Redistribution of Pb and other volatile trace metals during eruption, devitrification, and vapor-phase crystallization of the Bandelier Tuff, New Mexico

A diverse suite of micron-scale minerals was deposited from vapor during eruption and post-emplacement crystallization of the Bandelier Tuff, New Mexico. The mineral suite is rich in sulfides, oxides, and chlorides of both common and rare metals (e.g., Fe, Pb, Bi, Cu, Ag, Re), and oxides and silicates of incompatible elements (e.g., P, Zr, Y, Nb, Ba and LREE). Minerals preserved in glassy samples grew from magmatic vapor trapped during emplacement, or from vapor migrating along contacts with more impermeable rocks; minerals observed in devitrified samples also grew from crystallization of glass and vapor liberated during this process. In devitrified samples, mafic silicate phenocrysts were partially replaced by an assemblage dominated by smectite and hematite. n- to post-eruptive mineral assemblage observed in upper Bandelier Tuff (UBT) samples bears striking similarity to those deposited by cooling gases near active volcanic vents. However, several differences exist: (1) the mineral suite in the UBT is disseminated throughout the unit, and formed over a broad temperature range (> 700 to < 150 °C) at higher rock:gas ratios; (2) the highly evolved composition of the UBT yielded a greater abundance of minerals rich in incompatible elements compared to sublimates from less evolved volcanoes; and (3) the UBT has suffered over 1 million years of post-emplacement exposure, which resulted in solution (or local re-precipitation in fractures) of soluble compounds such as halite, sylvite, and gypsum. enriched toward the roof of the UBT magma body due to its affinity for the melt and vapor phases relative to crystals (Bulk Dpb < 0.2). Micron-scale Pb minerals appear to have grown from vapor exsolved during eruption, as well as vapor liberated during later devitrification. Additional Pb was scavenged by smectite and hematite that probably formed during the later stages of the devitrification and cooling process. Up to ten-fold increases in Pb concentrations are seen in zones of fumarolic concentration in the UBT, however, most bulk tuff samples have Pb values that appear to preserve magmatic values, indicating only very local trace-metal redistribution. The concentration of Pb and other heavy metals in micron-scale mineral coatings in porous tuff indicates that these metals could be readily mobilized and transported by acidic groundwaters or hydrothermal fluids, and thus locally concentrated into ore-grade deposits in long-lived systems.