Abstract

Major Miocene central Andean (lat 22°-34°S) ore districts share common tectonic and magmatic features that point to a model for their formation over a shallowing subduction zone or during the initial steepening of a formerly flat subduction zone. A key ingredient for magmatism and ore formation is release of fluids linked to hydration of the mantle and lower crust above a progressively shallower and cooler subducting oceanic slab. Another is stress from South American-Nazca plate convergence that results in crustal thickening and shortening in association with magma accumulation in the crust. Fluids for mineralization are released as the crust thickens, and hydrous, lower crustal, amphibole-bearing mineral assemblages that were stable during earlier stages of crustal thickening break down to dryer, more garnet-bearing ones. Evidence for this process comes from trace-clement signatures of pre-to postmineralization magmas that show a progression from equilibration with intermediate pressure amphibole-bearing residual mineral assemblages to higher pressure garnet-bearing ones. Mineralization over the shallowing subduction zone in central Chile (28°-33°S) is followed by cessation of arc volcanism or migration of the arc front away from the trench. Mineralization in the central Altiplano-Puna region (21°-24°S) formed above a formerly flat subduction zone as volcanism was reinitiating. Thus, hydration and crustal thickening associated with transitions in and out of flat-slab subduction conditions are fundamental controls on formation of these major ore deposits.