ABSTRACT

Sulfide samples (mostly chalcopyrite with some sphalerite and bornite) from the Ryusei vein of the Akenobe mine exhibit a narrow spread in δ³⁴S value ranging from -4.4 to -0.4‰. The range is lower than those of other Japanese sulfide deposits so far studied. The frequency diagram of δ³⁴S values for chalcopyrite indicates that the δ³⁴S distribution is bimodal; one peak at -3.0 to -2.5‰ and the other at -1.5 to -0.5 ‰. This suggests that chalcopyrite in the Ryusei vein was formed in at least two different stages of mineralization. Of 9 sphalerite-chalcopyrite pairs studied (including 2 pairs from different veins), 8 pairs are isotopically in disequilibrium; sphalerite is up to 2.7 ‰ lighter than coexisting chalcopyrite. From the isotopic and chemical equilibrium relationship between the aqueous and mineral species of the Cu-Fe-S-O system, δ³⁴S values of the ore-forming solutions were speculated to be rather similar to those of chalcopyrite (0∼-5 ‰).