Silicate liquid immiscibility and the formation of Fe (±Ti ±P ±F ±REE) ore deposits

The role of immiscibility between two silicate melts (ferrobasalt-rhyolite) on differentiation and ore-forming processes is under debate since decades. Clear evidence for the separation of two silicate melts in natural systems is rarely recorded in rocks because of the transient nature of immiscibility during cooling and solidification. Thus, experimental approaches are crucial to test the importance of melt immiscibility in magmatic and associated ore-forming processes. In this study, we propose to use a twofold approach to understand the role of silicate liquid immiscibility in the formation of Fe (±Ti ±P ±F ±REE) ore deposits: (1) we will study melt inclusions trapped in apatite of some major iron deposits which will give us direct constraints on the equilibrium liquid compositions and (2) we will perform a detailed experimental approach to understand the conditions for the onset of immiscibility (magma compositions, temperature, pressure, oxygen fugacity and fluid content), and the partitioning of major elements between paired melts. The role of melt immiscibility will be tested for three well known natural examples, all containing Fe- and P-rich associations and covering a broad compositional range: tholeiitic basaltic system (Bushveld complex), ferropicritic/high-Ti basaltic system (Emeishan large igneous province), and Kiruna-type systems (Vergenoeg deposit). In selected experiments, we will also study the effect of immiscibility on trace elements fractionation between Fe-rich and Fe-poor silicate melts. The results will provide crucial data allowing us to discuss quantitatively the role of melt immiscibility on differentiation processes in basaltic systems, on the formation of some economically important Fe ore deposits and on enrichment mechanisms of trace elements (especially REE).