Source peritectic crystal entrainment to mantle magmas produced Earth’s largest chromite deposit

The mechanisms responsible for the formation of the mineral deposits and complex layered stratigraphy in layered mafic intrusions from cratonic environments have remained elusive, largely because the nature of the mantle melting processes that generated the parental magmas are poorly understood. The largest chromium deposits reside within the mafic-ultramafic Rustenburg Layered Suite (RLS) of the Bushveld Complex, South Africa, as laterally continuous layers of chromitite. The RLS has complex, small-scale chemical stratigraphy (Mg#, An#, Sr(i), etc.) and has surprisingly evolved Sr-isotopic compositions. Due to the low solubility of chromium in the basaltic melts theorised as parental to the suite, several models attempt to explain the chromium enrichment. Perhaps the most plausible hypothesis yet proposed to account for the origin of the RLS chromitites, as well as the associated ferromagnesian silicate layered package, is that the suite was produced from hybrid magmas that formed though the assimilation of Kaapvaal craton rocks by komatiitic magmas. As komatiites arise by high degrees of mantle melting, they carry high concentrations of all strongly compatible elements that are enriched in the mantle, but have very low abundances in crustal rocks. However, average RLS compatible trace-element ratios are not similar to mantle values, with Cr/Ni and V/Ni indicating massive enrichment of chromium and vanadium over nickel. Using phase-equilibrium modelling techniques, this study investigated the possibility that the layering and chromitite formation in the RLS are a consequence of the entrainment of components of the magma source rocks. Results reveal a wedge-shaped domain in pressure-temperature space in the subcratonic mantle in which chromium-bearing orthopyroxene is produced as a peritectic product of incongruent melting of various fertile mantle source compositions. Given the ease of orthopyroxene nucleation and the high rates of plume-driven melt production apparent in the formation of large igneous provinces, entrainment of this orthopyroxene in the melts, on extraction from their mantle sources, seems unavoidable. During ascent, magmas with entrained peritectic orthopyroxene crystallise peritectic olivine and chromite due to reaction of the orthopyroxene with melt – a double-peritectic mechanism. These chromite- and olivine-bearing magmas intrude the upper continental crust as sills of crystal-melt slurry and can produce chromite and dunite layers by density separation immediately after emplacement, even if no further cooling occurs before melt drainage. If metasomatism of the mantle source by crustally-derived fluids (ideally ancient and of low volume) is accepted as plausible, then the very high ratios of chromium and vanadium to other compatible elements for average RLS is a superior fit with the formation of the suite by broadly basaltic melts that entrained peritectic orthopyroxene, rather than formation by reactive assimilation of crust by komatiitic magmas. Thus, this study presents a novel, chemically and thermodynamically constrained model that is a simple, first order, source-dependent alternative to the complex petrogenetic models of the current paradigm.