Constraints on the Genesis of Cobalt Deposits: Part II. Applications to Natural Systems
Notice bibliographique
Résumé
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.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,003 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,001 | 0,001 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,001 | 0,003 |
| Communication savante | 0,002 | 0,002 |
| Science ouverte | 0,001 | 0,001 |
| Intégrité de la recherche | 0,001 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,008 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».