MAFIC MAGMATISM ASSOCIATED WITH THE GENESIS AND EVOLUTION OF EL TENIENTE COPPER DEPOSIT, CHILE

El Teniente, located in the Andes of central Chile, is the world’s largest copper deposit. Copper was emplaced in multiple telescoped late Miocene and Pliocene magmatic-hydrothermal biotite, anhydrite, and tourmaline breccias hosted in middle Miocene mafic intrusive rocks. Felsic intrusions cut both these breccias and their mafic intrusive host rocks, and have truncated and redistributed pre-existing copper mineralization. Both felsic and mafic intrusive rocks were later cut by post-mineralization dikes of intermediate composition. Igneous activity ended in the area with the eruption of mafic lavas around the deposit, after which the Andean magmatic arc migrated >35 km to the east. The extensive history of mafic igneous activity at El Teniente is consistent with the dominantly mafic nature of Andean magmatism, which is generated by melting in the mantle wedge above subducting, dehydrating Nazca plate oceanic lithosphere. During the development of El Teniente, volatile-rich mantle-derived mafic magmas mixed within an open-system magma chamber below the deposit rather than erupting to the surface. Mafic magmas provided heat, water, sulfur, iron, metals and copper to the base of the system. Sulfur and metal-rich aqueous brines and vapor concentrated in and exsolved out of the upper part of this open magma chamber, producing brecciation, alteration and Cu-mineralization within the rocks above. This process generated multiple independent mineralized breccia complexes, beginning first with biotite breccias followed by both barren and mineralized anhydrite, tourmaline and rock flour breccias which emplaced the large tonnage, high-grade ore at El Teniente. As the input of mafic magmas into the chamber below the deposit decreased, smaller felsic and intermediate porphyry dikes intruded into the mineralized rocks and finally magmatic activity ceased.