Manganese crusts hydrothermal formed

Two types of metal-rich hydrogenous deposits are formed on the seafloor iron-manganese oxides and polymetallic sulfides. The iron-manganese oxides have been deposited as nodules, sediments, and crusts. They are enriched in various trace elements, such as manganese, iron, copper, cobalt, nickel, and zinc, making them a significant repository for some of these metals. Most of the metals in the polymetallic sulfides are of hydrothermal origin. These sulfides have been deposited as metalliferous sediments aroimd hydrothermal systems and as rocks that infill cracks within former... 

The best known forms of these precipitates are Bog ore, hydrothermal incrustations of manganese and iron, desert hamish, marine manganese nodules and crusts and marine red or brown clay. 

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