Alteration mapping, mineral trace element chemistry, and sulphur isotopes: implications for gold mineralisation at the Snowfield Au-Cu porphyry deposit in NW British Columbia
Bibliographic record
Abstract
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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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 | 0.002 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 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".