UThPb zircon dating of the 13.8-Ma dacite volcanic dome at Cerro Rico de Potosí, Bolivia

The temporal relationship between the extrusion of the Miocene dacite volcanic dome at Cerro Rico de Potosí, Bolivia, and the associated AgSn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit. The highly altered nature of the volcanic rock, however, has prevented previous efforts from definitively determining the emplacement age of the dome rather than subsequent episodes of alteration. The present study uses UThPb dating of sparse zircon contained in the dacite to demonstrate that, at most, only several hundred thousand years separate dome emplacement from main stage mineralization. cite contains a morphologically heterogeneous population of zircon with considerable variation in size, shape and color. Preliminary analyses revealed the presence of a significant component of inherited zircon, most likely as older cores that were overgrown by rims during the crystallization of the dacite magma. Morphological distinctions (shape, size, color) within the zircon population provided the basis for separating the zircon into fractions that reduced or enhanced the inherited component in composite grains. By analyzing the tips of zircon with very large length-to-width ratios, it was possible to constrain the crystallization age of the dacite to 13.8 ± 0.2 Ma. The dispersion of data on a concordia diagram indicates at least two ages of inherited zircon, one Mesozoic associated with yellow grains, and another Proterozoic associated with colorless grains. Such xenocrystic components have useful petrogenetic implications for the source and travel path of the magma.