Nevado Portugueza, a major center of calc-alkalic volcanism on the crest of the Western Cordillera of the Andes in central Peru, was active during Pliocene and perhaps Quaternary time. A large central volcano composed mostly of intermediate lavas formed about 3.5 to 4 m.y. ago. Explosive pyroclastic volcanism 2.4 + or - 0.2 m.y. ago produced the Atunsulla Tuff, a rhyodacitic ash-flow sheet of moderate volume, and created an east-northeast elongate collapse caldera about 7 km in maximum diameter.Steeply dipping fractures in the hydrothermally altered central part of the intracaldera tuff prism host thin veins containing base and precious metal mineralization dated at 1.90 + or - 0.08 m.y. This mineralization is probably related to postsubsidence magmatic activity reflected at the surface by a rhyodacite dome emplaced along the ring-fracture system 1.96 + or - 0.1 m.y. ago. Tetrahedrite-bearing veins, which appear to postdate collapse, are exposed at lower elevations directly east of the caldera. Hot water and sulfur present in workings in these veins may reflect episodically continuing activity of the volcanic center.Limited geochemical and Sr isotope data show that the postsubsidence lavas are chemically similar to the Atunsulla Tuff and suggest that these unevolved late-stage silicic rocks were not derived from magmas compositionally similar to the intermediate lavas that make up the earlier central volcano.

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