GENESIS OF GIANT CU-MO DEPOSITS IN THE ANDES OF CENTRAL CHILE (32-34°S) AS A RESULT OF RIDGE SUBDUCTION CAUSING UPLIFT, EROSION AND ARC MIGRATION

In Los Pelambres, Los Bronces-Rio Blanco and El Teniente, three of the world’s largest Cu-Mo deposits (>30x10⁶ metric tons of Cu), primary hypogene copper mineralization is concentrated around multiple magmatic-hydrothermal breccia centers. Breccia formation and mineralization in these three Miocene-Pliocene deposits formed over a period of >2 million years, during a time of significant tectonic and magmatic changes associated with the subduction of the Juan Fernández Ridge below central Chile. This caused the angle of subduction of the oceanic Nazca plate to shallow, which produced uplift and erosion and a decrease in sub-arc magma supply below the Miocene-Pliocene volcanic front, and ultimately the eastward migration of the Andean arc. Cooling of the large (>600 km³ based on the amount of Cu in these deposits), long-lived (>2 million years based on the time periods of breccias formation) magma chambers responsible for these systems was triggered by rapid uplift and erosional unroofing above, along with decreasing magma supply at the base of these chambers. Cooling produced crystallization of the mafic magmas at the bottom and exsolution of hydrous magmatic fluids from the silicic magma at the top of these magma chambers (~4-6 km below the surface). This generated multiple mineralized and unmineralized breccia pipes and associated veins over an extended period of time (>10 million years in the case of Rio Blanco-Los Bronces). Breccia mineralogy, alteration and mineralization evolved through time. The earliest breccias and alteration are dominated by actinolite-magnetite and are generally poor in copper sulfides. They are followed by more sulfide-rich biotite, tourmaline, anhydrite and finally by rock-flour breccias. Progressive uplift and erosion caused superimposition and telescoping of mineralized breccias and veins that resulted in the creation of these huge copper deposits.