AbstractVolcan Popocatpetl is a Quaternary stratovolcano located 60km southeast of Mexico City. The summit crater is the site of recent ash eruptions, excess degassing, and dacite dome growth. The modern cone comprises mainly pyroclastic flow deposits, airfall tephras, debris flows, and reworked deposits of andesitic composition; it is flanked by more mafic monogenetic vents. In least-degassed fallout tuffs and mafic scoria, transition metals are concentrated in phases formed before eruption, during eruption, and after eruption. Preeruptive minerals occur in both lavas and tephra, and include oxides and sulfides in glass and phenocrysts. The magmatic oxides consist of magnetite, ilmenite, and chromite; the sulfides consist of both (Fe,Ni)1-xS (MSS) and CuFe sulfide (ISS). Syn- and posteruptive phases occur in vesicles in both lavas and tephra, and on surfaces of ash and along fractures. The mineral assemblages in lavas include CuFe sulfide and FeTi oxide in vesicles, and Fe sulfide and CuFe sulfide in segregation vesicles. Assemblages in vesicles in scoria include FeTi oxide and rare FeCuSn sulfide. Vesicle fillings of FeTi oxide, Ni-rich chromite, Fe sulfide, Cu sulfide, and barite are common to two pumice samples. The most coarse-grained of the vesicle fillings are CuFe sulfide and Cu sulfide, which are as large as 50 in diameter. The youngest Plinian pumice also contains Zn(Fe) sulfide, as well as rare AgCu sulfide, AgFe sulfide, Ag bromide, Ag chloride, and AuCu telluride. The assemblage is similar to those typically observed in high-sulfidation epithermal mineralization. The fine-grained nature and abundance of syn- and/or posteruptive phases in porous rocks makes metals susceptible to mobilization by percolating fluids. The abundance of metal compounds in vesicles indicates that volatile exsolution prior to and/or during eruption played an important role in releasing metals to the atmosphere.