Sulfur isotope geochemistry of Besshi-type deposits

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The deposit of the Tsuchikura mine, located near Lake Biwa, occurs in an Early Jurassic non-metamorphosed accretionary complex that contains basaltic volcanic rocks, probably of Carboniferous age, and Carboniferous to Triassic chert. The Tsuchikura deposit consists of three deposits : the Eastern, Middle and Western deposits. Each deposit consists of a number of lenticular or fusiform orebodies. The size of the orebodies is very variable, ranging from pebble to 20,000 tons. Sulfur isotope ratios of sulfide minerals in the ore specimens from the deposit of the Tsuchikura mine are distributed in a range from -0.8 to +5.4*, mostly from +0.5 to 3.0*, close to the previous data (from +1 to +3*) by Miyake and Sasaki (1980). The distribution range is also similar to the major Besshi-type deposits of the Sambagawa terrain. Sulfur isotope fractionation between coexisting pyrite and chalcopyrite is not uniform. Of 11 pairs, 5 pairs show the normal fractionation (that means the delta^<34>S value of pyrite is higher than that of chalcopyrite), whereas 6 pairs show the reverse fractionation. This may be largely due to spatial rearrangement of the initial orebodies to the present form. Sulfide minerals mostly pyrite from host rocks (siltstones, sandstones, basaltic rocks) exhibit remarkably low delta^<34>S values ranging from -49.3 to -2.6*, distinctly different from the delta^<34>S values for sulfide minerals of ores. The low delta^<34>S values may indicate the bacteriogenic origin of host rock sulfides, although it seems peculiar that basaltic rocks also have extremely low delta^<34>S values. Ridge basalts usually have sulfur isotope ratios around 1* or so. The sulfides in the present samples may have been migrated from the nearby sediments. More detailed discussion will be done in near future, when sulfur isotope data are obtained for rock specimens of low sulfur contents.