High magma oxidation state and bulk crustal shortening:: key factors in the genesis of Andean porphyry copper deposits, central Chile (31-34°S)

The Andean segment between 31 and 34degreesS documents a unique Cenozoic tectono-magmatic evolution that involves the generation of three world-class Late Miocene porphyry copper deposits: Los Pelambres, Rio Blanco-Los Bronces and El Teniente. The genesis of these giant ore-deposits occurred during a major copper mineralization cycle that took place progressively from north to south, in close association with the emplacement of a series of calc-alkaline, highly oxidized granitoids (Fe2O3/FeO = ratio between 1 and 3). These granitoids were emplaced coevally with bulk shortening and appear to have fractionated along active steep, margin-oblique fault zones that may have played a key role in the exsolution process of mineralized hydrothermal fluids. An increasing contamination of the mantle source by components from altered oceanic crust beneath the arc could account for a rise in the oxidation state of the magmas without producing a significant increase in the Sr-87/Sr-86 initial ratio, as suggested by new and previously published geochemical data. The authors propose that this increasing supply of oceanic crust components to the Miocene magmas could be linked to the progressive subduction of the Juan Fernandez Ridge from north to south.