Occurrence and enrichment of cobalt in ferromanganese nodules from the Western Pacific

Abstract

Ferromanganese (Fe–Mn) nodules are an important potential Co resource, but the correlation between Co, Fe and Mn in Fe–Mn nodules is poorly understood. To investigate the occurrence and enrichment of Co in Fe–Mn nodules, we undertook bulk geochemical, TESCAN integrated mineral analyzer (TIMA), and sequential extraction procedures (SEP) of Co-rich nodules from the Western Pacific. In these nodules, the high Co contents are associated with Fe enrichment in the nodule layers. The Co is hosted mainly by amorphous Fe–Mn (Feox1) extracted in step 2 of the sequential extraction procedure. The pure Mn oxide phases (without Fe) and crystalline Fe oxide phases (Feox2 extracted in step 3 of the SEP; with almost no Mn) are not the carrier minerals for Co. In general, our study highlights the dual association of Co with Mn and Fe phases. The Co-rich nodules from the Western Pacific are typical hydrogenetic nodules, which consist mainly of Fe-vernadite. The Fe enrichment favored the formation of Fe-vernadite, which can adsorb positively charged, colloidal hydroxides of Co3+. As such, the Fe enrichment facilitated higher Co contents. This study provides new insights into Co enrichment in Fe–Mn nodules and can guide future mineralization models of deep-sea Co resources.