Ferromanganese nodules from the Central Indian Ocean Basin

Abstract

In order to delineate a mine site for ferromanganese nodules, extensive surveys were conducted in Central Indian Ocean Basin. Mapping of the basin by multibeam swath bathymetry (Hydrosweep) has revealed many new bottom relief features. These features have a strong influence on the movement and distribution of water masses, sediment transport, chemical activity, and enrichment of metals in the nodules and their distribution on the seafloor. Size of the nodules ranges from less than 2 cm to above 10 cm in more than 13 morphological facies in the basin. The highest abundance of nodules is generally associated with high relief areas like valleys, followed by hilltops and slopes. Abundance is least in abyssal plains. Nodules from the plains have the highest contents of Mn, Cu, and Ni, while those from hilltops have the lowest concentrations of these metals. There exists an inverse relationship between nodule abundance and their grade. Nodules are associated with practically all types of sediment, and their surface density varies from traces to more than 20 kg/m sup(2) in the basin. Genetically, three types of nodules are descernible in the basin - Type A, enriched in Mn, Cu, Ni, and Zn are concentrated in siliceous sediments. Type B, enriched in Fe and Co, are normally associated with red pelagic clays and are formed by accumulations mainly from sea water. These are represented by larger-sized nodules with smooth surfaces and containing grad-MnO sub(2) as the principal mineral phase. The third, Type AB, are mixed phase and represent both processes. Rare earth elements in the nodules are contained in a single authigenic phase comprised of Fe, P, and Ti sup(230)Th and sup(230)Th sub(exe) sup(232)Th measurements on nodules from the siliceous ooze region show an accumulation rate of 1.2 mm/my for the nodule tops and 1.9 to 3.2 mm/my for the bottoms