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

A geochemical and mineralogical study of hydrothermal deposits from 13 degrees N and 21 degrees N, East Pacific Rise was conducted in order to identify controls on the distribution of silver. The 349 degrees C hydrothermal fluid from the Southwest vent field (21 degrees N) is enriched in silver by approximately three orders of magnitude compared to seawater. This silver is shown to be transported as the dichlorosilver(I) complex, and to be undersaturated with respect to Ag2S. The absence of a separate silver phase such as acanthite in East Pacific Rise chimneys suggests that silver remains undersaturated with respect to Ag2S during cooling and mixing of the hydrothermal fluid with seawater. However, silver is present to some degree in sulfides occurring throughout the paragenesis of chimney growth, and is therefore controlled by its distribution in precipitating sulfides. Bulk silver content of subsamples from a zoned black smoker from the Southwest vent field near 21 degrees N is accounted for by silver contained in chalcopyrite, pyrite, marcasite and zinc sulfide. Galena has the highest silver concentrations (up to 0.95 w%), but is present in trace amounts and so does not make a significant contribution to the bulk silver content of the chimney. Bulk chemical analyses of 33 chimney samples gave silver concentrations of up to 206 ppm. Silver shows significant positive correlations with Pb, S, Sb, Cd, Mn and Au in chimneys from both areas. The silver content is also positively correlated with the morphological maturity of structures in which it is contained, being highest in basal mounds and lowest in immature anhydrite-rich chimneys. Thus, silver is continuously enriched in seafloor deposits during the lifetime of the hydrothermal system.