ABSTRACT

Sulfur isotope data of the Kuroko-ores from the No. 1 deposits of the Shakanai mine reveal that the δ³⁴S value of sulfide minerals decreases gradually towards the top of the sequence of the stratigraphic and mineralogical zonation of the ores. For example, δ³⁴S values of pyrite show a gradual decrease towards the top of the sequence: +8.2 to +7.2‰ in disseminated ores and veins in the footwall rocks; +6.2 to +5.4‰ in pyrite-ores; +5.5 to +4.6‰ in yellow-ores; +5.6 to +3.0‰ in black-ores; +4.6 to -6.0‰ in hematite-quartz layers. δ³⁴S values of barite in the black-ores are fairly uniform, giving +22∼s+23‰. Sulfur isotope temperatures obtained from coexisting sulfides from different types of ores are fairly consistent, giving temperatures around 250°C (from temperature scales by KAJIWARA and KROUSE, 1970), which is in good agreement with the_ maximum value of a temperature range indicated by fluid inclusion and mineral equilibria studies (ISHIZAKA, 1968; KAJIWARA, 1968). Applications of equilibrium concepts to the observed mineralogy and sulfur isotope data of the ores give the following information: 1) The major source of the ore-forming sulfur was Miocene sea-water sulfate with a δ³⁴S value of around +20‰. 2) Total sulfur content of solutions responsible for the ore-deposition was probably around 0.01 mole/Kg -H₂O, which is very close to the value of the present-day sea-water. 3) The observed trend of decreasing δ³⁴S values of sulfide minerals towards the top of the sequence can best be explained by the increase in fo₂, pH, and total sulfur content of solutions due to the increasing contribution of sea-water during the ore-deposition. It is thus suggested that the stratigraphic and mineralogical zonation of the ores was also caused by the same mechanism.