MAGMAGENESIS AND MINERALIZATION OF GIANT MIOCENE AND PLIOCENE MEGABRECCIA DEPOSITS IN THE ANDES OF CENTRAL CHILE

The world's largest Cu deposit, El Teniente, as well as Río Blanco-Los Bronces and Los Pelambres, two other giant Miocene and Pliocene deposit in the Andes of central Chile, contain a large part of their hypogene Cu in and around multiple magmatic-hydrothermal breccia pipes. Both high-grade Cu and different alteration assemblages are associated with individual breccias and not distributed concentrically around a central porphyry intrusion. Chronologic data confirm that mineralization occurred in conjunction with multiple breccia emplacement events, not intrusion of the small volume of felsic porphyries occurring in each deposit. Multiple breccias were emplaced over a >2.5 m.y. period during which evidence for coeval volcanic activity is absent. Roots of the mineralized breccia pipes are unknown, implying that magmatic fluids that produced these breccias were derived from magmas cooling >4 km below the paleosurface. Approximately 600 km³ of magma is required to supply the 100x10⁶ tons of Cu originally contained in each deposit. It is suggested that input of mantle-derived basalt into the base of large, long-lived, open-system magma chambers below each deposit provided heat for their progressive growth and persistence, as well as Cu, S and Cl-rich aqueous fluids which migrated to the tops of these chambers due to thermal gradients. As these chambers solidified, they exsolved the metal-rich aqueous fluids that generated multiple mineralized breccia pipes in each deposit. Crustal thickening, uplift and erosion speeded this crystallization and de-fluidization process, and caused telescoping of breccias, but isotopic data are consistent with derivation of all the igneous rocks and metals associated with each deposit from subduction-modified subarc mantle and suggest that formation of these deposits did not involve melting of continental crust. Each deposit contains small, late, barren or weakly mineralized felsic porphyry intrusions, but these are both too small to have been the source of the enormous amount of Cu in these deposits and they post-date the emplacement of the main mineralized breccias. They formed as a result of crystal-liquid fractionation within the same batholith-size mafic magma chambers the generated the mineralized breccias in each deposit.