Alteration mapping, mineral trace element chemistry, and sulphur isotopes: implications for gold mineralisation at the Snowfield Au-Cu porphyry deposit in NW British Columbia
Notice bibliographique
Résumé
The Snowfield deposit is a gold-enriched porphyry deposit with an unusually high gold/copper ratio, located in Northwest British Columbia along a trend shared by the Kerr-Sulphurets-Mitchell (KSM) Cu-Au porphyry deposits and the high grade Brucejack gold deposit. Snowfield contains 25.9 million ounces of measured and indicated gold resources at a cut-off grade of 0.30 grams of gold-equivalent per tonne. It is interpreted to be situated on the hanging wall of the Mitchel thrust fault and to be the upper continuation of the Mitchell deposit. The deposit has been subdivided into a copper-enriched stockwork zone and a copper-poor gold zone. Hyperspectral alteration mapping from ground-based and aerial surveys are used to provide an improved framework to understand the Snowfield deposit. Interpolation techniques, reported in Chapter 2, have been applied to the hyperspectral data from the handheld device, and yielded mineralogical patterns and alteration trends similar to those of the aerial map and provides a new approach for alteration mapping. Higher AlOH wavenumbers which are concentrated over and around the Stockwork Zone at Snowfield, show that the alteration resulted from more acidic hydrothermal fluids, compared to the alteration in the surrounding area. The distribution of pyrophyllite is used to define an advanced argillic alteration zone, which forms around the stockwork veins and the Au-Cu mineralisation. The distribution of muscovite and phengite are used to interpret the extensive phyllic alteration (quartz-sericite-pyrite). Chapter 3 reports textural relationships among gold and associated sulfide and silicate alteration minerals, the composition of gold and pyrite, and sulphur isotopes across the deposit. Gold grains occur as inclusions in pyrite, on pyrite and quartz grain boundaries, and as disseminated grains in the sericite altered host rock; gold is associated locally with chalcopyrite, tennantite, and bornite. The Au:Ag ratio of gold ranges from 0.7 to 375, with most of the values between 1-20. Inclusions in pyrite typically have higher Au/Ag ratios (>10), whereas in the gold in textural equilibrium with chalcopyrite the ratios are consistently <6. Pyrite compositions, show variation in Cu, Au, As, Co, Ni, Ag, Sb, Tl, Pb, Zn and Bi. Small homogeneous pyrite grains and pyrite with inclusions of electrum have low concentrations of all trace elements except Ni, and Co, whereas larger pyrite aggregates and grains with distinct rims and cores defined by inclusions and variation in chemistry, typically show enrichment in all elements. Sulphur isotopes in pyrite range from 2.3‰ to -15‰, with most values around -3‰ and a second smaller group of values around -8‰. We propose a model in which reduced hydrothermal fluids deposited some gold initially in association with potassic alteration of a diorite porphyry in the Gold Zone at relatively high temperature. This was followed by the main, high temperature Au-Cu porphyry event, which was related to emplacement of the dioritic Sulphurets intrusions, and formed the Stockwork Zone. Fluid phase separation (boiling or condensation) is evident in the Stockwork Zone, and lead to the formation of a dense stockwork of veins and advanced argillic alteration. Initially, gold was deposited with copper in the Stockwork Zone as a result of cooling and destabilization of the chloride complex. As the fluid continued to cool, the pH decreased, the oxidation state increased, and gold transported as a bisulphide complex was precipitated in association with the extensive quartz-sericite-pyrite alteration in both zones. These events were recorded by the textures and trace element chemistry of pyrite, which provide tools for exploration regionally; distinct textures paired with unique trace element signatures can be used to fingerprint the different mineralisation events, and to vector to the source of these mineralisation centres.
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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 | 0,001 | 0,002 |
| Études des sciences et des technologies | 0,002 | 0,001 |
| 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 ».