THE LEAD ISOTOPE COMPOSITION OF ORE MINERALS FROM PRECIOUS METAL-BEARING, POLYMETALLIC VEIN SYSTEMS IN THE COBALT EMBAYMENT, NORTHERN ONTARIO: METALLOGENETIC IMPLICATIONS
Bibliographic record
Abstract
The Cobalt Embayment is a large (~60,000 km 2 ), irregular domain of Paleoproterozoic (2.5–2.2 Ga) siliciclastic sedimentary rocks (i.e., the Huronian Supergroup) that unconformably overlies Archean basement rocks of the Abitibi greenstone belt. The Nipissing Diabase, regionally distributed complex of mafic sills and dikes intruded the Huronian sedimentary rocks ca. 2.2 Ga. The sedimentary rocks were subsequently affected by subgreenschist facies metamorphism and by a regionally distributed K and Na metasomatic events, at ca. 1.7 Ga, related to the Penokean orogeny. Although best known for the economically important Ag-Co veins of the Cobalt mining camp, the Cobalt Embayment also hosts numerous other hydrothermal, base and precious metal-mineralized calcite-quartz vein systems. The lead isotope compositions of pyrite and chalcopyrite from nine widely separated polymetallic vein systems yield a Pb-Pb errorchron age of 2236 ± 180 Ma ( n = 24). In contrast, the Pb isotope compositions of late-stage galena ( n = 11) from six regionally distributed vein systems yield a Pb-Pb secondary isochron age of ca. 1675 Ma. These Pb isotope data for the mineralized vein systems demonstrate the occurrence of at least two major episodes of hydrothermal fluid activity throughout the Cobalt Embayment, ca. 2220 Ma and ca. 1700 Ma, indicate that the emplacement of the Nipissing Diabase was the most likely driver of regional hydrothermal fluid circulation at ca. 2200 Ma, and provide supporting evidence for the basin-wide extent of the regional K and Na metasomatic event at ca. 1700 Ma, most likely related to the waning stages of the Penokean orogeny.
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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 | 0.001 |
| 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".