Field of formation of Besshi-type deposits-Their geochemical characterization

点击翻译按钮获取中文摘要

Besshi-type deposits have hitherto been studied mainly on those in the Sanbagawa belt. They underwent regional metamorphism up to eipdote-amphibolite facies, making it difficult to unravel their geological and geochemical primary features. Namely, ore deposits in the Sanbagawa belt are suitable for study of metamorphic features. Measurements of sulfur isotope fractionation between pyrite and chalcopyrite from the Besshi-type deposits in the Sanbagawa belt and some other weakly and non-metamorphosd terrains have revealed that highly metamorphosed deposits have a more uniform fractionation than weakly metamorphosed deposits (Watanabe et al., in print). Our recent laser-ablation analyzes showed that delta<@D134@>D1S values of sulfides in non-metamorphosed deposit are similar to those of hydrothermal deposits associated with oceanic ridges (Watanabe et al., in prep). Measured Rb-Sr and Sm-Nd isotope ratios of pillowed basalts metamorphosed in pumpellyite-actinolite facies condition yielded a Rb-Sr whole-rock isochron age of 107 (]SY.+-。[) 15 Ma with an initial ratio of 0.70401 (]SY.+-。[) 0.00006. This age is regarded as the metamorphic age. Initial epsilon values at T=107 Ma suggest that the most likely source for the basalts is considered as depleted oceanic mantle with an oceanic island-type isotopic signature (Watanabe et al., submitted). K-Ar ages were determined for mineral separates, ranging from ca.60 to 112 Ma, with the highest mode of ca.80 Ma (Watanabe et al., ditto). The Besshi-type mineralization in the Inner zone occurred related to basaltic volcanism in a back-arc basin under the influence of an island-arc in Late Carboniferous to Early Permian times, while that in the Outer zone did associated with basaltic volcanism in an intra-oceanic plate in Late triassic to Cretaceous times (Watanabe et al., submitted).