This deposit type groups together a wide range of deposit styles and mineral assemblages. The key process is precipitation from hydrothermal fluids passing through the host rock often sourced from, or powered by, volcanic activity. Ores can be found where minerals have been remobilised along fault planes, in veins, fissures and cracks, or as metasomatic replacement of host rocks. Some major examples of hydrothermal and volcanogenic deposits, most of which have been historically worked for cobalt, are listed in Table 3.
Table 3 Division and major examples of hydrothermal and volcanogenic cobalt deposits (Adapted from Smith, et al. 2001)
In all but one of these examples, cobalt is a by- or co-product of mining polymetallic ores.
Morrocan Bou Azzer Deposit
The Moroccan Bou Azzer deposit is a significant cobalt resource and hosts the only mine in the world where cobalt is produced as a primary product. Here cobalt arsenides were formed by serpentinisation of cobalt-rich peridotites in an ophiolite complex formed during the Pan-African deformation event (around 685 million years ago). These were further enriched by weathering processes and finally remobilised into economic concentrations during the Hercynian Orogeny. The metals were recrystallised and deposited into faults and fractures. The cobalt is principally found in the mineral skutterudite. High levels of copper, molybdenite and gold are also locally present at Bou Azzer.
Outokumpu District Deposit
The Outokumpu district in Sweden also produced cobalt as a primary product until operations ceased in 1994.
With Outokumpu-style deposits it is thought that economic concentrations of cobalt are created by the formation of a copper-rich proto-ore as a result of hydrothermal circulation at a mid-ocean ridge (Peltonen, 2006). This section of oceanic crust was thrust up over continental crust and the associated metasomatism caused the release of nickel- and cobalt-rich fluids. These mixed with the copper proto-ore minerals were further remobilised by regional deformation. Mineralisation is principally in the form of sulphides and cobalt can be found replacing nickel. There are 11 known deposits within the Outokumpu district with an estimated cobalt content of 500 000 tonnes (Smith, et al. 2001).
Reprinted with kind permission from the British Geological Survey. BGS © NERC. ALL Rights Reserved. 2016