These deposits are primarily worked for copper with cobalt as a by-product. They account for over 50% of world cobalt mine production. Major deposits are characterised by ore minerals contained within organic-rich pyritic shales and sandstones deposited in a near-shore lagoonal environment, under reducing conditions. These mineralised sediments are always found above oxidised terrestrial clastic sediments. The largest deposits are found within sequences deposited during the Upper Proterozoic (approximately 1500 to 600 million years ago) and Permian and Triassic periods (300 to 200 million years ago) due to the large areas of arid and semi-arid environments present at the Earth’s surface at these times.
Two of the largest and most well known deposits of this type are the European Kupferschiefer and the Central African Copperbelt.
Central African Copperbelt
The Central African Copperbelt is the world’s most important cobalt resource. It runs more than 500 kilometres across north-western Zambia and the south-east part of the Democratic Republic of Congo and is estimated to contain about six million tonnes of cobalt (Cailteaux, 2004), generally at grades between 0.17 per cent to 0.25 per cent cobalt. The mineralisation processes are not fully understood but the most widely accepted model of ore genesis is that metal concentration began with erosion of Palaeoproterozoic and Archaean basement terrains rich in cobalt, copper, nickel and gold and the influx of these sediments into a basin.
Metals were further concentrated by evaporation in an arid climate. Subsequently during a rise in sea level, oxidising mineralising brines from hyper-saline lagoons were mixed with interstitial reducing water, rich in organic compounds. The onset of a reducing environment within the sediments triggered crystallisation of syngenetic and early diagenetic copper and cobalt sulphides from sea-water sulphates (Cailteaux, 2004). Some of these deposits were later reworked by metamorphic processes, further increasing metal grades.
The mineralised sediments are all tidal and intertidal facies, principally dolomites, dolomitic shales, black shales and sandstones.
The European Kupferschiefer also contains major sediment-hosted base metal deposits. They are rich in copper and silver, with cobalt contents typically 20-30ppm and 100-300ppm in cobalt-enriched zones. The Upper Permian (around 250 to 256 million years ago) Kupferschiefer extends from north-east England across the North Sea to Poland with most deposits located in central Germany and Lower Silesia, Poland. Other important sedimentary cobalt deposits include Mount Isa, Australia and Kilembe, Uganda which are both also worked for their gold content (Smith et al., 2001).
Reprinted with kind permission from the British Geological Survey. BGS © NERC. ALL Rights Reserved. 2016