Geology and geochemistry of ferromanganese nodules in the Japanese Exclusive Economic Zone around Minamitorishima Island

A dense field of ferromanganese (Fe-Mn) nodules was discovered on a seamount approximately 300 km east of Minamitorishima Island, in the Japanese Exclusive Economic Zone. To evaluate its potential as a resource for critical metals, we describe results of a geological survey using the SHINKAI 6500 submersible (dive No. 6K 1207) during cruise YK10-05 of R/V Yokosuka in 2010, and geochemical analyses of collected samples. Submersible observations showed that spherical nodules 5-10 cm in diameter almost fully cover the region of high acoustic reflectivity. The large nodules generally consist of three concentric layers: the outermost mottled (sediment-filled) layer L0, the massive black layer L1, and the innermost porous (sediment-filled) layer L2. Elements including Fe, Ti, Co, As, rare earth elements other than Ce, Th, U, and Pb are concentrated in the nodule rim rather than the center. In contrast, Mn, Al, P, Ca, Ni, Zn, Y, Mo, Ce, and W are concentrated in the center, and decrease toward the rim. Geochemical and structural features indicate that the nodules are compositionally and morphologically similar to Fe-Mn crusts, suggesting that they owe their origin solely to prolonged hydrogenetic precipitation of Fe-Mn-(oxyhydr) oxides. As the nodules include metals of economic interest, especially Co, Ni, Mo, and W, this deposit should have high potential for future mining. The changes in nodule composition from the center to the rim may yield information on paleoceanographic events since early Oligocene time.