Jurassic extensional tectono-magmatism and associated mineralization of the El Faldeo polymetallic district, Chilean Patagonia: geochemical and isotopic evidence of crustal contribution

Abstract

The El Faldeo is the southernmost Andean mineral district and consists of Paleozoic polymetamorphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the Ibáñez formation. In addition, two epizonal intrusive units and related subvolcanic porphyries, sills and dikes which intrude both the metamorphic basement and the Ibáñez formation, are recognized: the Quebrada Colorada Granodiorite (QCG) and the Cerro Esmeralda Tonalite (CET). The polymetallic mineralization was formed in four stages and includes pyrite, arsenopyrite, gold, silver, sphalerite, galena and chalcopyrite hosted in felsic tuffs of the Ibáñez formation, hydrothermal breccias and epizonal intrusions. Field observations and geochronological data (U-Pb, Ar-Ar and fission track) on CET rocks suggest: (1) the development of subsiding pull-apart basins and coeval formation of spaces for magmatic emplacement and mineralization in an extensional tectonic regime in mid-Jurassic time; (2) a tectonic exhumation of the rocks of the district, probably triggered by the Chile ridge–trench interactions that started in Late Miocene time. Sr-Nd isotopic data from CET samples, and the Pb isotope characteristics of galenas from the second and third mineralization stages, are indicative of crustal contribution to the origin of the magmas and galena mineralization. Rocks of the Paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas.