Geochemical features of Fe-Mn micronodules in deep-sea sediments of the western North Pacific Ocean: Potential for co-product metal extraction from REY-rich mud

Rare earth element and yttrium (REY)-rich mud around Minamitorishima Island, Japan, in the western North Pacific Ocean has received significant attention as a novel REY resource. In REY-rich muds, industrially critical metals such as Co, Ni, and Mo occur in relatively high concentrations. We show that Fe-Mn micronodules separated from REY-rich muds are significantly enriched in these elements; we measured Co contents of almost 3000 ppm and Ni contents that exceed 39,000 ppm. Through experiments, we demonstrated that reductive chemical leaching can be used to extract major portions (generally >80%) of the metals associated with Fe-Mn (oxyhydr)oxides from REY-rich muds. The Fe-Mn micronodules were of an oxic diagenetic origin. A simple mass balance calculation indicated that the contribution of the Fe-Mn micronodules to the bulk Fe-Mn (oxyhydr)oxides in the sediment can amount to 8%–50% (median: 18%). The other metals associated with Fe-Mn (oxyhydr)oxides seem to be hosted in Mn-microparticles, probably of a hydrogenetic origin. We estimate that the most promising site for REY-rich mud development contains (9.3 ± 0.6) × 10² t, (1.6 ± 0.1) × 10³ t, and (2.8 ± 0.2) × 10² t of leachable Co, Ni, and Mo, respectively, in 1 km² × 0–10 m below seafloor. These amounts account for 0.66 ± 0.04%, 0.061 ± 0.004%, and 0.097 ± 0.007% of present annual global productions of Co, Ni, and Mo, respectively. The feasibility of extracting these metals as co-products during REY-rich mud development depends on the cost-efficiency of the recovery process, which ultimately depends on their market price in the near future.