Geochemistry of oceanic ferromanganese deposits

Abstract

Deposits of mixed manganese and iron oxides, with high concentrations of minor metals, cover large areas of the deep-sea floor. They occur as nodules , over a very wide depth range, but most abundantly on the abyssal sea floor in water depths between 4 and 5 km, and as unconsolidated sediments , rocks and crusts , which are restricted to areas of the world-wide active ridge system. The two types of deposit have different chemical compositions and are the products of different precipitation and accretion mechanisms on the abyssal sea floor. Ferromanganese nodules grow very slowly, they generally have Mn/Fe ratios greater than or equal to 1, and they contain high levels of Ni, Cu, Co, Ba, Pb, Zn and Mo. Regional, inter- and intra-ocean variations in composition are marked. Such variations can be easily mapped throughout the Pacific and are most plausibly explained by a combination of the variety of metal sources on the sea floor (including abyssal water, biological debris and perhaps volcanism), and by the different environmental conditions under which the nodules form. Some of the vai lability in the contents of Ni and Cu, two of the metals for which the nodules are considered a valuable resource, can be interpreted as a reflection of intensive metal recycling and diagenetic reaction at the sediment surface. Iron- and manganese-rich sediments, rocks and crusts accumulate quite rapidly, they have highly variable Mn/Fe ratios, and they contain a different suite of minor metals, generally at lower concentrations, compared with ferromanganese nodules. Compositional variations in this case are on the scale of an individual specimen as well as regional, although clear trends are not evident at the present time. They are thought to form by precipitation from the reaction products of newly extruded basalt and seawater; isotopic evidence also indicates that there is significant adsorption of metals from seawater by the poorly crystalline oxides produced by this reaction.