Precious metal-bearing epithermal deposits in western Patagonia (NE Lago Fontana region), Argentina

Precious metal-bearing quartz veins occur at the northeastern sector of the Lago Fontana region in southwestern Argentina, within the context of the Andean continental magmatic arc environment. The deposits and their associated alteration zones are spatially related to a Cretaceous calc-alkaline magmatism represented by silicic dikes and hypabyssal intrusions, and hosted by a Late Jurassic to Cretaceous volcano-sedimentary sequence. The veins and related veinlets crop out discontinuously, in general terms in a NW-SE belt. The primary vein mineral assemblage is composed mostly of pyrite ± galena ± chalcopyrite > hematite ± arsenopyrite in silica gangue minerals. Chemical analyses of grab samples from selected quartz veins show as much as 5.7 ppm Au and 224 ppm Ag, as well as elevated Pb, Cu, and Zn. Hydrothermal fluids caused an innermost silicification and adularia-sericite alteration assemblage, and an external propylitic halo.Sulfur isotope values measured for sulfides (δ34SH2Sδ34SH2S from −1.90 to +1.56‰), and oxygen and hydrogen isotopes measured on quartz crystals and extracted primary fluid inclusion waters (δ18OH2Oδ18OH2O = −2.85 to +5.40‰; δDH2OδDH2O = −106.0 to −103.4‰) indicate that mineralization probably formed from magmatic fluids, which were mixed with meteoric waters. Also, fluid inclusion data from quartz veins point out that these fluids had low salinity (1.7–4.2 wt% NaCl equiv.), and temperatures of homogenization between 180 and 325 °C.Mineralogical, petrographic and geochemical features for mineralized surface exposures indicate a typical adularia-sericite, low sulfidation epithermal system in the Lago Fontana area that represents a promising target for further exploration programs.

► Precious metal-bearing quartz veins occur at the NE sector of the Lago Fontana area, Chubut. ► Geologic features for surface exposures show a low sulfidation epithermal system. ► General geological characteristics encourage further regional exploration programs.