Deposition of a high-sulfidation Au assemblage from a magmatic volatile phase, Volcلn Popocatépetl, Mexico

Volcلn Popocatépetl (Popo) is a Quaternary stratovolcano located 70 km SE of Mexico City in the Trans-Mexican Volcanic Belt. The summit crater of Popo is the site of recent ash eruptions, degassing bursts, and dacite-dome growth. Fresh pumice samples collected on the flanks of Popo contain sparse, very fine-grained, base- and precious-metal phases that crystallized from magmatic volatiles trapped in vesicles. The assemblages consist of pyrite, sphalerite, chalcopyrite, tennantite, galena, enargite, magnetite, chromite, and barite; some samples also contain calaverite, stannite, AuCu telluride, Ag sulfide, AgCu sulfide, AgFe sulfide, Ag bromide, Ag chloride, Bi chloride, halite, kaolinite, and alunite. These assemblages are similar to those found in high-sulfidation epithermal Au deposits, and provide evidence consistent with a direct magmatic source for metals in those deposits. Bulk-sample concentrations of base and precious metals are not anomalously high and can be accounted for mainly by the presence of ore minerals in vesicles; this suggests that a high proportion of the metal content of the original magma was partitioned into a volatile phase which subsequently was trapped and condensed.