Constraints on the Genesis of Cobalt Deposits: Part II. Applications to Natural Systems

Abstract

AbstractIn a companion paper in this issue, the authors reviewed the properties of cobalt, its mineralogy, and the processes that concentrate it to exploitable levels. Using this information and knowledge of the geology of the principal types of cobalt deposits, the present paper assesses the conditions and controls of cobalt transport and deposition and develops/refines plausible models for the genesis of these deposits. Economic cobalt deposits owe their origins to the compatible nature of Co2+, which causes it to concentrate in the mantle, mainly in olivine, and to be released to magmas only after high degrees of partial melting (i.e., to komatiitic and basaltic magmas). Thus, there is a very close association between cobalt deposits and mafic and ultramafic rocks. Magmatic deposits, in which Co is subordinate to Ni, develop through sulfide-silicate liquid immiscibility as a result of the very strong preference of these metals for the sulfide liquid. Predictably, these deposits reach their highest grades where hosted by olivine-rich ultramafic rocks. Approximately 60% of the world’s cobalt resource is of hydrothermal origin and is contained in sediment-hosted copper deposits in the Democratic Republic of the Congo. Using a combination of thermodynamic data and geologic information, we have refined a model in which Co is leached from mafic and ultramafic rocks by oxidized, chloride-rich hydrothermal fluids, derived from evaporation, and deposited in response to decreasing fO2 in carbonaceous sediments that accumulated in intracratonic rift basins. Economic Co deposits also develop as hydrothermal vein systems, in which Co is the primary ore metal. In the only deposits of this type that are currently being exploited (Bou Azzer, Morocco), the source of the Co was an adjacent serpentinized peridotite. The ore fluid was an oxidized, high-salinity brine derived from evaporites, and deposition occurred in response to pH neutralization by the felsic to intermediate igneous host. The final major class of Co deposits is laterite-hosted and develops on olivine-rich ultramafic rocks or their serpentinized equivalents. Our thermodynamic modeling shows that Co is leached from an ultramafic source by mildly acidic fluids as Co2+ and is transported down the laterite profile, eventually concentrating by a combination of adsorption on Mn oxides, incorporation in the structure of absolane (an Mn oxide), and precipitation as heterogenite (HCoO2). The dissolution of cobalt at the surface and its deposition at depth are controlled mainly by pH, which decreases downward; oxygen fugacity, which also decreases downward, has the opposite effect, inhibiting dissolution of cobalt at the surface and promoting it at depth. It is our hope that this introduction to the economic geology of cobalt and the processes responsible for the formation of cobalt-bearing ores will help guide future studies of cobalt ore genesis and strategies for the exploration of this critical metal.