The Petrogenesis of the PGE-Rich UG2 and UG3 Chromitites at Winnaarshoek, Eastern Bushveld Complex: Intrusion of Chromite Slurries Contained Within Pyroxenitic Sills
Bibliographic record
Abstract
Abstract Numerous layers of chromitite occur in the Bushveld Complex, South Africa, each of which contains stratiform, or reef-type, PGE mineralization. The only chromitite mined exclusively for PGE is the UG2, one of the upper group layers. Here, we investigate the petrogenesis of the UG2, together with the subordinate UG3, UG3a, and UG3b chromitites, at Winnaarshoek in the Eastern Limb. The four layers of chromitite (widths of 0.13–0.92 m) occur in a 50 m-thick sequence of pyroxenites, norites, and anorthosites, together with minor harzburgites. The silicate matrix in the UG2 and UG3 is dominated by plagioclase, rather than the orthopyroxene and minor olivine found in many chromitites, including the UG3a and UG3b. Microtextures of the chromitites have been severely impacted by prolonged periods of annealing and compaction, and we have found no evidence of the chromite having nucleated in situ or having accumulated from gravitational settling. Closely spaced electron microprobe analyses have revealed the composition of the chromite is within analytical error in the UG2 and UG3. The UG3a and UG3b layers reveal minor height-related changes in the composition of the chromite. The composition of the internal silicates is similarly tightly constrained, as are the Sr and Nd initial isotopic ratios for the silicate host rocks (SrR0: 0.70634 ± 0.00035; εNd: −6.9 ± 0.9). The igneous layering developed from successions of intrusive events, magmas having been sourced from a deep crustal staging chamber and intruded into the shallow crust as thin sheets. Intrusion rates were insufficient to enable a magma reservoir to develop. The norites are related to the batch crystallization of a relatively felsic basaltic magma, the anorthosites having formed in situ, from low degrees of partial melting and metasomatism. The pyroxenites accumulated from a relatively mafic basaltic magma, emplaced as out of sequence sills that exploited competency contrasts between the substantially crystalline layers of norite and anorthosite. The mafic basaltic magma entrained xenocrysts of chromite in the staging chamber, sufficiently abundant as to form chromite slurries. The slurries were transported laterally by the pyroxenitic sills. The slurries were highly mobile, and in the case of the UG2 and UG3, they partially disaggregated the footwall anorthosites. The liberated crystals of plagioclase were inherited by the chromitites. The association between chromitite and anorthosite is a spatial feature and does not imply cotectic crystallization of chromite and plagioclase. The UG3 chromitites are adjudged to be the lateral equivalent of the UG2 Leader seams, the separating and coalescing of which are described from several localities. Field relationships, including the preservation of pyroxenitic partings and anorthositic xenoliths at internal contacts in the coalesced seams, are consistent with each chromitite having accumulated from a chromite slurry with a unique composition. The composition of the chromite and the mineralization were established at depth, the PGE having been transported as microxenocrysts, which piggybacked on the xenocrysts of chromite. The PGE mineralization is entirely orthomagmatic, having been concentrated in the chromite slurries independent of an immiscible sulphide melt or hydrothermal fluid.
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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 | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| 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 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".