2015 年 49 巻 2 号 p. 65-78
Besshi-type deposit is a Cu–Zn (±Ag±Au) volcanogenic massive sulfide deposit occurring worldwide in the accretionary complexes, which is derived from seafloor mineralization related with a mid-ocean ridge volcanism. There are numerous Besshi-type deposits including the type locality in the Japanese accretionary complexes and they are one of the main Cu producers for the Japanese industry until 1980s. Despite of the long research history over 100 yr, the depositional environment and geological reason why Besshi-type deposits are unevenly distributed in the Japanese accretionary complexes are still uncertain due to the lack of direct age constraints for the timing of Besshi-type sulfide deposition on a paleo-seafloor. Here, we report the Re–Os isochron ages of Besshi-type deposits in the Sanbagawa, Northern Shimanto and Hitachi Belts. Eleven typical Besshi-type deposits in the Sanbagawa Belt yielded Re–Os isochron ages around 150 Ma (148.4±1.4 Ma from the composite isochron) in the Late Jurassic time. Since the depositional site of these Besshi-type deposits is truly pelagic, voluminous massive sulfide deposition/preservation in the Late Jurassic was considered to be triggered by intense hydrothermal activity with a concomitant Late Jurassic oceanic anoxic event. The three Besshi-type deposits in the Northern Shimanto Belt have been dated to be Late Cretaceous to Paleogene, which was closely associated with the Kula Ridge subduction beneath the Japanese Island. The Re–Os isochron age of the Fudotaki deposit in the Hitachi Belt was 533±13 Ma, indicating that the Hitachi Fudotaki deposit is the oldest ore deposit in the Japanese Island and was produced by bimodal volcanic activity on a back-arc area between the China block and the subduction belt of the Panthalassa Ocean.
