The chemical and mineralogical analyses of manganese-rich and iron-rich oxide crusts, of probable hydrothermal origin, from several locations on active spreading centers and seamounts in the Pacific and Atlantic indicate that these crusts originate from a uniform depositional process. These crusts are characterized by extreme Fe/Mn ratios, Fe and Si contents which co-vary, generally very low concentrations of most trace metals and the rare-earth elements, and variable contents of Zn and the volatile elements Hg, As, and Sb. Co is markedly depleted relative to Cu and Ni, and Zn is enriched, resulting in characteristically low Co/Zn values. These compositions can be accounted for by the fractional precipitation of Fe, SiO2, and Mn and incorporation of Zn, As, Sb, and Hg from hydrothermal solutions, and adsorption of minor amounts of Co, Cu, Ni, Pb, Ba, and rare-earth elements from sea water.

Mineralogical analyses by ultra-slow scan X-ray diffraction show the manganese-rich crusts to be composed of nearly pure, well-crystallized birnessite and/or todorokite. Iron-rich crusts consist of either amorphous hydrated iron oxide and silica or iron-rich, low-aluminum nontronite.

Analyses of a suite of thin ferromanganese crusts formed on ocean-ridge out-croppings of basalt and of a number of Pacific manganese nodules indicate that ferromanganese crusts are enriched in Fe and Si and depleted in trace metals relative to nodules and are similar in chemical composition to East Pacific Rise metalliferous sediments.

The similarity of the ferromanganese crusts to metalliferous sediment suggests that they have a common origin. It is believed that most of the Fe, SiO2 and Mn entering sea water in hydrothermal solutions precipitates as colloidal SiO2 and hydrated Fe and Mn oxides which are advected by bottom currents and deposited as crusts and sediments. Trace-element contents in these deposits result from adsorption from sea water onto the Fe and Mn colloids during advection.

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