Japan Oil, Gas and Metals National Corporation (JOGMEC) reported ore reserves calculation results for two seafloor hydrothermal deposits in addition to discoveries of seven new deposits and evaluation results for rare earth elements (REE) contained in seafloor mud since 2014.
As to seafloor hydrothermal deposits consisting of sulfide minerals, footwall of the ore bodies are altered pyroclastic rocks and siltstone accompanied by clay minerals of chlorite, illite, kaolinite, smectite and sulfate minerals. The assay results of vertical drill holes penetrating ore bodies indicate that copper is rich in the lower part, whereas lead, zinc, gold and silver are rich in the upper part. There are differences between the Hakurei site with pyrrhotite in back-arc basin and the Gondou site without pyrrhotite in volcanic front. For example, sulfur fugacity of the formation environment of the Gondou site is similar to a known kuroko deposit, but the fugacity of the Hakurei site is relatively lower.
As to REE in other types of marine deposits such as manganese nodule, cobalt-rich crust and REE-bearing mud, REE contents normalized by seawater show positive anomalies of cerium in common. However, there are differences among them as follows. Positive correlations between the contents of P2O5 and total REE exist for REE-bearing mud and manganese nodule, but the correlation is not present for cobalt-rich crust and trend of the positive correlations are different between REE-bearing mud and manganese nodule.
