ORIGIN OF PLATINUM-GROUP MINERALS IN MAFIC AND ULTRAMAFIC ROCKS: FROM DISPERSED ELEMENTS TO NUGGETS
Bibliographic record
Abstract
Based on the studies of paragenetic assemblages of platinum-group minerals (PGM) from placers and chromitite ores associated with zonal-ring dunite intrusions in the Aldan Shield, on the Siberian platform, a novel model for the PGM formation is offered, which implies separation of a Cr-rich oxide liquid with significant concentration of the platinum-group elements (PGE) from a picritic magma. The presence of PGM alloys in sulfide-poor chromitite ores from layered intrusions and zonal gabbro–dunite massifs indicates that PGE mineralization is genetically related not just to sulfide formation but to chromite formation as well. A hypothetical (MgO + FeO + CaO) – SiO2 – (Cr2O3 + Fe2O3 + Al2O3) triangular diagram shows that fractional crystallization of olivine results in the enrichment of the residual melt in Cr, PGE, a range of metals, and easily fusible and volatile elements, and in magma separation into a light silicate-rich liquid and a dense PGE–Cr-rich oxide liquid. In fluid-saturated magma, a Cr-enriched oxide melt completely separates from a silicate melt, its crystallization temperature is lower, and the solubility of PGE is higher. It is likely that the presence of PGE in the Cr-rich oxide liquid takes the form of sybotactic domains, clusters surrounded by ligands of easily fusible and volatile elements, which prevents the PGE from removal via an untimely crystallization from the melt as micronuggets. Crystallization of a PGE-, Cr-rich oxide melt can be traced in the schematic chromite – Os – Pt ternary diagram proposed, which shows a wide field of immiscibility existing between chromite and the PGE. Owing to the low solubility of PGE in chromite, crystallization of the Cr-rich melt results in accumulation of PGE in the interstitial liquid. With IPGE prevailing over PPGE, the crystallization trend does not reach the immiscibility field, ending in synchronous crystallization of chromite and small grains of Pt–Ru–Ir–Os alloys. In case of Pt dominance, the crystallization trend reaches the immiscibility field, Pt gradually accumulates in the residual melt, with final crystallization occurring at a triple eutectic point, which creates favorable conditions for the formation of large Pt nuggets from tens of grams to a few kilograms in weight, which are characteristic only of PGE–chromite deposits of the Uralian–Alaskan and Aldanian types.
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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 | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 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".