Sulfosalt melts and heavy metal (As-Sb-Bi-Sn-Pb-Tl) fractionation during volcanic gas expansion: the El Indio (Chile) paleo-fumarole

High-sulfidation vein gold deposits such as El Indio, Chile, formed in fracture arrays 650°C following higher temperature vapor deposition of crystalline pyrite and enargite. The sulfosalt melt aggressively corroded the earlier enargite and pyrite and hosts clusters of distinctive euhedral quartz crystals. The crystallizing sulfosalt melt also trapped an abundance of vugs within which heavy metal sulfide and sulfosalt crystals grew together with K-Al silicates and fluorapatite. These data and their geologic context suggest that, in high-temperature fumaroles on modern active volcanoes, over 90% of the arsenic content of the primary magmatic vapor (perhaps 2000 mg kg−1) was precipitated subsurface as sulfosalt. Subsurface fractionation may also account for the range of exotic Pb-Sn-Bi-Se sulfosalts observed in fumarole sublimates on active volcanoes such as Vulcano, Italy, as well as on extra-terrestrial volcanoes such as Maxwell Montes, Venus