Cobalt and nickel enrichment in ferromanganese nodules: insights from supergene Mn-oxides deposits in South China

  Cobalt and nickel are technologically critical metals used in rechargeable battery electrodes, superalloys for gas turbines and jet engines, catalysts, and resistant alloys. Numerous studies have demonstrated that cobalt and nickel often accumulate in ferromanganese nodules on the seafloor, but their enrichment behaviors in Mn-oxide nodules from terrestrial surface environments are poorly constrained. Large amounts of ferromanganese nodules occur in laterite profiles of the supergene Mn-oxide deposits that are widely distributed in South China. These nodules contain significant amounts of cobalt (10-1280 ppm, mean 308.58 ppm) and nickel (66-5000 ppm, mean 997.02 ppm) that can be potentially produced as a by-product.

  52 ferromanganese nodules from three supergene Mn-oxide deposits in South China have been studied by optical microscope, scan electron microscope, bulk chemical analysis, electron probe analysis, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine the minerals, textures, mineral association, and distribution of the cobalt-nickel in Mn-oxides. Petrographic observations demonstrate that Mn-oxides in the collected ferromanganese nodules consist mainly of hollandite-cryptomelane, pyrolusite and lithiophorite. All pyrolusite generally contain minor Co (10.96-526.68 ppm) and Ni (27.43-1253.56 ppm). The early lithiophorite grains occur as fine-grained to cryptocrystalline and contain high Co (560.11-3010.57 ppm) and Ni (3182.77-20135.47 ppm), while the late lithiophorite crystals are often coarse-grained and have relatively high Co (29.52-2302.38 ppm) and Ni (518.78-10906.86 ppm). The mixed hollandite-cryptomelane minerals contain relatively high Co (23.43-1311.12 ppm) and Ni (27.66-2294.98 ppm).

  In lithiophorite, Co and Ni primarily incorporation into a manganese layer sheet. They are also predominantly enriched in the octahedral of hollandite-cryptomelane structure, whereas Ni is probably incorporated in hollandite-cryptomelane to a lesser extent by forming outer-sphere complexes within the tunnels. Sharp chemical gradients across different Mn-oxide species, along with irregular reaction fronts, indicate that repeated leaching and reprecipitation are the main mechanisms for the incorporation of Co and Ni. The fractionation of these elements may be attributed to low f_O2 and high pH, which favor higher Co/Ni ratios in hollandite-cryptomelane. The lithiophorite with lower Co/Ni ratios precipitated which reflected an increase in Ni concentrations in fluids under conditions of high f_O2 and low pH. Differences in weathering intensity in South China likely contributed to the development of diverse Mn-oxide varieties, while local physicochemical changes influenced the partitioning of elements within individual deposits.