The Petrogenesis of the PGE-Rich UG2 and UG3 Chromitites at Winnaarshoek, Eastern Bushveld Complex: Intrusion of Chromite Slurries Contained Within Pyroxenitic Sills
Notice bibliographique
Résumé
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.
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
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Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,001 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule tête enseignante, pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».