THE REGIONAL TECTONIC SETTING OF THE GIANT ANTAMINA CU-ZN SKARN DEPOSIT, NORTH-CENTRAL PERU

The largest known Cu-Zn skarn system, with 559 Mt at 1.24% Cu, 1.03% Zn, Antamina is located at 9°32’S, 77°03’W, within and adjacent to a monzogranite stock intruding Cretaceous carbonates in the upper Eocene, Incaic, Marañon fold and thrust belt. Plagioclase and Al-in-hornblende thermobarometry of the granitoid rocks supports an abrupt decrease in crystallization pressure from ca. 3 to 0.5 kb during hydrothermal activity at ca. 10 Ma. Moreover, late phreatic breccias and directly associated veined endoskarn host ca. 58% of the Cu ore, implying abrupt decompression, critical to development of this giant deposit. The deposit lies on a NE-trending structural arch, the Querococha Arch (new term) coinciding with a cross-strike discontinuity where the strike of the segmented Marañon belt changes from NNW to N. Carboniferous-through-Cretaceous stratigraphic relationships indicate that the arch was high in the Jurassic, or throughout the Late Paleozoic - Early Mesozoic. Incaic folds plunge SSE to the S of the arch, but N to the N. The composite 13.3 to 16.6 Ma Carhuish pluton lies athwart the arch, whereas broadly coeval Huaraz Group volcanics lie at similar altitudes to the N and S. Magmatic quiescence in the Cordillera Occidental , at ca. 10 Ma, separated the terminal hypabyssal activity of the Huaraz Group, at 14.2 Ma, and the initial intrusion of the Cohup granodiorite, at 8.2 Ma. Only scattered Cu-Au hydrothermal centers of this age are known in the Cordillera Negra, but the Antamina stock is one of an apparent swarm of intrusives within the arch. At 10 Ma, the WSW margin of the Carhuish pluton was uplifted from ca. 3 kb along a precursor to the Pliocene-Quaternary Cordillera Blanca range-front fault, experiencing ca. 90° rotation about a horizontal NNW axis. Vital synmineralization uplift at Antamina and synchronous rotational uplift of the Carhuish pluton are both genetically related to upper middle Miocene extension, and also imply segmentation along the Querococha arch. Subduction of the Inca Plateau and Nazca Ridge, whose combined eastern front underlay the Cordillera Blanca at ca. 10 Ma, assuming today’s plate motion vectors, increased coupling between the oceanic and continental plates and caused the Quechua II contraction that effectively terminated the pre-Cordillera Blanca Batholith phase of plutonism and extension.