Fine scale study of major and trace elements in the Fe-Mn nodules from the South China Sea and constraints on their formation processes

The marine Fe-Mn polymetallic nodules contain relatively high concentrations of Mn, Cu, Ni, Zn, Co and rare earth elements plus yttrium (REY), with a growing economic potential interest in their exploitation. To determinate the metallogenic processes and occurrence phases of the economic metals in the Fe-Mn nodules, we have undertaken high resolution mineralogical and geochemical studies of Fe-Mn nodules collected from the South China Sea (SCS).
The whole-rock mineralogical and chemical compositions of the SCS Fe-Mn nodules indicate hydrogenetic origin. The Mn mineral phases mainly are composed of nanocrystalline vernadite with interlayered 10 Å and 7 Å phyllomanganates, such as todorokite, birnessite, and buserite. Fe(-Ti) oxides/hydroxides are intergrown and essentially X-ray amorphous feroxyhyte and goethite. But we recognize two main types of internal microlayers in the SCS Fe-Mn nodules: Layer type A of suboxic diagenetic precipitates with extremely high Mn/Fe ratio and concentrations of Cu, Ni, Zn, Ba, Li and Mg; Layer type B of oxic hydrogenetic accretions with low fractionation of Mn and Fe and high contents of Co, REY, Ti, Sr and Pb. Furthermore, the elemental mapping indicates that the enrichment of Co and REY mainly associated with Fe mineral phases rather than Mn mineral phases, which are enriched in Mg, Cu, Ni, Zn, Li and Ba. Two mineralization processes and distributions of metals in the individual microlayers respectively are controlled and occurred by the different mineral phases. The increasing occurrence of 10 Å and 7 Å phyllomanganates present in the Layer type A are typically enriched in trace metals such as Ni, Cu, Zn, Li, Ba, and Mg, whereas the metals associated with the Layer type B include Co, Ti, Pb, Sr, REY, which might be carried by the intergrown of Fe(-Ti) oxyhydroxides and vernadite. Thus, hydrogenesis is more beneficial to the enrichment of Fe, Co, Ti, Sr, Pb and REY, while diagenesis is more favorable for the enrichment of Mn, Ni, Zn, Cu, Li, Ba and Mg during the metallogenic processes of the SCS nodules.