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

Abstract

In 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.

Cobalt distribution in the Boleo stratiform sediment-hosted ore deposit: a transtension rift basin in the central Baja California peninsula, Mexico

“…Moreover, hydrothermal mobilization of cobalt can lead to ore grade concentration if the ore fluids are saline and oxidized. Cobalt can effectively be deposited as a sulfide mineral in lower temperatures in response to a sudden decline in fugacity (Vasyukova and Williams-Jones, 2022). 415…”

“…However, it is not unreasonable to think that rift-transitional basalt and basaltic andesites rocks of Cerro Juanita (9.66 +-0.21 Ma) and Cerro Sombrero Montado (8.84 +-0.16 Ma) (Busby et al, 2020), may have also provided additional heat source since deprivation of mantle lithosphere by rifting or the opening of a slab window can last 420 millions of years (Busby et al, 2020). Furthermore, a rift setting can induce migration of hydrothermal brine fluids by compaction upward and outward from rifts, causing leaching of mafic rocks with a high degree of partial melting and cobalt enrichment due to fractional crystallization (Ochoa-Landín, 1998;Vasyukova and Williams-Jones, 2022). In addition, saline water composition can control the maximum concentrations of copper-cobalt with sediments derived from basalt provenance (Haynes and Bloom, 1987).…”

Metal-rich organic matter and hot continental passive margin: drivers for Devonian copper-cobalt-germanium mineralization in dolomitized reef-bearing carbonate platform

“…In the context of a hot, extensional passive margin (Hitzman 1986 ; Hitzman et al 1986 ), we discuss whether short-term transgression-regression pulses (Shalev and Yechieli 2007 ) in a warming Frasnian climate (Joachimski and Buggisch 2003 ; Joachimski et al 2009 ) could have been an additional contribution to the up-and-down flux (Shalev and Yechieli 2007 ) of mineralizing brines in dolomitized carbonaceous limestone. Trangression-regression cycles induce “fluid pumping” that causes fluid pressure changes and modifies the flow of oxidized, evaporitic residual brines in marginal and rift basin (Vasyukova and Williams-Jones 2022 ), i.e., recession of sea level may cause brine discharge whereas rise of sea level makes brines sink through faults and migrate through tilted blocks (Shalev and Yechieli 2007 ).…”

“…1 ), glacio-eustasy is non-existent and third-order sea level changes of 15–35 m in the Middle to Late Devonian are documented (Witzke 2011 ; Smith et al 2019a , b ; Kabanov and Jiang 2020 ) although sea-level changes up to 90–145 m from the late Middle Devonian (Givetian) to Late Devonian (Famennian) may have been possible (Witzke 2011 ; Ver Straeten et al 2019 ). Therefore, in this context and considering the “fluid pumping” model (Shalev and Yechieli 2007 ; Vasyukova and Williams-Jones 2022 ), we conceptualize that succeeding transgression-regression cycles at the climax of Devonian greenhouse conditions may have contributed to the dynamics of flow of pulsed mineralizing fluids for sediment-hosted Cu-Co sulfide mineralization in reef-bearing carbonate platform.…”

Sphalerite as a non-traditional critical metal source: Correlative microscopy (EPMA, EBSD, and APT) of cobalt-enriched sulfides from the sediment-hosted copper-cobalt Dolostone Ore Formation deposit, Namibia

“…The decomposition of linnaeite is also supported by the presence of cobaltpentlandite inclusions within the host pyrite and accessory subhedral cobaltite overgrowing linnaeite (Results 4.1 and Bertrandsson Erlandsson et al, 2022). Thermodynamic stability modeling of Co-S-O and Co-S-O-As has demonstrated that decreasing fS 2 and/or fO 2 conditions will result in the transformation of linnaeite → cobaltpentlandite → cobaltite (Vasyukova and Williams-Jones, 2022;Williams-Jones and Vasyukova, 2022). As sphalerite and chalcopyrite formed during the decomposition of linnaeite, i.e., elevated rather than decreased fS 2, it is more likely that a decrease in fO 2 is responsible for breakdown of linnaeite and the subsequent formation of sphalerite, chalcopyrite, cobaltpentlandite, and cobaltite.…”