Rapid transition to long-lived deep crustal magmatic maturation and the formation of giant porphyry-related mineralization (Yanacocha, Peru)

Abstract

The Yanacocha magmatic field (northern Peru) hosts the largest high sulfidation gold deposit on Earth. Mineralization is associated with porphyritic intrusions distributed along a NE-trending magmatic structural corridor. Eight of these intrusions investigated in this study range in age from 12.4 to 8.4 Ma and show systematic chemical and isotopic changes through time. They are interpreted to derive from hydrous mafic magmas evolving through amphibole–clinopyroxene ± garnet fractionation and lower crust melting (leaving a garnet residue) at deeper levels, which led to variably strong adakite-like signatures, and through plagioclase–amphibole fractionation at shallower levels, both accompanied by crustal assimilation and recharge (recharge assimilation fractional crystallization, RAFC, processes). Systematic geochemical and isotopic changes with intrusion ages, coupled with plagioclase zoning and amphibole geobarometry, suggest that the evolution of the magmatic system occurred through interaction of mantle-derived melts with an increasing length of the crustal column and propagation from deep towards shallower crustal levels through time. This was probably the result of a steadily increasing compression that has progressively slowed down magma ascent forcing magmas to evolve at a series of intermediate level chambers between the lower and upper crust. Increased compression might have been related to the onset of subduction of the buoyant Inca oceanic plateau, estimated to occur at ∼ 12 Ma, i.e., the same time of the onset of the rapid transition from “normal” to adakite-like signatures. The giant Yanacocha ore system developed in coincidence with the ∼ 3.6–4.0 Ma-long intrusion of the adakite-like magmas (12.4/12.0–8.4 Ma) formed by the above processes into a small upper crustal volume and peaked during the last ∼ 2.4 Ma (10.8–8.4 Ma) of magmatic activity after a ∼ 1.4 Ma long (12.4–11.0 Ma) maturation of magmas at deep crustal levels. Further investigation is needed to understand whether the association of adakite-like magmas with ore, which is typical of other giant porphyry-systems, is the result of the build-up of incompatible volatiles and metals in oxidized magmas that evolve under high-pressure conditions, of recycling of lower crustal sulfide-rich cumulates, and/or of a long-lived, focused transfer of magmas from deep to shallow crustal levels.