Ben Hayes, Reza Maghdour-Mashhour, Lewis D. Ashwal, Albertus J.B. Smith, Henriette Ueckermann, Jaco Vermeulen
{"title":"Melt infiltration in a crystal mush and pegmatoid formation in the platiniferous Merensky Reef, Bushveld Complex, South Africa","authors":"Ben Hayes, Reza Maghdour-Mashhour, Lewis D. Ashwal, Albertus J.B. Smith, Henriette Ueckermann, Jaco Vermeulen","doi":"10.1007/s00126-024-01278-z","DOIUrl":null,"url":null,"abstract":"<p>Giant mafic-ultramafic layered intrusions of Archaean-Proterozoic age are the fossilised remnants of huge injections of silicate magma in the Earth’s crust and are our most important repositories of platinum-group elements. Magmatic PGE-rich ore deposits, such as the Merensky Reef, are typically hosted in stratiform reefs at the contacts between ultramafic and feldspathic cumulates. The Merensky Reef is commonly characterised by coarse-grained and pegmatoidal textures that may provide important clues to its origin. We present textural and in situ geochemical data for Merensky pegmatoids at Styldrift Mine (Impala Bafokeng) in the Western Bushveld Complex of South Africa. This region is adjacent to an inferred magmatic feeder zone to the Bushveld. The Merensky pegmatoids are characterised by (i) amoeboid olivine inclusions in zoned orthopyroxene megacrysts with increasing molar Mg# of orthopyroxene towards olivine, (ii) fine-grained chains of orthopyroxene in compositional equilibrium with adjacent orthopyroxene megacrysts, (iii) increasing molar Mg# of orthopyroxene megacrysts and increasing molar An with decreasing <sup>87</sup>Sr/<sup>86</sup>Sr<sub>i</sub> (at 2.06 Ga) of plagioclase oikocrysts in pegmatoids laterally across a 10-km section distal to the feeder, and (iv) highly variable molar An and initial <sup>87</sup>Sr/<sup>86</sup>Sr<sub>i</sub> of interstitial plagioclase proximal to the feeder. We interpret the coarse-grained and pegmatoidal textures, their dissolution-reprecipitation features, and lateral chemical variations as the product of lateral melt infiltration and mixing in a crystal mush. We suggest that the platiniferous Merensky Reef was not formed at the base of a large melt-filled magma chamber but was instead the product of non-sequential magma emplacement that rejuvenated the crystal mush.</p>","PeriodicalId":18682,"journal":{"name":"Mineralium Deposita","volume":null,"pages":null},"PeriodicalIF":4.4000,"publicationDate":"2024-05-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Mineralium Deposita","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.1007/s00126-024-01278-z","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
引用次数: 0
Abstract
Giant mafic-ultramafic layered intrusions of Archaean-Proterozoic age are the fossilised remnants of huge injections of silicate magma in the Earth’s crust and are our most important repositories of platinum-group elements. Magmatic PGE-rich ore deposits, such as the Merensky Reef, are typically hosted in stratiform reefs at the contacts between ultramafic and feldspathic cumulates. The Merensky Reef is commonly characterised by coarse-grained and pegmatoidal textures that may provide important clues to its origin. We present textural and in situ geochemical data for Merensky pegmatoids at Styldrift Mine (Impala Bafokeng) in the Western Bushveld Complex of South Africa. This region is adjacent to an inferred magmatic feeder zone to the Bushveld. The Merensky pegmatoids are characterised by (i) amoeboid olivine inclusions in zoned orthopyroxene megacrysts with increasing molar Mg# of orthopyroxene towards olivine, (ii) fine-grained chains of orthopyroxene in compositional equilibrium with adjacent orthopyroxene megacrysts, (iii) increasing molar Mg# of orthopyroxene megacrysts and increasing molar An with decreasing 87Sr/86Sri (at 2.06 Ga) of plagioclase oikocrysts in pegmatoids laterally across a 10-km section distal to the feeder, and (iv) highly variable molar An and initial 87Sr/86Sri of interstitial plagioclase proximal to the feeder. We interpret the coarse-grained and pegmatoidal textures, their dissolution-reprecipitation features, and lateral chemical variations as the product of lateral melt infiltration and mixing in a crystal mush. We suggest that the platiniferous Merensky Reef was not formed at the base of a large melt-filled magma chamber but was instead the product of non-sequential magma emplacement that rejuvenated the crystal mush.
期刊介绍:
The journal Mineralium Deposita introduces new observations, principles, and interpretations from the field of economic geology, including nonmetallic mineral deposits, experimental and applied geochemistry, with emphasis on mineral deposits. It offers short and comprehensive articles, review papers, brief original papers, scientific discussions and news, as well as reports on meetings of importance to mineral research. The emphasis is on high-quality content and form for all articles and on international coverage of subject matter.