Re-Os dating of the Kuroko ore deposits from the Hokuroku district, Akita Prefecture, Northeast Japan.

Abstract

Re-Os dating method was applied to the Kuroko ore deposits of the Hokuroku district, Japan. The Re and Os concentrations of four Kuroko, three Oko and one Keiko ores vary from 4.1 to 233 ppb and from 9.1 to 647 ppt, respectively, and their ^<187>Os/^<188>Os ratios range from 0.84 to 1.43. Most reliable isochron age is 14.32±0.51 Ma, which is consistent with previously reported ages based on stratigraphical, paleontological and K-Ar studies. The initial ^<187>Os/^<188>Os ratio of 0.7563±0.0047 falls within the range of the middle Miocene seawater values, suggesting seawater origin of the Kuroko Os.