Fractionation of Noble Metals in Serpentinite-Hosted Hydrothermal Systems

Abstract

Data on the speciation and mechanisms of transport of noble metals (NM) (Au, Ag, Pd, Pt, Rh, Ru, Ir) in oceanic hydrothermal systems were considered and summarized. The contents (0, n–10n ppb) and distribution of NM and Re in the seafloor rocks of the Logatchev and Ashadze hydrothermal fields of the Mid-Atlantic Ridge were determined. The dominance of light chalcophile elements (Au, Pd, Ag, and Re) and high Pd/Ir values were observed in samples taken in close proximity to hydrothermal structures, and the highest Pd/Pt values were obtained for the same samples. Such PGE fractionation may be due to element redistribution during magmatic and postmagmatic transformations under the influence of hydrothermal fluid. According to the obtained data, Au, Pd, and Re could migrate in the form of dissolved complex compounds with inorganic and organic ligands, as well as through pseudocolloid transport of complex and nanometer-sized forms of Au, Pd, and Ag on organomineral particles based on Fe oxyhydroxides. Iridium-group PGE are probably mainly sorbed by organomineral particles from seawater (and its derivatives formed during water percolation through crustal materials). The presence of a positive Rh anomaly in chondrite-normalized PGE distribution patterns was confirmed.