Fractionation of sulfur isotopes and selenium between coexisting pyrite and chalcopyrite from the Hitachi deposits, Ibaraki Prefecture, Japan

抄録

Sulfur isotopes and selenium/sulfur ratios were determined for coexisting pyrite and chalcopyrite from the Kieslager-type deposits of the Hitachi mine, Ibaraki Prefecture, Japan. All pyrite-chalcopyrite pairs analyzed exhibit the normal isotope fractionation, i.e., ³⁴S is more enriched in pyrite than in chalcopyrite. Selenium is more enriched conversely in chalcopyrite than in pyrite. The mean δ³⁴S differences between pyrite and chalcopyrite are 0.84±0.44‰ (11) for the ore bodies of the epidote-amphibolite facies and 0.54±0.24‰ (12) for those of the amphibolite facies. It seems too optimistic, however, to regard the observed differences in the mean value between the two facies as indicating the difference in metamorphic temperature between the two facies, because the magnitude of isotope fractionation is rather variable even within the single ore bodies. Partition of selenium between pyrite and chalcopyrite was found to be virtually the same for both the facies. The mean distribution coefficients of selenium, D_cp-py, are 4.26±0.85 (12) for the epidote-amphibolite facies and 4.44±1.60 (13) for the amphibolite facies. Near-equilibrium trend in sulfur isotope fractionation and rather consistent partition of selenium between pyrite and chalcopyrite strongly suggest that selenium equilibrium is also approached between the two minerals in the Hitachi deposits.