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 polymeta- morphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the IbaÂnÄ ez formation. In addition, two epizonal intrusive units and related subvolcanic por- phyries, sills and dikes which intrude both the meta- morphic basement and the IbaÂnÄ ez formation, are rec- ognized: the Quebrada Colorada Granodiorite (QCG) and the Cerro Esmeralda Tonalite (CET). The polyme- tallic mineralization was formed in four stages and in- cludes pyrite, arsenopyrite, gold, silver, sphalerite, ga- lena and chalcopyrite hosted in felsic tu s of the IbaÂnÄ ez formation, hydrothermal breccias and epizonal intru- sions. Field observations and geochronological data (U- Pb, Ar-Ar and ®ssion track) on CET rocks suggest: (1) the development of subsiding pull-apart basins and co- eval 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 contri- bution to the origin of the magmas and galena miner- alization. Rocks of the Paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas.