Sulfur isotope geochemistry of Besshi-type deposits

土什库拉矿床位于琵琶湖附近，赋存于早侏罗世未变质的增生杂岩中，其中含有玄武岩火山岩，可能是石炭纪的火山岩和石炭纪至三叠纪的硅质岩。土仓矿床由三个矿床组成：东部、中部和西部。每个矿床由许多透镜状或梭形矿体组成。矿体的大小变化很大，从鹅卵石到2万吨不等。土仓矿矿石样品中硫化物矿物的硫同位素分布范围为-0.8~+5.4*，大部分为+0.5~3.0*，与Miyake和Sasaki(1980)的数据(+1~+3*)相近。其分布范围也与桑巴加瓦地体的主要别石型矿床相似。共存的黄铁矿和黄铜矿的硫同位素分馏不均匀。在11对矿物中，5对呈正向分馏(即黄铁矿的34&gt；S值高于黄铜矿)，6对呈反向分馏。这在很大程度上可能是由于初始矿体在空间上重新排列成现在的形式。硫化物矿物主要是赋存于围岩(粉砂岩、砂岩、玄武岩)中的黄铁矿，表现出极低的S值介于-49.3~-2.6*之间，与矿石中硫化物矿物的S值明显不同。尽管玄武岩也具有极低的Delta^&lt；34&gt；S值似乎很奇怪，但低的Delta^&lt；34&gt；S值可能指示了寄主岩石硫化物的细菌成因。山脊玄武岩的硫同位素比值通常在1*左右。目前样品中的硫化物可能是从附近的沉积物中迁移而来的。在不久的将来，当获得低硫含量的岩石样品的硫同位素数据时，将进行更详细的讨论。

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.