研究業績

Although there is increasing recognition that long-lived magmatic-hydrothermal systems are essential components of large-scale mineralization, relatively little is known about the source processes that enable such activity, especially in areas of average crustal thickness. To address this issue, we investigated the sources of ore-forming elements and drivers of prolonged ore-forming activity (>1 million years) in the Toyoha polymetallic intermediate-sulfidation deposit, located at the northern end of the Northeast Japan Arc. We have presented Pb isotopic evidence that in the southeastern part of the deposit, where the hydrothermal activity lasted substantially longer (>1 million years) than the nearby (<10 km) volcanic activities, sulfide minerals from the ore had a specific metal source in addition to the wall rocks and the magmas observable as nearby extrusive rocks. The additional source was likely latent intrusions similar to the isotopically enriched volcanic rocks ~20 km southeast of the deposit, which incorporated abundant components from the subducting sediment. Based on regional geological, geochemical (major, trace elements and Sr-Nd-Pb isotopes) and geochronological data, we propose that seamount subduction induced the extra sediment input into the mantle wedge, which generated additional magma batches and helped to sustain magmatic-hydrothermal activity in the study area. The large tonnage, particularly of Pb, Zn, Ag, and In, in Toyoha was likely a result of the productive source, which enabled long-lived magmatic-hydrothermal activity, and an effective structure for the transport and trapping of ore-forming materials.