Isotope Geochemistry of the Northeast Zone, Mount Polley Alkalic Cu-Au-Ag Porphyry Deposit, British Columbia: A Case for Carbonate Assimilation
Bibliographic record
Abstract
Mount Polley is a Late Triassic (~205 Ma) alkalic porphyry Cu-Au-Ag deposit (226.3 thousand tonnes (t) Cu, 21.5 t Au, and 65.1 t Ag), hosted by silica-undersaturated to silica-saturated monzonitic intrusions of the Mount Polley Complex, located in British Columbia, Canada. The Northeast ore zone at Mount Polley is hosted by magmatic-hydrothermal breccia. Copper and precious metals occur in sulfide minerals primarily as coarse- to fine-grained breccia cement. Local wall rocks include equigranular to porphyritic diorite, monzodiorite, and monzonite. Alteration, breccia cement, and veins of the Northeast ore zone formed in five paragenetic stages: prebreccia (stage 1), brecciation and main-stage mineralization (stage 2), late-stage mineralization (stage 3), unmineralized postbreccia dikes and veins (stage 4), and epithermal-style veins (stage 5). Intense pervasive K and Fe metasomatism calcite and calc-silicate alteration occurred prior to brecciation caused by the intrusion of megacrystic K-feldspar-phyric monzonite. Stage 2 fluids were silica undersaturated, high temperature (>350C), CO2 enriched, and of near-neutral to alkaline pH. Potassic, sodic, and calc-potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4. Evidence for more acidic and lower-temperature conditions is preserved in stage 5 veins. The δ34Ssulfide isotope compositions of stages 2 and 3 chalcopyrite, pyrite, and bornite range from −7.1 to +1.4. Sulfur isotope compositions of anhydrite and gypsum are mostly between 6.2 and 9.8. These values, together with the presence of hematite, are consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid. Limited sulfur isotope geothermometry indicates that Cu sulfides precipitated at temperatures from ~480 to ~250C. Hydrothermal calcite occurs in all paragenetic stages at Mount Polley. Calcite δ13C values range from −0.2 to −10.5, and δ18O values from 4.0 to 20.9. The enriched C-O isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid. Interaction of the fluid and/or magma with limestone is considered a likely process to explain the C and O isotope signature. Lead isotope data suggest mixing of mantle and crustal sources during mineralization. Main-stage chalcopyrite and pyrite as well as late-stage galena have 206/204Pb values of 18.77 to 18.92, 207/204Pb of 15.56 to 15.59, and 208/204Pb of 38.22 to 38.32. Strontium isotope data (0.703310.70371) provide evidence of a strongly depleted mantle source of Sr with minor crustal input. Epsilon Nd values for main-stage apatite range between 5.9 and 6.5, also indicating a depleted mantle source. Stage 5 carbonate 206/204Pb values of 18.96 to 19.04, 207/204Pb of 15.57 to 15.59, and 208/204Pb of 38.26 to 38.36 suggest superposition of an epithermal system onto the Northeast ore zone, potentially as late as ~100 m.y. after breccia formation. The data presented are consistent with the hypothesis that the silica-undersaturated alkalic Mount Polley Complex formed due to carbonate assimilation prior to mineralization. This process can explain the δ13C-δ18O isotope data, calcite precipitation concurrent with Cu-Au mineralization, and silica undersaturation of the magma. The CO2 released during assimilation of carbonate also could have promoted magmatic-hydrothermal brecciation. Silica-undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate-bearing substrate.
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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 | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".