Sulfosalt melts: Evidence of high-temperature vapor transport of metals in the formation of high-sulfidation lode gold deposits

In enargite-gold (high-sulfidation) vein deposits, magmatic fluid is considered responsible for the transport of metals and sulfur into the depositional regime. New data from Field-Emission SEM analyses of sulfosalt mineral assemblages (primarily enargite and tennantite) from El Indio, Chile, and Summitville, Colorado, provide direct evidence of high-temperature deposition, including the following: (1) the preservation of delicate euhedral quartz assemblages in sulfosalts, (2) a range of discrete Sb-rich sulfosalt, quartz, feldspar, and flourapatite vug-filling minerals, and (3) symplectic sulfosalt-chalcopyrite textures that are arguably quenched melts. Together, these features indicate formation from the vapor phase at high temperatures. Furthermore, euhedral quartz crystals from El Indio contain high-temperature, vapor-rich fluid inclusions. Combined, these observations are interpreted as suggestive of deposition in response to vapor-phase decompression within fracture arrays that may be considered the analogues of the feeder fractures beneath large low-grade silver-gold deposits such as Yanacocha, Peru.