JURASSIC EXTENSIONAL TECTONO-MAGMATISM AND ASSOCIATED MINERALIZATION OF THE EL-FALDEO POLYMETALLIC DISTRICT, CHILEAN PATAGONIA - GEOCHEMICAL AND ISOTOPIC EVIDENCE OF CRUSTAL CONTRIBUTION

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 beds of the Iba nez formation. In addition, two intrusive units and related subvolcani c porphyries and dikes which intrude both the metamorphic basement and the Ibanez formation, are recorganized: the Quebrada Colorada Granodi orite (QCG) and the Cerro Esmeralda Tonalite (GET). The polymetallic m ineralization was formed in four stages and includes pyrite, arsenopyr ite, gold, silver, sphalerite, galena and chalcopyrite hosted in felsi c tuffs of the Ibanez formation, hydrothermal breccias and epizonal in trusions. Field observations and geochronological data (U-Pb, Ar-Ar an d fission track) on CET rocks suggest: (1) the development of subsidin g pull-apart basins and coeval formation of spaces for magmatic emplac ement and mineralization ill an extensional tectonic regime in mid-Jur assic time; (2) a tectonic exhumation of the rocks of the district, pr obably triggered by the Chile ridge-trench interactions that started i n Late Miocene time. Sr-Nd isotopic data from CET samples, and the Pb isotope characteristics of galenas from the second and third mineraliz ation stages, are indicative of crustal contribution to the origin of the magmas and galena mineralization. Rocks of the Paleozoic metamorph ic basement are prime candidates for the crustal source components of the analyzed galenas.