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

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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.