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 duri ng eruption and post-emplacement crystallization of the Bandelier Tuff , New Mexico. The mineral suite is rich in sulfides, oxides, and chlor ides of both common and rare metals (e.g., Fe, Pb, Bi, Cu, Ag, Re), an d oxides and silicates of incompatible elements (e.g., P, Zr, Y, Nb, B a and LREE). Minerals preserved in glassy samples grew from magmatic v apor trapped during emplacement, or from vapor migrating along contact s with more impermeable rocks; minerals observed in devitrified sample s also grew from crystallization of glass and vapor liberated during t his process. In devitrified samples, mafic silicate phenocrysts were p artially replaced by an assemblage dominated by smectite and hematite. The syn- to post-eruptive mineral assemblage observed in upper Bandel ier Tuff (UBT) samples bears striking similarity to those deposited by cooling gases near active volcanic vents. However, several difference s exist: (1) the mineral suite in the UBT is disseminated throughout t he unit, and formed over a broad temperature range (> 700 to < 150 deg rees C) at higher rock:gas ratios; (2) the highly evolved composition of the UBT yielded a greater abundance of minerals rich in incompatibl e elements compared to sublimates from less evolved volcanoes; and (3) the UBT has suffered over 1 million years of post-emplacement exposur e, which resulted in solution (or local re-precipitation in fractures) of soluble compounds such as halite, sylvite, and gypsum. Pb was enri ched toward the roof of the UBT magma body due to its affinity for the melt and vapor phases relative to crystals (Bulk D-Pb < 0.2). Micron- scale Pb minerals appear to have grown from vapor exsolved during erup tion, as well as vapor liberated during later devitrification. Additio nal Pb was scavenged by smectite and hematite that probably formed dur ing the later stages of the devitrification and cooling process. Up to ten-fold increases in Pb concentrations are seen in zones of fumaroli c concentration in the UBT, however, most bulk tuff samples have Pb va lues that appear to preserve magmatic values, indicating only very loc al 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 gro undwaters or hydrothermal fluids, and thus locally concentrated into o re-grade deposits in long-lived systems.