On sediment-starved deep ocean floors, generally 4 to 5.5 kilometres deep, concretions rich in manganese, cobalt and nickel can form around small fragments of debris and organic material. The nodules concentrate minerals from siliceous oozes and from the water column. Their occurrence in proximity to mid-ocean ridges suggests a volcanogenic source for mineral enrichment. They may occur as densely packed sheets covering areas of hundreds of square kilometres with cobalt grades in the range of 1 to 2.5 per cent.
Cobalt-rich crusts form in a similar way with minerals precipitating out of sea-water, possibly with the aid of bacterial activity, into crusts up to 25 centimetres thick. Cobalt-rich crusts form at shallower depths (around 0.4 to 4 kilometres) than manganese nodules on the flanks and summits of seamounts (ISA, 2007). Accretion rates for cobalt-rich crusts and nodules are some of the slowest natural processes on the planet and can be as slow as one to six millimetres of growth every one million years.
It is estimated that the resources of cobalt in these nodules and crusts may be comparable with land-based resources. However, due to political uncertainties regarding ownership of the oceans and problems associated with extraction in deep-sea water, these resources have so far remained uneconomic.
Reprinted with kind permission from the British Geological Survey. BGS © NERC. ALL Rights Reserved. 2016