Melt infiltration in a crystal mush and pegmatoid formation in the platiniferous Merensky Reef, Bushveld Complex, South Africa

IF 4.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Mineralium Deposita Pub Date : 2024-05-27 DOI:10.1007/s00126-024-01278-z
Ben Hayes, Reza Maghdour-Mashhour, Lewis D. Ashwal, Albertus J.B. Smith, Henriette Ueckermann, Jaco Vermeulen
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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.

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南非布什维尔德岩群铂髓质梅伦斯基礁水晶蘑菇和埙状岩层中的熔融渗透
太古宙-新生代的巨型黑云母-超黑云母层状侵入体是地壳中硅酸盐岩浆巨大喷射的化石遗迹,是我们最重要的铂族元素储藏地。富含铂族元素的岩浆矿床,如梅伦斯基礁,通常赋存于超基性岩积和长石岩积接触处的层状岩礁中。梅伦斯基礁通常具有粗粒和辉绿岩质地的特征,这可能为其起源提供了重要线索。我们展示了南非西布什维尔德复合体 Styldrift 矿(Impala Bafokeng)的梅伦斯基伟晶岩的纹理和现场地球化学数据。该地区毗邻推断的布什维尔德岩浆给矿带。梅伦斯基辉绿岩的特征是:(i) 带状正长辉石巨晶中的amoeboid橄榄石包裹体,正长辉石的摩尔Mg#向橄榄石方向递增;(ii) 与相邻正长辉石巨晶成分平衡的正长辉石细粒链;(iii) 正长辉石巨晶的摩尔Mg#递增,摩尔An随87Sr/86Sri(2.06 Ga时)的递减而递增。(iv) 在馈源附近,间隙斜长石的摩尔安和初始 87Sr/86Sri 变化很大。我们将粗粒和柱状纹理、其溶解-再沉淀特征以及横向化学变化解释为晶体泥浆中横向熔体渗透和混合的产物。我们认为,含铂的梅伦斯基礁并不是在一个充满熔融物的大型岩浆腔底部形成的,而是非连续性岩浆喷出的产物,它使晶泥重新焕发了活力。
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来源期刊
Mineralium Deposita
Mineralium Deposita 地学-地球化学与地球物理
CiteScore
11.00
自引率
6.20%
发文量
61
审稿时长
6 months
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.
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