New approaches in understanding layered intrusions : field-based and analytical evidence from the Bushveld and Monchegorsk complexes

• Main Author: Karykowski, Bartosz
• Published: Cardiff University 2017
• Subjects: QE Geology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.732297

The formation of layered intrusions remains one of the most important, yet unresolved issues in the study of mafic-ultramafic systems, although they are of major economic significance, hosting more than 80 % of the world's platinum-group element (PGE) resource. In many layered intrusions, PGE mineralisation is associated with stratigraphic intervals that are characterised by pronounced igneous layering. Thus, the origin of layering and the emplacement mechanism of individual layers are closely related to the formation of PGE deposits. In this study, field-based and analytical evidence from the Bushveld Complex of South Africa and the Monchegorsk Complex in Russia is used to gain a better understanding of the small- and large-scale processes associated with the emplacement of layered intrusions. Detailed examination of drill core and field exposures suggest that sill-like intrusions of crystal mushes play an important role in the formation of layered intrusions, especially in the lower ultramafic portions of large complexes. In contrast, the in situ Sr isotope analysis of plagioclase from the upper portion of the Bushveld Complex indicates that the more mafic portions may also crystallise in situ from crystal-poor magmas, which can also undergo mixing. Moreover, mineralogical and microtextural work based on high-resolution elemental mapping highlights the importance of melt migration at different stages of cumulate solidification as a consequence of displacement by convecting interstitial liquids and compaction. Further, broadly stratiform PGE mineralisation in the Monchegorsk Complex cannot always be explained by a classic PGE reef model, in which the mineralised horizon marks the transition from sulfide-undersaturated to sulfide-saturated conditions. It is more likely that preformed sulfides were entrained in crystal mushes and emplaced into a semi-consolidated cumulate pile at different levels of the layered intrusion. Ultimately, thermal modelling shows that a multi-stage emplacement history of a complex should be regarded as highly prospective with respect to PGE-Ni-Cu mineralisation.