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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Sulfur isotope study of the Kuroko-ores of the Shakanai No. 1 deposits, Akita Prefecture, Japan

Yoshimichi Kajiwara
Geochemical Journal, Vol. 4, No. 4, P. 157-181, 1971

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.

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