A series of scientific drilling at the submarine hydrothermal fields were conducted to decipher mineralization process of the hydrothermal deposits in the middle Okinawa Trough. By using LWD technique, in-situ measurements of temperature, flow-in of the fluid, electromagnetic nature in the boreholes were successfully achieved. Further, the physical properties of the drilled formations showed good agreement with those of cored samples analyzed on-board and shore-based.
Submarine ore-bodies recently attract broad interest among governmental, industrial, academic communities under circumstances that any activity directing sustainable development is desired. Following this stream of the trend, several countries and companies applied properties of submarine mining even in distant high-seas far away from the shorelines.
Thus in Japan, within the eleven governmental funded R&D projects under an umbrella of SIP, a project focused on submarine resources named as "Next-Generation Technology for Ocean Resources Exploration (Zipangu in the Ocean)" has been launched in 2014. This project not only focuses hardware developments to be utilized in exploration of the submarine resources but also does scientific studies of formation processes of submarine resources and assessment technique for ecological impacts. In the project, hydrothermal sulfide deposits is mainly targeted among three types of submarine resources besides Fe-Mn oxide crusts enriched in Co or REY-rich mud.
Even though such situation, there are still large arguments on the formation model of the submarine hydrothermal deposits (e.g. Tornos et al., 2015 and references therein), especially on those having large volumes of the consealed type, i.e. ore bodies buried beneath the seafloor.
Prior to the commencement of the "Zipangu in the Ocean" project, the IODP Expedition 331 scientific drilling was conducted in 2010. During the expedition, an occurrence of vast sub-seafloor hydrothermal fluid reservoir was suggested beneath the Iheya-North Knoll, Okinawa Trough (Takai et al., 2012). Further, a seismic reflection analyses suggests that such a reservoir may extend a kilometer area indicated as a reverse polarity reflector (Tsuji et al., 2012). This view is also supported by dense heat flow measurement on the Knoll (Masaki et al., 2011). In the expedition, the heterogeneous sub-seafloor nature beneath the hydrothermally active area prohibited the core samples from being retrieved in high yield, down to a few percent in the holes (Takai et al., 2012), which is critical shortcomings to establish the model of the ore genesis.
