Graduate Thesis Or Dissertation

Longo Dissertation.pdf

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  • The Yanacocha Mining District in northern Perú is considered the largest group of high-sulfidation style epithermal gold deposits in the world. District-scale geologic mapping coupled with detailed 40Ar/39Ar geochronology, geochemistry and petrography establish the volcanic history of the area and analyze the temporal and spatial evolution of volcanism, hypogene advanced argillic alteration, and Au-Cu-Ag mineralization. Volcanic rocks spanned from ~19.5 to 8.4 Ma and evolved temporally from andesite (60% SiO2) to rhyolite (71% SiO2). Rocks older than 15 Ma erupted from pre-Yanacocha volcanic centers outside the district. Volcanic rocks of the Yanacocha Volcanic Field erupted in the district from 14.5 to 8.4 Ma contemporaneous with magmatichydrothermal quartz-alunite that is directly associated with the deposition of gold ore. Alunite ages define several discrete pulses of hydrothermal activity that spanned 5.4 m.y., from 13.6 to 8.2 Ma. Magmatism and hydrothermal activity progressed northeast through time across the district. Five periods of magmatic activity with volcanism and six pulses of hydrothermal activity have been interpreted from the volcanic stratigraphy and age data. Volcanism alternated between dominantly effusive and explosive stages. Eruptions began 14.52±0.13 Ma with the Lower Yanacocha andesite lavas in the west and ended by 11.22±0.08 Ma with the explosive eruption in the east of the dacitic San Jose ignimbrite. Magmatism then shifted to highly oxidized dacite to rhyolite magmas characterized by domes and isolated intrusions of porphyry plugs, and ended in the final explosive eruption of the Negritos rhyolite ignimbrite at 8.43±0.04 Ma. Volumes and rates of eruption peaked from 14.5 to 11.2 Ma during eruptions of the Lower Yanacocha andesite and the San Jose ignimbrite, and then decreased dramatically after ~11 Ma. Alunite ages indicate hydrothermal activity began in the west at the Cerro Negro Oeste and Quilish deposits from 13.56±0.24 to 12.64±0.61 Ma, and migrated east to the Carachugo and Maqui Maqui deposits where separate pulses developed from 11.01±0.09 to 10.73±0.05 Ma and 10.24±0.14 to 9.95±0.14 Ma. Activity then became centered at the Cerro Yanacocha deposit from 9.25±0.10 to 8.22±0.46 Ma. Comparisons of volcanic and alunite ages suggest that most gold deposition events correlate with periods of low volcanic eruption rates. Rates of gold deposition increased dramatically after ~11 Ma when eruptive output decreased and magmatic SiO2 content increased. The 40Ar/39Ar dates demonstrate that magmatic and hydrothermal activity lasted 2.2 and 4.8 m.y. longer than previously hypothesized for Yanacocha.
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