2020 Volume 73 Pages 74-82
The development of seabed mineral resource has been attracting attention as there is a growing concern about the future supply of metal resources. Seafloor massive sulfide deposits, which bear high grade metals including zinc, lead, and copper are formed by precipitation of metal elements from seafloor hydrothermal fluid circulation systems. Many active seafloor hydrothermal systems have been located at around volcanic seamounts within Japanese EEZ, which strongly suggests high potential for metal resources on the seafloor in this area. The "Next-generation technology for ocean resource exploration" was conducted as an SIP Project of the Cabinet Office from 2014 to 2019 with a goal of technology development/transfer of systematic exploration method for ocean mineral resources. "Integrated Survey System for Concealed SMS Deposits" are established by combining various survey methods and "Survey protocol for seafloor massive sulfide deposits" are organized to define target areas using three-step approach; regional survey, semi-detailed survey and detailed survey. The exploration trials conducted in 2018 by private sectors were successful in identifying unknown subseafloor ore body based on the newly developed survey protocol. As another important operation, drilling campaigns using D/V Chikyu were conducted three times in active hydrothermal fields in the Okinawa Trough. Analyses of the obtained sediment cores and logging data revealed that occurrence of sulfide minerals is likely to be controlled by sedimentary strata, and that hydrothermally altered sediments of specific physical properties are distributed across wide region so that detected by geophysical exploration methods. Synthesis of these obtained results would be facilitated by comparison with knowledge from explorations of Kuroko-type VMSDs in northeast Japan. Progress in geophysical exploration technology is not only useful for effective mineral resource exploration but also applicable to get crucial information for understanding the genesis of seafloor massive sulfide deposits.
