Compositional Variations of Apatite, Fractionation Trends, and A Pge-Bearing Zone In the Kivakka Layered Intrusion, Northern Karelia, Russia
Bibliographic record
Abstract
Abstract The ∼3.5 km thick Kivakka complex (2.45 Ga) in northern Karelia hosts a zone of disseminated Pd-Pt-Ni-Cu mineralization near its base. The zone is related to a special or “critical horizon” and displays relative maxima in bulk-rock concentrations of Cu, Ni, Co, and Cr, in the ratios Cu:Ni, Cu:Zr, and 100Fe 2 O 3 /(FeO + Fe 2 O 3 ), and elevated levels of Cr in ortho- and clinopyroxene, along with higher levels of incompatible Ti, V, and Zr. Early crystallizing minerals (highly magnesian pyroxenes and An-rich plagioclase) are associated in this zone with 1–10% (commonly 3 to 5%) base-metal sulfides (BMS) and interstitial phases [nearly pure Ab, chlorapatite, zircon, Fe-(Ti) oxides, etc .]. Apatite is characterized by extensive variations in Cl (both intragranular and intergranular), which attain the composition of chlorapatite (>6 wt.% Cl), not observed below the zone. The compositional trend of the apatite series is extensive; the content of F generally increases upward, whereas Cl decreases, with the observed maximum (3.3% F) near the top of the intrusion. The PGE-rich zone is inferred to have formed at the “critical level” as a consequence of a combination of several factors. Factor 1 represents an abrupt change in the sequence and character of cumulus phases; a massive crystallization of cumulate An 75–81 is accompanied by a sharp decrease in the amount of orthopyroxene with cessation of crystallization of olivine. As a likely result, the remaining melt became substantially enriched in Cr. Alternatively, the Cr enrichment is related to an influx of a new portion of magma. Factor 2 represents a rise in levels of oxygen fugacity, f (O 2 ). Factor 3 is displayed by the sharp increase in Cl in the intercumulus apatite; the fractionated melt well above this zone appears to be depleted in Cl. The sudden drop in Cl is attributed to degassing of the melt, from which a Cl-rich volatile phase exsolved, causing fractionation of Cl from F. Factor 4 involves an increase in levels of sulfur fugacity, f (S 2 ), resulting in the deposition of the disseminated BMS in association with tiny grains of Pd-Pt bismuthotellurides and sperrylite. Thus, the formation of PGE mineralization appears to have proceeded in Kivakka as a sequence of processes and events, rather than as a result of a single factor or process during fractional crystallization of the complex.
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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 | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".