ABSTRACT

Re-Os dating method was applied to the Kuroko ores from of the Wanibuchi Mine, southwestern Japan. Two whole-rock Kuroko ores and two yellow color fragments separated from one Kuroko ore sample were analyzed. Their Re and Os concentrations range from 3.52 to 50.6 ppb and from 20.1 to 92.9 ppt, respectively, and their ¹⁸⁷OS/¹⁸⁸Os ratios vary from 0.911 to 1.609. These data yield an isochron age of 18.44 ± 0.60 Ma with initial ¹⁸⁷Os/¹⁸⁸Os of 0.621 ± 0.013. The age is older than the inferred Kuroko formation age of northeastern Kuroko deposits, and also about three Ma older than the time of the clockwise rotation of southwestern Japan (ca. 15 Ma). The Wanibuchi Kuroko deposits are considered to have formed at the waning stage of the first phase (28–18 Ma) of the rifting and seafloor spreading in the Japan Sea area. The initial ¹⁸⁷Os/¹⁸⁸Os ratio might reflect the contemporaneous local seawater ratio that was more affected by mantle-derived Os resulted from the volcanic activities of Japan Sea formation than the typical oceanic Os ratio.