Subaqueous ferromanganese deposits (FMD), except nodules, are commonly classified into hydro-genic and hydrothermal types. According to others, both types are formed as a result of coprecipi-tation of Fe-Mn colloids that absorb chemical elements, including nonferrous metals (NFM), from sea-water. It is assumed that the hydrothermal deposits should be depleted in NFM relative to the hydrogenic counterparts, because the duration of contact of colloids, which produce the hydrothermal deposits, with seawater is much shorter. To subdivide FMD, the diagram (Cu + Ni + Co)× 10 - Fe - Mn proposed by Bonatti et al.is widely used.
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