The origin of discordant Fe-rich wehrlite bodies and the role of metasomatism and liquid immiscibility in the evolution of the Bushveld Igneous Complex and its platinum deposits

南非的布什维尔德火成岩复合体是地球上最大的几种战略贵金属和工业金属的天然资源库，即铂族、铬和钒。复杂的规模（数百公里宽），以及有限的地表暴露，特别是在高度工业化的西部地区，阻碍了对复杂及其惊人的矿产财富的了解。尽管存在诸多障碍，而且由于在科学和经济上的重要性，几十年来，地球科学家一直把布什维尔德作为一个独特的自然实验室，在那里探索和测试了岩浆位置、分化和矿化的理论。岩浆结晶与交代过程在形成该杂岩和其他大型强分异侵入岩中火成岩多样性的关系是尚未解决的主要问题之一。该项目通过研究一套极富铁橄榄辉石岩（wehrlite）的起源，解决了这一重大问题的重要方面。这些岩体也被称为富铁超镁铁质伟晶岩（IRUP），据信是由交代熔体或来源和成分未知的流体对先前存在的辉长岩堆积岩进行的大规模取代反应产生的。有人提出，这种替换是由侵入体顶部硅酸盐液体不混溶产生的致密富铁硅酸盐熔体向下渗透造成的。然而，在Bushveld复合体和其他镁质侵入物中，大规模的液体不混溶性是有争议的，因此Fe-wehrlite组的起源仍然不清楚。该项目的目标是测试交代液体和硅酸盐液体不混溶之间的可能联系，并评估交代转化的地球化学影响，包括Fe-Ni-Cu硫化物和铂族元素的重新分布。拟议的工作方案将天然岩石的定向地球化学和岩石学研究与天然岩石和合成玻璃混合物的熔融实验结合起来。实验目的是重现取代反应，并对交代流体的组成建立更好的相平衡约束。替代铁韦氏体的起源及其相关反应不仅具有重要的理论意义，而且具有重要的实际意义，因为这种替代破坏了铂族矿体，给采矿带来了重大问题。

The Bushveld Igneous Complex in South Africa is the Earths largest natural repository of several strategic precious and industrial metals, namely the platinum-group, chromium and vanadium. The sheer scale of the complex (hundreds of kilometres across), as well as the limited surface exposure, particularly in the highly industrialised western sector, have obstructed understanding of the Complex and its fabulous mineral wealth. Despite the obstacles and because of the great scientific and economical importance, geoscientists have used Bushveld for several decades as a unique natural laboratory, where theories of magma emplacement, differentiation and mineralization have been explored and tested. One of the major unresolved problems is the relationship between magmatic crystallization and metasomatic processes in shaping the diversity of igneous rocks in the Complex and other large, strongly differentiated intrusions. This project addresses important aspects of that major problem by studying the origin of a suite of extremely Fe-rich olivine-pyroxene rocks (wehrlite). The bodies, also known under the name of iron-rich ultramafic pegmatites (IRUP), are believed to result from large-scale replacement reactions imposed on pre-existing gabbronoritic cumulate rocks by metasomatizing melts or fluids of unknown origin and composition. It has been proposed that the replacement was caused by downward percolation of dense, Fe-rich silicate melts produced by silicate liquid immiscibility in the apical part of the intrusion. However, large-scale liquid immiscibility in the Bushveld Complex and in other mafic intrusions is contentious, so the origin of the Fe-wehrlite suite remains unclear. The goal of the project is to test the possible link between metasomatizing liquids and silicate liquid immiscibility and to assess the geochemical effects of the metasomatic transformation, including re-distribution of Fe-Ni-Cu sulphides and the platinum-group elements. The proposed working programme combines targeted geochemical and petrographic studies of natural rocks with melting experiments on mixtures of natural rocks and synthetic glasses. The experimental objectives are to reproduce the replacement reactions and establish better phase equilibria constraints on the compositions of metasomatizing fluids. The origin of the replacement Fe-wehrlite and the associated reactions are not only of great theoretical interest but also of serious practical importance because the replacement disrupts the platinum-group ore bodies and cause significant problems for mining.