GEOCHEMICAL JOURNAL

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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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The distribution coefficients of Sr and Ba were measured when Mg-poor calcite, Mg-rich calcite or aragonite was precipitated in the calcium bicarbonate solutions containing strontium or barium in trace amounts, MgCl₂, NaCl and/or Na-citrate. The abnormally high values of the distribution coefficients were observed very often at the early stages of carbonate precipitation. The conditions, under which such high values are measured, are investigated. And the use of distribution coefficients measured in the laboratory for interpretation of the trace element contents of marine carbonate skeletons is discussed.

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