METAL-RESIDENCE SITES IN LAVAS AND TUFFS FROM VOLCAN-POPOCATEPETL, MEXICO - IMPLICATIONS FOR METAL MOBILITY IN THE ENVIRONMENT

Volcan Popocatepetl is a Quaternary stratovolcano located 60 km southe ast of Mexico City. The summit crater is the site of recent ash erupti ons, excess degassing, and dacite dome growth. The modern cone compris es mainly pyroclastic now deposits, airfall tephras, debris flows, and re-worked deposits of andesitic composition; it is flanked by more ma fic monogenetic vents. In least-degassed fallout tuffs and mafic scori a, transition metals are concentrated in phases formed before eruption , during eruption, and after eruption. Preeruptive minerals occur in b oth lavas and tephra, and include oxides and sulfides in glass and phe nocrysts. The magmatic oxides consist of magnetite, ilmenite, and chro mite; the sulfides consist of both (Fe,Ni)(1-x)S (MSS) and Cu-Fe sulfi de (ISS). Syn- and posteruptive phases occur in vesicles in both lavas and tephra, and on surfaces of ash and along fractures. The mineral a ssemblages in lavas include Cu-Fe sulfide and Fe-Ti oxide in vesicles, and Fe sulfide and Cu-Fe sulfide in segregation vesicles. Assemblages in vesicles in scoria include Fe-Ti oxide and rare Fe-Cu-Sn sulfide. Vesicle fillings of Fe-Ti oxide, Ni-rich chromite, Fe sulfide, Cu sulf ide, and barite are common to two pumice samples. The most coarse-grai ned of the vesicle fillings are Cu-Fe sulfide and Cu sulfide, which ar e as large as 50 mu in diameter. The youngest Plinian pumice also cont ains Zn(Fe) sulfide, as well as rare Ag-Cu sulfide, Ag-Fe sulfide, Ag bromide, Ag chloride, and Au-Cu telluride. The assemblage is similar t o those typically observed in high-sulfidation epithermal mineralizati on. The fine-grained nature and abundance of syn- and/or posteruptive phases in porous rocks makes metals susceptible to mobilization by per colating fluids. The abundance of metal compounds in vesicles indicate s that volatile exsolution prior to and/or during eruption played an i mportant role in releasing metals to the atmosphere.