Role of Liquid Immiscibility and Sequential Extraction in the Evolution of the Bushveld Layered Mafic Intrusion, RSA

Abstract

The Bushveld layered mafic intrusion is the world’s largest magmatic body and hosts much of the known platinum, chromium, titanium, and vanadium reserves. The origin and magmatic evolution of the intrusion is still the source of much controversy. Surprisingly, the rocks of the 2.05 By intrusion are relatively undeformed and unaltered and are available for critical geochemical and petrologic analysis. Recently, we have proposed a new model for the origin of layering in mafic intrusions by analyzing the whole-rock major element variations within individual layers of the much smaller Skaergaard intrusion of Greenland. Our model invokes liquid immiscibility of evolved interstitial melt trapped in successive convective boundary layers of the crystallizing magma chamber. Complementary liquids of vastly different densities (with subsequent contrasting mobilities) results in an easily recognized, diagnostic geochemical ‘fingerprint’. We have compiled a rich dataset collected from scattered published sources that span the stratigraphy through most of the 8 km-thick Bushveld body. Our analysis shows that immiscibility may have played a role in both the early (Lower and Critical Zones) and final (Upper Zone) stages of the crystallizing pile, but that crystallization of the bulk of the intrusion (Main Zone) was not influenced by liquid immiscibility processes.