Revisiting the type area VMS deposit of Besshi, SW Japan: In situ trace element chemistry, isotopes and Re Os age of sulfides
Abstract The Besshi Cu rich volcanogenic massive sulfide (VMS) deposit occurring in the type area of Besshi, central Shikoku, Southwest Japan comprises tabular orebodies closely associated with mafic schist in the Cretaceous Sambagawa high P/T metamorphic belt. Pyrite, chalcopyrite, and sphalerite are the dominant sulfide minerals, together with minor amounts of pyrrhotite and bornite. The associated greenstone samples show geochemical features between N MORB and E MORB. The d34S values of sulfides from the greenstones and Cu ores are in the range of 2.10 to 7.11%0, indicating deposition from hydrothermal fluids with a heterogeneous metal source. Re Os analysis of sulfides show initial (187Os/188Os)i ratio of 1.14 0.86 and yield an isochron age of 147.9 4.2 Ma, considered as the timing of sulfide deposition on the paleo seafloor. The lithological association, geochemical features of the host greenstones and isotopic data from the sulfides indicate that the Besshi type ores formed in a pelagic setting, and are possibly associated with hydrothermal activity related to mid ocean ridge volcanism and can be correlated to a major oceanic anoxic event.