The nature and origin of the bonanza-grade Brucejack epithermal Au-Ag deposit, northwestern British Columbia
Notice bibliographique
Résumé
Hydrothermal veins supply much of the Earth’s gold. Their propensity to contain ultra-high-grade, or ‘bonanza’, concentrations of gold that are millions of times greater than the concentration of gold in the Earth’s crust makes them important targets for exploration and resource development. The mechanisms by which such hyper-enrichment occurs are enigmatic. The accepted wisdom is that this enrichment reflects the saturation and precipitation of gold as crystals from hydrothermal fluids. Laboratory experiments and measurements of active hydrothermal systems, however, have shown that the solubility and concentration of this noble metal in typical ore-forming fluids are, respectively, exceptionally low. For example, gold concentrations in the fluids responsible for epithermal mineralisation are typically on the order of 10-30 ppb, which are far too low to explain concentrations of 10’s of thousands grams per tonne Au in veins in some deposits. The formation of such high-grade veins by direct precipitation of native gold or electrum from the ore fluids would require that individual fractures remain open for unreasonably long periods of time (e.g., >100,000 years) or that fluid fluxes be extraordinary. A potential solution to the paradox of ultra-high-grade gold ore formation, in a geologically realistic time-frame, is offered by the physical transport of gold in the solid state as electrically charged nanoparticles (i.e., colloidal suspensions) which flocculate to produce aggregates of gold. This study investigates gold mineralisation from the Valley of the Kings (VOK) and Eastern Promises (EP) zones of Brucejack, a large and exceptionally high-grade epithermal gold-silver deposit in British Columbia’s Stewart-Eskay District, to evaluate this possibility. Images obtained using transmitted electron microscopy show that: (1) gold commonly occurs as < 1-10 nm spherical nanocrystals of electrum embedded within a calcite matrix; and (2) larger (100-500 nm) particles of electrum, also embedded in calcite, are composed of hundreds of nanoparticles, each displaying distinct crystal lattice plane orientations. These images provide compelling evidence for the formation of colloidal suspensions of gold (electrum) and their flocculation. To better understand how flocculation occurs in a hydrothermal system, this study also examines the trace element and sulphur isotope signatures of pre- and rare syn-electrum pyrite at Brucejack. Secondary ion mass spectrometry (SIMS), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and electron microprobe wavelength-dispersive X-ray spectroscopy (EMP-WDS), reveal that the Brucejack hydrothermal system experienced a series of abrupt changes in fluid chemistry, which preceded and ultimately coincided with the onset of ultra-high-grade gold-silver mineralisation. These changes, which include the occurrence of extraordinarily negative δ34S values (e.g., -36.0‰) in zones of pre-electrum, auriferous, arsenian pyrite, followed by sharp increases of δ34S in syn-electrum zones of non-arsenian pyrite, were caused by vigorous, episodic boiling and subsequent inundation of the hydrothermal system by seawater. This influx of seawater was an essential step in creating bonanza electrum mineralisation by triggering the aggregation of colloidal gold suspensions through the addition of cationic flocculants and cooling
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Comment cette classification a été obtenuedéplier
Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| 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 | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 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 tête enseignante, 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 ».