Paired Sulfur Isotopic Belts: Late Cretaceous-Paleogene Ore Deposits of Southwest Japan

摘要

Sulfides from late Cretaceous-Paleogene ore deposits of the Inner Zone of Southwest Japan were analyzed for ~(34)S/~(32)S ratios (delta ~(34)S_(CDT) values) on 5 samples from the Ryoke Belt, 18 from the Sanyo-Naegi Belt, 15 from the Uetsu- Kanto District and 47 samples from the Sanin-Shirakawa Belt. Together with the published data, the average delta ~(34)S values are calculated for individual deposits of the whole region; 109 localities for the ilmenite-series granitic belt and 56 localities for the magnetite-series granitic belt. The sulfur isotopic ratios vary regionally rather than with sulfide species or types and commodities of the ore deposits. The delta~(34)S values are generally negative in the fore-arc, ilmenite-series belt, and positive in the back-arc, magnetite-series belt. This regional variation is considered to reflect the ilmenite-/magnetite-series pairing of the genetically related granitic or volcanic activities, in which the positive delta~(34)S ore sulfur was derived from a deep igneous source, but the negative 6~~S sulfur was originated in biogenic sulfur of the accreted sedimentary complex. Within each terrane, delta~(34)S values and mineral commodities vary locally, which is called domains in this paper. A few negative delta~(34)S minima are observed in the Kinki, Hiroshima and Mino domains. These anomalies may have been brought by the most sediments-dominant granitic magmas generated within the accretionary sedimentary complex. The paired delta~(34)S belts, called the Japanese-type pattern, may be unique to island arcs where their fore-arc sediments have been accreted in the arc- front associated with the back-arc rifting.