Genesis and evolution of the Natividad Au-Ag(-Ge) epithermal deposit in southern Mexico: Unravelling the final chapters of the Sierra Madre del Sur metallogeny

Abstract

The final magmatic arc—Miocene in age—of the long-lived Sierra Madre del Sur igneous province in southern Mexico hosts several epithermal deposits that have hitherto received little attention. The Natividad Au-Ag(-Ge) epithermal deposit is one of them and holds a mining record of > 200 years. Here, we present the first petrogenetic and metallogenic study on the Natividad deposit, in which we use a multi-methodological approach to assess the genesis and evolution of the deposit and its associated igneous rocks. Natividad is mostly hosted by Oligocene–Miocene arc-related dacites (U-Pb zircon dates of 22.8–24.7 Ma), whose geochemical features suggest lower-crustal differentiation (i.e., involvement of garnet) at the base of a thickened continental crust. The mineralization occurs in three multi-stage quartz + carbonate veins and consists of Fe-poor sphalerite (<5 mol. % FeS), galena, chalcopyrite, acanthite, and marcasite (sulfide-dominated stage) overprinted by pearceite-polybasite, pyrargyrite-proustite, tetrahedrite-group minerals, electrum, and argyrodite (Ag8GeS6; Ag-sulfosalt-dominated stage). Geothermometry and fluid inclusion data for ore-bearing assemblages reveal fluid temperatures of 170°–400 °C and salinities of 14.6–19.5 wt% NaCl equiv. δ18O from 0.8 ‰ to 7.5 ‰ in mineralizing fluids suggests mixtures of magmatic brines and meteoric waters, while δ13C from –8.4 ‰ to 2.2 ‰ indicates the recycling of organic carbon from the meta-sedimentary basement. Further, δ34S from –3.2 ‰ to –0.3 ‰ in sulfides attest to magmatic S. These results align with an intermediate-sulfidation epithermal model, whereby upwelling magmatic ore-bearing brines precipitated the ores due to conductive cooling, dilution, and, locally, boiling. Our study highlights the metallogenic potential of Miocene magmatism in the eastern Sierra Madre de Sur.