Re-Os Geochronological Evidence for Multiple Paleoproterozoic Gold Events at the Scale of the West African Craton
Notice bibliographique
Résumé
The Archean-Proterozoic craton of West Africa hosts numerous gold deposits, which are spatially and temporally related to the Eburnean orogeny that took place between 2250 and 1980 Ma, and included multiple deformation events. The majority of these gold deposits are located along shear zones. The structural history is relatively well established for most gold deposits, but absolute timing of the mineralization is commonly lacking. Five deposits hosted in the Baoulé-Mossi domain, located in the southwestern part of the West African craton, were studied to better constrain timing of gold events: Nassara and Kiaka in Burkina Faso and Wassa, Damang, and Obuasi in southern Ghana. Gold mineralization was structurally characterized at each deposit, and dated by the Re-Os method on pyrite, arsenopyrite, and pyrrhotite grains that were coeval with the gold deposition. Combined structural and Re-Os geochronological constraints allow two groups of gold deposits to be distinguished. Early orogenic gold formed during the Eoeburnean orogeny, i.e., between 2190 and 2125 Ma, as represented by Kiaka1 at 2157 ± 24 Ma, and Wassal at 2164 ± 14 Ma. These early gold occurrences are expressed as early disseminated gold enrichment (1-3 g/t Au) at Kiaka, and gold hosted in folded and boudinaged quartz veins that are transposed along the primary S1 foliation at Wassa. These Re-Os ages on the early sulfides at Wassa and Kiaka are significant because they definitively provide the first direct age constraints on gold-only Eoeburnean mineralization in the Birimian of West Africa. Late orogenic gold, hosted by brittle structures and with higher gold grades (up to 60g/t at Kiaka), formed during late Eburnean deformation (D3-D5 events) between 2120 and 2000 Ma, as represented by the Re-Os dating of these ores at Wassa at 2055 ± 18 Ma and Obuasi at 2045 ± 40 Ma. Similar-style ores dated at Nassara at 2131 ± 99 Ma and Damang at 2080 ± 135 Ma, but both with large uncertainties, and an undated younger event at Kiaka, are also suggested to be part of this later mineralizing episode. The high uncertainties on the Nassara and Damang ages may be directly linked to the low rhenium and osmium contents of the studied samples. These results highlight the polyphase character of the widespread gold mineralization in the West African craton. Although the late stages of the Eburnean orogeny constitute a prolific period for the formation of highgrade gold mineralization, identification of less well-studied early-stage gold deposits, which can also contain large quantities of gold, is critical for mineral exploration in the West African craton.
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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 | 0,000 | 0,001 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,002 | 0,002 |
| Études des sciences et des technologies | 0,001 | 0,001 |
| Communication savante | 0,001 | 0,001 |
| Science ouverte | 0,000 | 0,001 |
| Intégrité de la recherche | 0,000 | 0,000 |
| 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 ».