Composition characteristics and genetic mechanism of ferromanganese crusts in the southern section of the Kyushu-Palau Ridge

Ferromanganese crusts (also known as cobalt-rich crusts or polymetallic crusts) are important strategic resources of minerals rich in key metals (Co, Cu, Mn, Ni, Ti, V, REE, Y, and Zn), which is essential for both human being’s daily life and high-tech industries. We conducted mineralogical and geochemical analyses of nine ferromanganese crust stations obtained from depths of 1900～2600 m in the southern section of the Kyushu-Palau Ridge, analyzed the mineral composition, major and trace element contents, and rare earth element (REE) contents of the ferromanganese crusts, and explored their genetic types. Results show that the mineral composition of the ferromanganese crusts was dominated by vernadite, and contained a large amount of amorphous iron oxide/hydroxide. The average contents of Mn, Fe, Co, Ni, and Cu were 16.15%, 15.38%, 0.32%, 0.33%, and 0.10%, respectively. The average CaO/P₂O₅ ratio was 5.93, indicating that the ferromanganese crust samples from the southern section of Kyushu-Palau Ridge had not undergone phosphatization. The ferromanganese crusts were significantly enriched in REYs, with an average content of 1194 μg/g. Light REEs were significantly enriched, and the Post-Archean Australian Shales (PAAS) normalized distribution pattern was relatively flat, showing a positive Ce anomaly and no Eu anomaly, and the REEs in the ferromanganese crusts may have originated from seawater. The mineral composition and genetic discrimination diagrams of the ferromanganese crusts indicated that they were of hydrogenetic origin and were not significantly affected by diagenesis.