The Mineralogy, Geochemistry, and Metallurgy of Cobalt in the Rhombohedral Carbonates
Carbonate ores of cobalt are a significant but under-recognized fraction of the global Co resource. Cobalt forms spherocobaltite (CoCO3, calcite group), whose complete solid solution with isostructural magnesite, MgCO3, is described here for the first time. Cobalt-rich dolomite, Ca(Mg,Co)(CO3)2, and Co-rich calcite, (Ca,Co)CO3, can accommodate up to 20 mol.% Co and up to 2 mol.% Co, respectively. Cobalt has also been reported as a minor substituent of other calcite-group carbonates and as a major constituent of the non-rhombohedral carbonates comblainite, Ni4Co3+ 2 (OH)12(CO3)·3H2O (hydrotalcite supergroup), and kolwezite, (Cu,Co)2(CO3)(OH)2 (poorly understood, possibly rosasite group). Cobalt carbonates are most common in the supergene zones of Cu-Co sulfide ore deposits, especially the Central African Copperbelt. A study focused on the Tenke-Fungurume district (TFM) in the Copperbelt found Co-rich dolomite, Co-rich magnesite, spherocobaltite, and kolwezite. Cobalt-rich dolomite occurs as Co-rich bands in supergene dolomite and as individual
Co-rich dolomite crystals filling void spaces. Members of the magnesite-spherocobaltite solid solution occur as crystals filling void spaces in rocks and as microscopic inclusions with kolwezite in supergene chalcocite (Cu2S) replacing primary carrollite (CuCo2S4). The formation of Co-rich carbonates remains enigmatic. Evidence from Bou Azzer indicates that they can form under specific hypogene conditions, but in general Co-rich carbonates form from supergene processes. Dedolomitization has been proposed as a mechanism of formation for the analogous Zn carbonates, but there is no evidence of dedolomitization in the TFM cobalt carbonates. Most of them appear to have precipitated directly from pockets of Co-(Mg)-(Cu)-carbonate-enriched solution trapped within oxidizing hypogene sulfides. Cobalt carbonates pose a serious metallurgical problem. Most carbonate ores are processed by solvent extraction using acid. Solubility calculations indicate that the Co in carbonates is less soluble than Mg, Fe, and Ca by 3 to 4 orders of magnitude. Thus, acid leaching will liberate all other ions from carbonate ores before releasing appreciable Co. Furthermore, many of the Mg-rich spherocobaltites in this study were initially misidentified as Co-rich dolomite, which is far more soluble than spherocobaltite. This may cause Co recoveries to be lower than predicted at many Central African Copperbelt mines.