Enrichment and constraints of critical metals in ferromanganese crusts from 13°20'N seamount of the southern Kyushu-Palau Ridge

Ferromanganese crusts are highly enriched in a wide variety of critical metals including Co, Cu, Mn, Ni, Ti, V, REE, Y, and Zn. Study of their enrichment in the crusts and the geological constraints is important for future development and utilization of them at seafloor. Recently, ferromanganese crust samples were acquired from 13°20′N seamount of the southern Kyushu-Palau Ridge, and analyzed in mineralogy and element geochemistry, as well as for electron probe microanalysis. Results show that the mineralogical and chemical composition of the samples are relatively uniform, and the crusts have not suffered from obvious phosphatization, which indicates that the crusts are characterized by one hydrogenetic crustal layer. Critical metals with high content such as Co and Ni are mainly enriched in vernadite. Co mainly exists in the lattice of vernadite due mainly to surface oxidation of vernadite. Ni is enriched in the crusts by replacing and occupying lattice vacancies of Mn as Co does, and a large amount of Ni exists in the form of adsorption. Ti, V, and REY are enriched in the iron oxyhydroxide components dominated by feroxyhyte by surface complexation, crystal lattice entry, and co-precipitation. Cu and Zn are lack of crystal lattice entry ability; the Cu content in seawater is very low and the adsorption of Zn is weak, thus resulting in their dispersed distribution and low content in the samples. This study reveals that the crusts started growing in the late Miocene and show no obvious growth break; the accumulated enrichment degree of critical metals in these samples is lower than that in the highest potential areas of the global ocean due to insufficient continuous growth time. However, the ideal water depth conditions, low deposition rate, stable tectonic environment, suitable water depth distribution of the oxygen minimum zone, and long distance from macroscale input of the non-metallogenic material into the study area are favorable for continuous growth and enrichment of critical metals in these crusts in the future.