Always the bridesmaid, never the bride: cobalt geology and resources

Abstract Cobalt is one of the least familiar metals and with a crustal abundance of only 25 ppm it is also one of the rarest. With the exception of the Bou Azzer mine current production is entirely a secondary product of copper and nickel mining. Thirty-four cobalt minerals have been recognized—principally sulphides, selenides, arsenides, sulpharsenides, carbonates, sulphates and arsenates. The main ore minerals of cobalt are the sulphides cobaltite, linnaeite and carrollite and the hydrated oxide asbolane. Cobaltiferous pyrite is a further source. Workable deposits generally contain 0.1-0.4% Co and belong to one of four geologically distinct types: (1) sediment-hosted—largely Precambrian—typified by the copperbelts of Congo Democratic Republic and Zambia, which since the 1970s have contributed between 25 and 50% of the world's mine production; (2) mid-Tertiary to Recent nickel-rich lateritic deposits generated by tropical and subtropical weathering of peridotitic rocks, most notably in New Caledonia, Cuba and Australia; (3) primary magmatic Ni-Cu sulphide concentrations, such as Sudbury, Noril'sk, Voisey's Bay and Bushveld; and (4) a more diverse group attributable to hydrothermal and volcanogenic processes, the most important of which in terms of current and future production are the ophiolite-hosted Co-As deposit at Bou Azzer, Morocco, and the epigenetic Cu-Au-Co concentrations of the Idaho Cobalt Belt, U.S.A. The last group also includes the formerly important Outokumpu-type massive sulphide and five-element (Ni-Co-Ag-As-Bi), vein-type deposits. The currently quoted figure for global cobalt reserves of 4 500 000 t may be overstated since it does not take into account the effects of political uncertainty in the Congo Democratic Republic. However, even at proposed increased levels of production (70 000 t /year, against the current figure of 28 300 t/year) reserves have a life of at least 60 years. Some potential exists in the longer term for cobalt extraction from offshore reserves of manganese nodules.