The San Alberto Lead-Zinc Ore Body at Cerro de Pasco Mine, Cerro de Pasco, Peru
AuthorAscencios C., Alejandro
KeywordsCerro de Pasco Peru
mineral deposits genesis
Geology -- Peru -- Cerro de Pasco
Ore deposits -- Peru
Committee ChairLacy, W. C.
MetadataShow full item record
PublisherThe University of Arizona.
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AbstractThis thesis presents briefly the geology of the Cerro de Pasco district to acquaint the reader with the general geological setting of the district. A study of ore controls for a typical lead-zinc replacement body at the world famous Cerro de Pasco mine in Peru, 102 km northeast of Lima, was undertaken for purposes of better understanding. The particular body selected, the San Alberto Ore Body, occurs as a northeast extension of a main mass of pyrite, known as the "pyrite body", and is enclosed in Triassic- Jurassic limestone. Primary ore controls were determined to be a "Y"-like intersection formed by NS Longitudinal Faulting with a NE striking bedding fault. Resultant brecciation created the necessary permeable conditions whereby ore fluids were channeled away from the pyrite body into the limestone for ore emplacement. Three phases of hydrothermal rock alteration were identified as silicic alteration, chloritic alteration and an outer zone of bleaching and recrystallization. One peculiarity is found in the mineral composition of the silicic alteration, and a hypothesis is presented to explain it. The paragenetic sequence given for hypogene and gangue minerals was determined from the examination of more than 150 thin and polished sections.
Degree ProgramGraduate College
Degree GrantorUniversity of Arizona
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STRUCTURAL AND GEOCHEMICAL EVOLUTION OF A MINERALIZED VOLCANIC VENT AT CERRO DE PASCO, PERURogers, Ralph David (The University of Arizona., 1983)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.