SULFOSALT MELTS AND MELT INCLUSIONS IN ENARGITE: EVIDENCE OF HIGH TEMPERATURE VAPOR TRANSPORT OF METALS IN THE FORMATION OF HIGH SULFIDATION LODE GOLD DEPOSITS

It is well established through stable isotope studies that enargite-gold “high sufidation” deposits are derived from the expansion of gases released from shallow magma reservoirs in volcanic systems. Fluid inclusion homogenization temperatures are however equivocal for the early pyrite-enargite stage of lode formation due to uncertainties inherent in infra-red fluid inclusion techniques, the rarity of primary inclusions, and the uncertainties associated with low density fluid inclusion homogenization measurements.

Enargite from a number of lode deposits consistently show evidence of solid state unmixing processes such as “disease texture” and symplectic intergrowths of luzonite-famatinite and enargite. We have shown using Field Emission SEM that distinctive domain textures characterize enargite from El Indio (Chile), Summitville (Co), Chinkuashih (Taiwan) and Lepanto (Philippines) that correspond to subtle variations in antimony-arsenic ratios and relative abundances of vapor inclusions or tubules, some of which contain Sb-end member famatinite. Samples from El Indio reveal domains characterized by graphic intergrowths consistent with unmixing from sulfosalt melt at temperatures above 600oC. Enargite most commonly forms after deposition of well crystallized, sometimes zoned, pyrite with the latter becoming unstable relative to enargite melt. Doubly-terminated quartz crystals commonly grow into the sulfosalt melt, and in some samples become extensively corroded and replaced by enargite. Sulfosalt melt inclusions may also be preserved in these euhedral crystals and fluid inclusions confirm formation from low salinity, low density fluid with subsequent secondary inclusions also preserved.

These observations are consistent with pyrite-enargite formation from magmatic vapor expansion through brittle fracture arrays in previously altered brittle and impermeable silica-alunite altered rocks. They contrast with the common assumption of “epithermal” temperatures of 250+50oC for deposition, the latter applying to later stages of mineralization. Structurally-controlled high grade lode deposits represent the “feeder” structures for shallow low grade, bulk tonnage gold deposits such as Pascua-Lama and Yanacocha.