Distribution of sulfur isotopes in the Iwami Kuroko deposits, Shimane Prefecture, Japan

Abstract

Sulfur isotope study of the Iwami Kuroko deposits shows that sulfide minerals from the disseminated ores, the bedded ores, the mudstones and the gypsum ores have δ³⁴S values ranging from +1.2 to +5.5‰, from -0.8 to +5.4‰, from -7.7 to -1.29‰, and from -15.3 to +5.4‰, respectively. Pyrites from the bedded ores exhibit a wider spread in the δ³⁴S value (-0.8 to +5.4‰) than pyrites from the disseminated ores (+3.1 to +5.5‰), and chalcopyrites of the bedded ores have δ³⁴S values ranging from +0.9 to +2.2‰, which are lower than those for chalcopyrites of the disseminated ores (+2.1 to +4.6‰). δ³⁴S values for barites from the bedded ores and the mudstones and gypsums from the gypsum ores are fairly uniform, ranging from +19.5 to +23.2 ‰. Sulfur isotope temperatures calculated for sphalerite-galena pairs of the disseminated ores are fairly constant, giving the values of around 290°C, which are not so much different from 250°C obtained for most sulfide pairs from the Kuroko deposits in Northeast Japan (KAJIWARA and DATE, 1972). Other sulfide pairs including pyrite and/or chalcopyrite, however, give so much variable isotope temperatures. The observed isotopic distribution in the disseminated ores and the bedded ores may be explained by the assumption that pyrite and chalcopyrite were preferentially contaminated by biogenic sulfur during and/or after the ore deposition. Low and variable δ³⁴S values obtained for pyrites from the mudstones and the clay of the gypsum ores suggest that these pyrites are of biogenic origin.