OCCURRENCE AND DISTRIBUTION OF PRECIOUS METALS IN SULFATE-RICH HYDROTHERMAL PRECIPITATES FROM THE EDMOND VENT FIELD, CENTRAL INDIAN RIDGE

Here, we report the first documented occurrence of native gold, silver and copper in seafloor hydrothermal sulfides from the Edmond vent field, Central Indian Ridge. Anhydrite-dominated fragments of sulfide chimneys have significantly higher contents of precious metals (up to 73.9 ppm Ag and 1.5 ppm Au) than Fe-rich massive sulfides and silica-rich hydrothermal precipitates. Coarse-grained native gold (up to 10 µm), mainly associated with anhydrite, amorphous Fe-oxyhydroxides and secondary copper minerals, have been identified in sulfate-rich ore samples by SEM/EDS observation. The presence of irregular-shaped, fine-grained native copper is closely associated with alterd chalcopyrite, late-stage zinc sulfides and minor digenite or chalcocite, exhibiting signs of secondary dissolution, recrystallization and reprecipitation. Native silver occurs as fissure-filling veinlets in low-temperature, argentiferous luzonite (up to 6.7 wt.% Ag determined by EPMA), or as submicron-sized inclusions trapped within the cavities of porous sphalerite and chalcopyrite. By using HRTEM combined with HAADF-STEM imaging, we have determined that silver is also present in significant quantities as discrete nanoparticles (5 – 10 nm) in Sb-bearing luzonite grains. In addition, tiny electrum particles with high Ag/Au ratios, mostly as submicroscopic inclusions in chalcopyrite and luzonite, tend to occur close to grain boundaries with Fe-poor sphalerite. In contrast to abundant Ag-Au alloys typically found in primary sulfide minerals, the absence of detectable silver in relatively coarse gold grains deposited on the outer surfaces of weathered sulfides indicates that secondary enrichment of gold and silver has probably occurred after precious metals were co-precipitated directly from hydrothermal fluids during the deposition of chalcopyrite-sphalerite-sulfosalt assemblages. Conductive cooling and mixing of vent fluids with oxidized seawater, together with post-depositional reworking or supergene processes in the Edmond VMS deposit are considered to be important for the remobilization and local reconcentration of early-formed precious metals, and may have been responsible for the formation of high-purity, secondary native Au and Ag in sulfate-rich hydrothermal precipitates.