New geological, geochronological and geochemical characterization of the San Dimas mineral system: Evidence for a telescoped Eocene-Oligocene Ag/Au deposit in the Sierra Madre Occidental, Mexico

摘要

The San Dimas district is a historical world class Ag/Au epithermal deposit located in the Sierra Madre Occidental (SMO) of western Mexico. San Dimas has been classified as a classic low to intermediate sulfidation Ag/Au epithermal deposit (quartz + adularia + sericite type) developed during a single hydrothermal pulse of similar to 10 Ma, associated to the emplacement of intermediate intrusive bodies of Eocene K-Ar ages. However, this metallogenetic model includes several ambiguities, such as K-Ar cooling ages incompatible with the local and regional magmatic pulses, as well as wide differences in Ag-Au ratios between individual veins, which were also emplaced in two different structural systems. Based on a detailed study of mineralized veins, including new petrographic observations, new geochronological data (zircon U-Pb, adularia Ar-40/Ar-39, and titanite fission track ages, FT), zircon trace-element composition, and geochemistry of gold and silver minerals, we demonstrate that San Dimas exhibits multiple mineralization events developed during different magmatic and tectonic episodes from Late Cretaceous to early Oligocene. The earliest episode is represented by Late Cretaceous copper - gold porphyry mineralization associated with fertile and oxidized magmas during the development of the Laramide arc. The second, most abundant ore mineralization is represented by a Ag-dominant vein system (adularia + rhodochrosite type), which developed into east-west striking fractures. New Ar-40/Ar-39 ages on adularia as well as new titanite FT ages for this mineralization event, together with one published Ar-40/Ar-39 age indicate the formation of Ag rich veins at similar to 41-40 Ma, shortly after the final emplacement of the Piaxtla batholith (similar to 45 Ma), which formed from multiple fertile and oxidized magma intrusions in an extensional environment. The third mineralization episode is represented by Au-dominant epithermal veins (quartz sericite type), which were emplaced at similar to 31 Ma into north-south to NNE-SSE striking fractures. At regional scale these fractures also host rhyolitic domes associated with the first silicic ignimbrite flare up of the SMO in an extensional setting. The formation of the San Dimas district with its exceptionally rich Ag-Au mineralization is interpreted to be related to an extension-related late Eocene-early Oligocene regional uplift that allowed the overprinting of originally deeper Ag veins by a shallower Au mineralizing event.