EVOLUTION OF VOLCANISM AND THE ASSOCIATED HIGH-SULFIDATION EPITHERMAL GOLD DEPOSITS AT YANACOCHA, PERU

Yanacocha is a world-class mining district in northern Peru that contains >50 Moz of gold. Ore deposits developed as multiple centers associated with hydrothermal quartz and quartz-alunite alteration contemporaneous with Miocene volcanic host rocks. This study investigates the nature of volcanism and its relation to the gold ores by integrating geologic field mapping, ⁴⁰Ar/³⁹Ar ages on igneous rocks and hydrothermal alunite, and petrologic analyses to define the stratigraphy and the timing of volcanism and mineralization.

Following ~5 m.y. of precursor Calipuy volcanism, Yanacocha magmatism spanned 6 m.y. from 14.5 to 8.4 Ma contemporaneous with episodic gold mineralization in five discrete stages over the last 5.4 m.y. The Yanacocha magmas represent a calc-alkaline suite that evolved from early hydrous andesite to late water-rich, oxidized, and sulfate-rich dacite and minor rhyolite. Yanacocha volcanics form a cogenetic series of lavas and pyroclastic rocks with a cumulative volume of ~88 km³. Early volcanism was characterized by multiple and widespread andesite stratovolcanoes and domes, whereas late eruptions of dacite ignimbrites and domes were confined to the center of the district. Some of these centrally located dacite domes and porphyry plugs form the youngest, volumetrically smallest, and most silica-rich igneous rocks in the district. Most gold (>30 Moz) was introduced at this time in zones of massive and vuggy quartz, quartz-alunite, and quartz-pyrophyllite alteration, associated with pyrite±enargite-tennantite-covellite. This record represents one of the largest and longest-lived magmatic-hydrothermal gold districts documented.

These data support a hypothesis that the enormous gold deposits at Yanacocha developed during long durations of subaerial magmatism with compositional diversity and low eruption rates in a favorable structural setting. Low eruption to intrusion ratios decreased further in the late stages. Magmas with highly oxidized sulfur contents evolved from andesite to rhyolite in one or more inferred upper crustal granitoid magma chambers.