ABSTRACT

The reduced partition function ratio predicts that among sulfide minerals in isotopic equilibria the heavy isotope (³⁴S) is enriched in the following order: galena < chalcopyrite < sphalerite < pyrite. This order also exists among coexisting sulfides in natural deposits. The isotopic composition of pyrite at hydrothermal temperatures is estimated to be very close to that of gaseous hydrogen sulfide. The isotopic property of aqueous sulfide ions is determined by the relative concentrations of the three species H₂S, HS^-, and S^2-, all of which have different reduced partition function ratios. Therefore, the pH of hydrothermal solution is as important a parameter as the temperature in determining the isotopic ratio of a sulfide mineral. δS* of pyrite, for instance, in isotopic equilibrium with a solution at 500°K having a δS of 0.0‰, would be +5.0‰ at pH 9.5, +2.0‰ at pH 8.0, and 0.0‰ at pH 4.0. Some natural examples are discussed which may indicate a change with time in the pH as well as in the isotopic raito of hydrothermal solutions during mineralization. The isotopic exchange reaction between sulfide and sulfate is also discussed.