Convergent margin Ni-Cu-PGE-Cr ore systems: U-Pb petrochronology and environments of Cu-PGE versus Cr-PGE mineralization in Alaskan-type intrusions
Bibliographic record
Abstract
Magmatic Ni-Cu-PGE deposits hosted by ultramafic-mafic intrusions in convergent margin or supra-subduction/ post-subduction tectonic settings are becoming an increasingly important global resource. In the northern Cordillera, this class of intrusions is restricted to the accreted arc terranes of the continental margin of North America where the Alaskan-type subclass is particularly prevalent. This report describes recent research on the Tulameen and Polaris Alaskan-type intrusions in British Columbia that contain orthomagmatic Ni-Cu-PGE mineralization. Preliminary U-Pb geochronological results and trace element analyses of zircon from the Tulameen intrusion are interpreted using an integrated petrochronological approach to distinguish crystallization from post-crystallization events and to constrain the duration of magmatism. Magmatic Cu-PGE mineralization (predominantly chalcopyrite-bornite) in the Tulameen intrusion is hosted by hornblende-magnetite-bearing ultramafic rocks near its periphery. Preliminary results suggest that emplacement and crystallization of the Tulameen intrusion occurred ca. 204-205 Ma (latest Triassic), coincident with a 6 million year period centred on 205 Ma that produced the most important porphyry Cu-Au deposits in British Columbia. The Cu-PGE mineralization constitutes a unique deposit type for Alaskan-type intrusions and has some attributes in common with stratiform or 'reef-style' mineralization in tholeiitic layered intrusions. Magmatic Ni-Cu-PGE sulphides (mainly pyrrhotite, chalcopyrite, minor pentlandite) in the Early Jurassic (ca. 186 Ma) Polaris intrusion are hosted by the more evolved ultramafic and mafic rocks. Remobilized ultramafic cumulates form a distinctive component of both intrusions: plastically deformed and dismembered olivine-chromite cumulates are widely distributed in dunite, and what are interpreted as synmagmatic avalanche deposits are well exposed at Tulameen. The remobilization of pre-existing cumulate sequences is rationalized in terms of an open-system magma recharge model that accounts for some of the key textural features exhibited by ultramafic rocks in Alaskan-type intrusions.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.002 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| 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 teacher head, 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".