noble gases isotope analyses suggest that the San Dimas deposit consist of two different mineralization styles: 1) Ag-dominant epithermal Eocene veins that occurred at temperatures up to ~350 °C developed at ca. 2–3 km depth, associated to the final stages of intrusion of the Piaxtla batholith, with FI dominated by a crustal component, and 2) epithermal low sulfidation Au-dominant Oligocene veins which were developed at 250 °C, at shallower depths (< 1 km), associated to the feeding fractures of rhyolitic domes developed at the end of the main ignimbrite flare up of the SMO, with FI showing crustal fluids variably mixed with a magmatic component. Our results highlight the importance of a multidisciplinary approach, such as field observations, geochronological and geochemical studies, to better understand the complexity of the hydrothermal magmatic processes involved in the formation of many Mexican ore deposits and their proper classification.