The nature and origin of the bonanza-grade Brucejack epithermal Au-Ag deposit, northwestern British Columbia
Bibliographic record
Abstract
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
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.005 | 0.001 |
| Scholarly communication | 0.002 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.005 | 0.001 |
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".