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

Abstract

Abstract Many marine Fe-Mn polymetallic nodules may contain high concentrations of Mn, Cu, Ni, Zn, Co and rare earth elements plus yttrium (REY). To determine the occurrence and enrichment processes of these metals in the Fe-Mn nodules, we conducted high resolution mineralogical and geochemical studies on the Fe-Mn nodules collected from the South China Sea. In-situ EPMA and LA-ICP-MS analyses and elemental imaging were performed on individual microlayers of the nodules to reveal the major and trace element distributions. The whole-rock mineralogical and chemical compositions of the Fe-Mn nodules indicate a hydrogenetic origin. The Mn mineral phases are mainly composed of nanocrystalline vernadite with interlayered 10\u202fA and 7\u202fA phyllomanganates, such as todorokite, birnessite, and buserite. Fe(-Ti) oxides/hydroxides are intergrown and essentially X-ray amorphous feroxyhyte and goethite. We recognize two main types of microlayers: Type-A layer (suboxic diagenetic precipitates) containing a high Mn/Fe ratio and high concentrations of Cu, Ni, Zn, Ba, Li and Mg, and Type-B layer (oxic hydrogenetic accretions) with low fractionation of Mn and Fe and high contents of Co, REY, Ti, Sr and Pb. Furthermore, elemental mapping indicates that the enrichments of Co and REY are mainly associated with Fe mineral phases rather than Mn ones, which are enriched in Mg, Cu, Ni, Zn, Li and Ba. Two mineralization processes and metal distributions in the individual microlayers are identified in the Fe or Mn mineral phases. The occurrence of 10\u202fA and 7\u202fA phyllomanganates in Type-A layers are commonly enriched in trace metals such as Ni, Cu, Zn, Li, Ba, and Mg, whereas the metals for the Type-B layers include Co, Ti, Pb, Sr, REY, which may be carried by the intergrowing Fe(-Ti) oxyhydroxides and vernadite. Thus, hydrogenesis and diagenesis facilitate the enrichment of Fe-Co-Ti-Sr-Pb-REY and Mn-Ni-Zn-Cu-Li-Ba-Mg, respectively, during the mineralization of the South China Sea nodules.