Vein Formation and Deformation in Greenstone Gold Deposits
Notice bibliographique
Résumé
Abstract Veins are common components of greenstone gold deposits. Their analysis is one key aspect in understanding the sequence of events leading to the formation or deformation of gold deposits. This analysis is essential for the determination of controls on mineralization and ore-forming processes, and for the prediction of the geometry and plunges of deposits and orebodies. Many greenstone gold districts have experienced a common structural evolution: D1 thin skin-style shortening and D2 thick skin-style shortening are largely responsible for the structural trend and penetrative fabrics in a district, whereas D3 and D4 transcurrent deformation are largely focused along preexisting major fault zones. A majority of greenstone gold deposits consists of quartz-carbonate veins in or adjacent to high-angle reverse, and less commonly transcurrent, shear zones, viewed as splays or subsidiaries of major, complex, belt-scale fault zones. In other deposits, veins simply overprint gold mineralization and provide important information about the postore deformation history. Three main types of veins occur in greenstone gold deposits and each records small increments of bulk strain. Laminated fault-fill veins form by slip along the central parts of active shear zones in low-angle di-lational bends, or less commonly by extensional opening of foliation planes. Extensional and oblique-extension veins form within or adjacent to shear zones, at high angles to foliation and elongation lineation. They represent opening and filling of extensional and hybrid extensional-shear fractures, respectively. In more competent host rocks, extensional veins can form arrays of en echelon planar or sigmoidal veins, or of stacked planar veins, and can also combine into multiple sets to form stockwork and breccia bodies. Multiple types and sets of auriferous veins commonly combine to form variably complex vein networks, especially in large deposits. These vein networks record deposit-scale bulk incremental strain, with axes of elongation and shortening that can be compared with those of the main deformation increments in the district as a further way of constraining their timing of formation. The formation of vein networks in many districts is compatible with D2, and in a number of others with D3, reflecting their formation in contractional or transcurrent deformation regimes, likely involving subhorizontal compressional stress under high fluid pressures. Veins in many districts also systematically display evidence of overprinting deformation, in the form of folds, boudins, striated vein margins, and a number of internal vein textures such as recrystallized quartz and stylolites. Overprinting deformation is a natural consequence of vein formation in active shear zones, but it can also result from overprinting of veins by a younger increment of regional deformation. This can lead to local shear zone reactivation or wholesale folding or boudinage of a deposit. The confident determination of the structural timing of veins in deposits is critical but challenging, and is at the center of divergences of interpretation of the origin of many greenstone gold deposits. A number of guidelines are offered to help distinguish pre-orogenic veins and deposits from those with syn- to postorogenic timing.
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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 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,002 | 0,001 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 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 ».