Successfully determined formation age of the oldest ore deposit in the Japanese Island ー Re-Os isotope dating revealed formation age of the Hitachi deposit ー


Dr. Tatsuo Nozaki and Dr. Katsuhiko Suzuki, Research and Development (R&D) Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Asahiko Taira, President), and Prof. Yasuhiro Kato, School of Engineering, The University of Tokyo, successfully determined the primary formation age of the Hitachi deposit (copper-zinc-lead sulfide deposits classified into volcanogenic massive sulfide) in the Ibaraki Prefecture. To directly date the constituent sulfide minerals included in sulfide ores, the Rhenium-Osmium (Re-Os) isotope dating method was applied to them and revealed that the primary formation age of the Hitachi VMS deposit is estimated to be the Cambrian (485.4 to 541 million years ago), not Early Carboniferous (323.2 to 358.9 million years ago) as before. The Hitachi VMS deposit is, therefore, classified as the oldest dated ore deposit in Japan.

Our results support the ideas of previous studies, indicating that there is a large age gap (hiatus) in the geologic record of 150 million years and a wide distribution of the Cambrian beds in the Hitachi area. It is expected to provide an important age constraints to reconstruct the initial tectonic history of the Japanese Islands.