Ferromanganese oxide concretions from the Indian Ocean exhibit considerable mineralogical and chemical variation, often differentiated on a regional basis. Nodules richest in Mn, Ni, Cu and Zn are found in sub-equatorial basin areas, but enrichments of some of these metals also occur in nodules elsewhere, particularly where there is evidence of major biogenic contributions to their underlying sediments. Fe, CO and Pb are most enriched in concretions from sea-mounts and the mid-ocean ridge system, but also attain high values in some basin areas. Local variations in concretion composition have been found off southern Africa, and are due to both biological and continental influences.

There are several major controls on the compositions of the concretions. Source variations influence composition in that Mn, Ni and Cu show evidence of diagenetic enrichment in deposits after being cycled through the biogenic component of sediments, whereas, Fe, CO and Pb appear to be supplied mainly by normal seawater. Environmental influences, including bottom water characteristics, affect the degree of dissolution of biogenic remains and this affects the sediment type and the supply of metals to the deposits and also their mineralogy, which in turn affects trace metal uptake. In the Central Indian Basin, source and environmental influences combine to produce ‘ore grade’ nodules.

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