A reactive in-situ crystallization origin for the UG2 Chromitite of the Bushveld Complex, South Africa?
Bibliographic record
Abstract
The UG2 Chromitite of the ~2 Ga Bushveld Complex (South Africa) is one of the largest platinum-group element (PGE) deposits on Earth. It is ~1 m thick and can be traced for virtually the entire circumference of the eastern and western lobes of the Bushveld. Despite its economic importance and a plethora of studies devoted to understanding its petrogenesis, there is no consensus on how the UG2 body formed (see recent studies by Robb & Mungall, 2020; Latypov et al. 2023; Maier & Barnes, 2024). Models invoking fractional crystallization, in situ crystallization, gravity settling within crystal-rich slurries and crustal contamination have all been proposed. Recent studies on chromitite formation in other layered intrusions have demonstrated significant mineral chemical and textural disequilibrium features associated with chromitite layers, attributed to a reactive origin for these bodies (Hepworth et al. 2020). This study aims to test the hypothesis that reaction between incoming magma and anorthositic footwall triggered crystallization of some or all of the massive UG2 Chromitite. Specifically, we focus on microtextural variations at the mm-to-cm-scale across silicate to massive chromitite contacts in the UG2 and associated leader seam from a drill core sampled on the Magazynskraal farm in the western lobe of the Bushveld intrusion. We have carried out petrographic analysis and quantitative textural approaches including crystal size distribution (CSD), dihedral angle measurements, as well as mineral chemical and in-situ 87Sr/86Sr laser ablation inductively coupled-plasma mass spectrometry (LA-ICPMS) in plagioclase samples leading up and into the base of the UG2 Chromitite. We also studied materials at the contacts of the overlying leader seam. Petrographic observations reveal disequilibrium textural relationships in the footwall pyroxenite; e.g., chromite is typically separated from orthopyroxene by thin (µm-scale) rims of plagioclase. Interstitial plagioclase is chemically zoned and the anorthite content of the plagioclase shows a distinct increase (An58 to An92) near chromite. CSD analysis yields mainly log-linear plots suggesting in-situ crystallization with some evidence for postcumulus textural modification. Apparent chromite-chromite-plagioclase dihedral angle measurements reveal median values (for a given ~1 cm thick interval) of 65-75° and suggest that textural equilibrium has not been achieved. The 87Sr/86Sr variability points to the interaction between isotopically distinct signatures at the postcumulus stage. Our combined observations suggest disequilibrium between mineral phases and hint at reactive (dissolution-reprecipitation) crystallization in the UG2 Chromitite. Robb & J. Mungall (2020). Earth and Planetary Science Letters 534, 116084. Latypov et al. (2023). Lithos 460–461, 107374. D. Maier & S.-J. Barnes (2024). The Canadian Journal of Mineralogy and Petrology 62(5), 731-745. Hepworth et al. (2020). Nature Geoscience, 13(5), 375–381.
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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 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| 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".