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 the altered geometry and thermal structure of the subducting oceanic plate and the development of large-scale deformations in the plate causing it to fracture and form a slab window allowing asthenosphere injection through the gap in the slab. In many cases, such processes trigger the melting of oceanic lithosphere of almost any age and the formation of metalliferous 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, the ore-bearing complexes of which contain igneous rocks with adakitic geochemical characteristics. Present-day adakites of Ecuador, associated with low-angle 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 an admixture of copper), pyrrhotite, chalcopyrite, chalcocite, galena, and exhibit enrichment in silver and mercury relative to the upper continental crust. Metalliferous adakites are assumed to be magmatic precursors of copper-gold-silver mineralization at convergent boundaries of lithospheric plates in areas of flat and/or oblique subduction development.