INFLUENCE OF FLAT-SLAB SUBDUCTION ON THE EVOLUTION OF THE EL TENIENTE CU DEPOSIT, CHILE

El Teniente, the world's largest Cu deposit, is one of three world-class Cu deposits located in the central Chilean Andes, at the southern margin of the flat-slab segment of the subducting Nazca plate. The formation of El Teniente is temporally related to decreasing subduction angle during the Late Miocene and Pliocene that resulted in 1) the compression of the continental lithosphere causing the cessation of local volcanic activity, 2) the development of a large, deep (>6 km), and long-lived (~3 m.y.) magma chamber promoting the enrichment of volatiles and metals at its apex, and 3) the eventual cooling and solidification of this chamber as the magmatic arc migrated to the east. High-grade (>1.5% Cu) mineralization at El Teniente is concentrated in biotite-, anhydrite-, and tourmaline-bearing magmatic-hydrothermal breccia pipes that were emplaced along lithologic and structural weaknesses as the increasing volatile pressure within the magma chamber exceeded the confining lithostatic pressure. Later igneous breccias that intrude into and are genetically related to these mineralized breccia pipes contain primary igneous anhydrite (up to 20 modal %) that crystallized contemporaneously with Na-plagioclase (An2-7), quartz, biotite, K-spar, Fe-oxides and Cu-sulfides. Primary igneous anhydrite has been described in the extrusive products of El Chichon, Pinatubo, and Lascar volcanoes, but occurs in much lesser quantities. The high modal abundance of primary igneous anhydrite in the plutonic matrix of igneous breccias at El Teniente suggests that the highly oxidized, sulfur- and Cu-rich magmas that formed them were under sufficient confining pressure (>1.3 kbar) to prevent devolatilization, which we attribute to lithospheric compression as a result of decreasing subduction angle.