STRUCTURAL AND GEOCHEMICAL EVOLUTION OF A MINERALIZED VOLCANIC VENT AT CERRO DE PASCO, PERU

Abstract

Najor Cu-Pb-Zn-Ag mineralization is associated with a dissected volcanic vent, approximately 2 Km in diameter, at Cerro de Pasco, Peru. Vent fill material, including volcaniclastic agglomerate, bedded and welded tuff, and rootless blocks, document more than one kilometer of subsidence of the floor of the vent. A concave-upward conical fold distorts the vent-fill material, thus recording a late structural collapse. Resurgent igneous activity is evidence by quartz monzonite porphyry dikes and a group of interpenetrating dacite domes intruded along the vent margin. Hydrothermal ore deposition occurred during the final stage in the evolution of this volcanic system. The volcanic system is best described as a caldera system that has experienced igneous resurgence. Mineralization is associated with three distinctive fracture sets. Fractures in Set I parallel the strike and dip of the vent margin and formed as a result of slumping along the vent margin. Fractures in Set II occur between the vent margin and the Longitudinal Fault and formed as a result of strike-slip motion along that fault. Fractures in Set III have a consistent east-west strike, converge at depth to a quartz monzonite porphyry dike, and formed in response to emplacement of the dike. Alteration zones have developed parallel to fracturecontrolled flow channels. Quartz-alunite-kaolinite is developed closest to vein mineralization. This assemblage grades into quartz-phyllosilicate-kaolinite away from the veins, and this latter zone in turn grades into calcite-chlorite-epidote further from the veins. Hydrothermal solutions were localized by the permeable zones along the vent margin. The first hydrothermal minerals deposited were pyrite and silica. These formed a large massive sulfide replacement body. Subsequently, ~300°C solutions equilibrated with the early formed pyrite and continued to ascend upward and outward into what would become the main ore zone. As these solutions moved into the oxidized environment of the upper levels of the volcanic vent they deposited pyrite and enargite in veins and generated hydrogen ions that hydrolitically altered the adjacent wallrocks. Changes in solution composition accompanying alteration are discussed.