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.