Silver in sulfide chimneys and mounds from 13 degrees N and 21 degrees N, East Pacific Rise

The 349 degrees C hydrothermal vent from the SW vent field (21 degrees N) is enriched in Ag by approximately three orders of magnitude compared to sea-water. This Ag is shown to be transported as the dichlorosilver(I) complex, and to be undersaturated with respect Ag (sub 2) S. The absence of a separate Ag phase such as acanthite in these chimneys suggests that the Ag remains undersaturated with respect to Ag (sub 2) S during cooling and mixing of the hydrothermal fluid with sea-water. However, Ag is present to a limited extent in sulphides throughout the paragenesis of chimney growth, and is therefore controlled by its distribution in precipitating sulphides. Bulk Ag content of subsamples from a black smoker is accounted for by the Ag in chalcopyrite, pyrite, marcasite and ZnS; galena has the highest Ag (up to 0.95 wt.%), but is only present in trace amounts. Bulk chemical analyses of 33 chimney samples gave Ag up to 206 ppm; the Ag content correlates with Pb, S, Sb, Cd, Mn and Au in chimneys from both areas, and is also positively correlated with the morphological maturity of the containing structures, being highest in basal mounds and lowest in immature anhydrite-rich chimneys. Silver is thus continuously enriched in seafloor deposits during the lifetime of the hydrothermal system.