Adakites of the Andean Active Margin: Relationship with Anomalous Subduction Processes and Copper–Gold–Silver Mineralization

Areas of anomalous (flat and/or oblique) subduction are characterized by altered geometry and thermal structure of the subducting oceanic plate and development of large-scale deformations in the slab, causing its break-off and formation of a slab window, which triggers the ascent of hot asthenospheric mantle through the gap in the slab. In many cases, such processes result in the melting of oceanic lithosphere of almost any age and the formation of metal-bearing adakite melts. Areas of anomalous subduction of the aseismic Carnegie, Nazca, Iquique, and Juan Fernandez ridges are associated with large, giant, and supergiant porphyry–Cu–Au–Mo and epithermal Cu–Au–Ag deposits in the Andean metallogenic belt, where the ore-bearing complexes contain igneous rocks with adakitic geochemical characteristics. Present-day Ecuadorian adakites, associated with flat subduction of the aseismic Carnegie Ridge beneath the Andean margin, contain microinclusions of precious metals (platinum, gold, silver) and their alloys with copper and zinc, chlorides and sulfides of silver (occasionally with minor copper), pyrrhotite, chalcopyrite, chalcocite, and galena and exhibit enrichment in silver and mercury with respect to the upper continental crust. Metal-bearing adakites are assumed to be magmatic precursors of copper–gold–silver mineralization at convergent plate boundaries in areas of flat and/or oblique subduction development.