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/⁸⁶Sr_i (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/⁸⁶Sr_i 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.

太古宙-元古代的巨型镁铁质-超镁铁质层状侵入体是地壳中大量硅酸盐岩浆注入的化石残留物，是我们最重要的铂族元素宝库。富含 PGE 的岩浆矿床，例如 Merensky 礁，通常赋存于超镁铁质和长石堆积物接触处的层状礁中。梅伦斯基礁的普遍特征是粗粒和伟晶状纹理，这可能为其起源提供重要线索。我们提供了南非西部布什维尔德综合体 Styldrift 矿（Impala Bafokeng）的 Merensky 伟晶石的结构和原位地球化学数据。该地区毗邻推测的布什维尔德岩浆供给区。 Merensky 伟晶石的特征是 (i) 分区斜方辉石巨晶中的变形橄榄石包裹体，斜方辉石的 Mg# 摩尔数向橄榄石方向增加，(ii) 细粒斜方辉石链与相邻斜方辉石巨晶的成分平衡，(iii) Mg# 摩尔数增加斜长石巨晶的数量和摩尔 An 的增加，随着 ⁸⁷ Sr/ ⁸⁶ Sr（在 2.06 Ga 时）的减少，斜长石卵晶横向穿过馈线远端 10 公里的部分，并且（ iv) 高度可变的摩尔 An 和靠近饲养层的间质斜长石的初始 ⁸⁷ Sr/ ⁸⁶ Sr。我们将粗粒和伟晶结构、它们的溶解-再沉淀特征以及横向化学变化解释为横向熔体渗透和混合在晶糊中的产物。 我们认为，含铂的梅伦斯基礁并不是在一个巨大的充满熔体的岩浆房的底部形成的，而是非连续的岩浆侵位的产物，使水晶糊重新焕发了活力。