Abstract: Fluid inclusion evidence for an epithermal-porphyry link at the Mount Milligan porphyry Cu-Au system, Quesnel Terrane, British Columbia, Canada
Bibliographic record
Abstract
Based on detailed field studies, genetic links have been made between high sulfidation epithermal Au and porphyry Cu-Au systems but a connection between low sulfidation epithermal and porphyry systems is less clear, owing partly to the lack of preservation of fluid inclusions in the transitional environment between these two deposit styles. Recent studies have demonstrated that epithermal mineralization may be precipitated by the cooled, aqueous vapour phase that is produced by boiling of metal-rich intermediate density fluids in the porphyry environment. At the Mount Milligan porphyry system of central British Columbia, Canada, mineralized andesitic and trachytic volcanic rocks occurring distally to the main Cu-Au porphyry zones contain abundant alteration veins that form narrow stockworks. The veins are comprised of quartz ± carbonate ± pyrite ± chlorite ± tourmaline and are younger than the porphyry stage mineralization based on cross-cutting relations. Bulk analyses of mineralized rock have yielded two distinct trends of mineralization. Primary and pseudosecondary fluid inclusions in massive quartz and freestanding quartz crystals lining the walls of the alteration veins occurring in the volcanic host rocks to the porphyry system. The inclusions contain a two-phase aqueous fluid (L + V) at room temperature. Microthermometric analysis of 60 inclusions from 6 different veins indicate fluid salinities ranging from 4.2–14.6 NaCl wt% eq (n = 60) and homogenization temperatures (to liquid) between 117 and 270 °C. Individual assemblages representing single veins show much narrower ranges in salinity and homogenization temperature. The homogenization temperatures in individual veins are consistent with the temperatures of chlorite crystallization in the veins determined by Al-in-chlorite compositional thermometry (by electron microprobe). Analyses of single inclusions from the veins by LAICPMS (ETH Zurich) show that the low to moderate salinity fluids contained extremely high dissolved concentrations of B, As, and Sb. Ratios of As/Sb in the inclusions overlap closely with bulk ratios in the associated epithermal mineralization in the volcanic rocks. However, the inclusions contain unusually low concentrations of metals expected to be mobile as chloride complexes in hydrothermal solutions (e.g., Pb, Zn, Fe, and Ag) compared to saline fluids documented in other Cu-Au porphyry systems. The inclusions may represent a condensed vapour phase resulting from early boiling since boiling is a viable mechanism to fractionate chloride-complexed metals (into brine) from bisulfide-complexed metals (into vapour). Fluid inclusion assemblages in alteration veins at the Mount Milligan porphyry system preserve evidence of the transport of a lowto moderate-salinity, aqueous fluid with chemical characteristics consistent with a contracted magmatic vapour phase that may have mixed at a shallow level with meteoric water causing the precipitation of pyrite-rich low sulfidation epithermal veins. Considering the distinct metal concentrations in the fluid inclusions, the tourmaline and anomalous As and Sb contents in bulk rocks, or elevated B, As, and Sb in fluid inclusions in apparently barren alteration veins in prospective volcanic terranes in the Canadian Cordillera may indicate proximity to alkalic porphyry-style mineralization or associated epithermal systems.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".