The occurrence of gold in sulfide deposits of the TAG Hydrothermal Field, Mid-Atlantic Ridge
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Sulfide deposits in the TAG Hydrothermal Field include both active and relict hydrothermal mounds, with high-temperature black smokers, lower-temperature white smoker chimneys, and coarse recrystallized massive sulfides. On the active TAG mound, black smoker chimneys consist mainly of pyrite-chalcopyrite assemblages (up to 24 wt.% Cu), with concentrations of gold from 0.03 to 1.7 ppm Au. Zn-rich sulfides from the mound are commonly gold-rich, with an average of 9.5 ppm Au and a median value of 6.0 ppm Au (n = 15). White smokers from a low-temperature vent complex on the mound have gold contents of up to 42 ppm Au. In the white smokers, gold is present as submicroscopic particles or as ‘’invisible” gold within fine-grained dendritic sphalerite that comprises the bulk of the chimneys. The significant enrichment of gold within the lower-temperature white smoker complex and the effective separation of Zn and Cu in hydrothermal precipitates at the surface of the mound are interpreted to reflect a strong thermal gradient within the central zone of upflow. Massive, Cu-Fe-Zn sulfides from a nearby relict sulfide mound, the MIR Mound, also are gold-rich. Concentrations of gold in the MIR sulfides reach 15.5 ppm Au, with an average of 7.6 ppm and a median value of 7.7 ppm (n = 13). These samples are distinctly coarse-grained and show evidence of extensive hydrothermal recrystallization and replacement, as well as of overprinting by multiple hydrothermal events. Native gold in these samples occurs mainly as free grains up to 4 mu m in diameter, occupying open spaces in the massive sulfides, in late microfractures, and along grain boundaries between coarse recrystallized sulfides. Ion-microprobe analyses of pyrite and chalcopyrite indicate background concentrations of less than 2 ppm Au as invisible gold in the coarse-grained sulfides. The most abundant gold occurs in a late-stage, sphalerite-rich vein that cuts earlier massive pyrite; similar veins may have been feeders for gold-rich white smokers once present at the surface of the mound (i.e., similar to those on the active TAG Mound). A history of high-temperature venting in the TAG Hydrothermal Field, spanning more than 50,000 years, has resulted in extensive hydrothermal reworking of the sulfide deposits. This process is considered to be important for the remobilization and local reconcentration of early-formed gold and may have been responsible for the formation of relatively coarse-grained, high-purity native gold in recrystallized massive sulfides from the MIR Mound.