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 δ34S values of sulfides from the greenstones and Cu ores are in the range of 2.10 to 7.11‰, 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.