Sulfur isotope study of the Yatani Au-Ag and Zn-Pb vein-type deposits in Japan

Abstract

Sulfur isotopic compositions of sulfide minerals from the Yatani deposits which occur in the Green tuff region of Japan were determined separately for those from Zn-Pb and Au-Ag veins. δ³⁴S values of Au-Ag veins are higher than those of Zn-Pb veins. The values for Zn-Pb veins and Au-Ag veins are ca. 0.5–4.5 ‰ and ca. 3–6 ‰, respectively. This difference in δ³⁴S of Zn-Pb veins and Au-Ag veins is difficult to be explained by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H₂S rich fluid (deep fluid) with SO₄^2- rich fluid (shallow fluid) is the most likely process for the cause of this difference. This fluid mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H₂S rich fluid.