Petrology, geochemistry, and U-Pb (zircon) age of the quartz-feldspar porphyry dyke at the Lake George antimony mine, New Brunswick: implications for origin, emplacement process, and mineralization
Bibliographic record
Abstract
A steeply west-dipping, north-northwest-trending, biotite-bearing quartz-feldspar porphyry dyke is associated with the Lake George granodiorite, a cupola of the Pokiok Batholith. These Early Devonian intrusions are spatially and temporally related to W-Mo-Au and Sb-Au mineralization in the area. The porphyry dyke yielded an age of 420.8 +5.9/-4.0 Ma (U-Pb zircon) and is inferred to be coeval with or slightly older than the Lake George granodiorite (412 +5/-4 Ma, U-Pb zircon). The variably altered dyke contains subhedral to anhedral (rounded) quartz, plagioclase, and orthoclase phenocrysts (0.5 to 30 mm; 30 to 60 vol. %), and numerous angular to subrounded xenoliths of altered intrusion carapace and quenched dyke rocks, as well as local Kingsclear Formation metasedimentary rocks. The dyke is similar to the Lake George granodiorite in phenocryst population, volcanic arc (I-type) affinity, average Zr (157 ppm), TiO2 (0.54 %), La/Yb ratio (13), and REE (129 ppm). The average Au content is lower in the dyke (20 ppb) than in the granodiorite (32 ppb), the abundance of gold is related to minor magmatic hydrothermal chalcopyrite and pyrrhotite (< 2 %). Based on textural evidence, including autobrecciation, coupled with rheological and thermal modeling calculations for the 5 m-wide dyke, it is suggested that fluidization (vapour exsolution) of the magma was key to its rapid emplacement as a quenched glass-gas mixture (tuffisite). The S abundance and S isotopic signature of the dyke, relative to Au and Cu abundance, support reduced I-type magmatic volatiles involved in dyke emplacement and cogenetic autometasomatic alteration. RÉSUMÉ Un dyke de porphyre quartzo-feldspathique renfermant de la biotite, orienté vers le nord-nord-ouest et s’inclinant abruptement vers l’ouest, est associé à la granodiorite du lac George, une coupole du magmatisme du batholite de Pokiok. Ces intrusions régionales du Dévonien précoce sont spatialement et temporellement apparentées à une minéralisation de W-Mo-Au et de Sb-Au dans le secteur. Le dyke de porphyre a accusé un âge de 420,8 +5,9/-4,0 Ma (U-Pb sur zircon) et on le suppose du même âge ou légèrement plus âgé que la granodiorite du lac George (412 +5/-4 Ma, U-Pb sur zircon). Le dyke altéré de façon variable abrite des phénocristaux hypautomorphes à allotriomorphes (arrondis) de quartz, de plagioclase et d’orthoclase (0,5 à 30 mm, 30 à 60 % en vol.) ainsi que de nombreux xénolites angulaires à subsphérique d’une carapace d’intrusion altérée et de roches de dykes noyées, en plus de roches métasédimentaires locales de la Formation de Kingsclear. Le dyke s’avère analogue à la granodiorite du lac George du point de vue de la population de phénocristaux, de l’affinité avec l’arc insulaire (de type intrusif), de la concentration moyenne de Zr (157 ppm), du TiO2 (0,54 %), du ratio de La/Yb (13) et des ÉTR (129 ppm). La teneur moyenne en Au est plus faible dans le dyke (20 parties par milliard) que dans la granodiorite (32 parties par milliard); son abondance est apparentée à la présence restreinte de chalcopyrite et de pyrrhotite hydrothermales magmatiques (< 2 %). Selon les indices texturaux, notamment l’autobréchification, conjugués à des calculs de modélisation rhéologique et thermale du dyke (5 m), on peut supposer que la fluidisation (exsolution à l’état de vapeur) du magma a contribué à sa mise en place rapide sous forme d’un mélange de verre-gaz (tuffisite) noyé. L’abondance de S et la signature isotopique du S du dyke, comparativement à l’abondance d’Au et de Cu, appuient la participation réduite d’éléments volatils magmatiques de type intrusif à la mise en place du dyke et à l’altération autométasomatique cogénétique. [Traduit par la rédaction]
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".