Silicate liquid immiscibility and the formation of Fe (±Ti ±P ±F ±REE) ore deposits
Final Report Abstract
During this project, different aspects of the late-stage evolution of the Bushveld Complex, which formed the Upper and Upper Main Zone of this economically and scientifically important intrusion, were investigated. The findings of this study provide a better understanding of the formation mechanisms affecting the magmatic sequence of the upper part of the intrusion. It is shown that the Upper and Upper Main Zone of the Bushveld Complex evolved from an andesitic parental magma containing small amounts of H2O and not basaltic liquid as previously proposed. The first cumulates have the mineral assemblage plagioclase + low-Ca-pyroxene + clinopyroxene, and the cumulate series is marked by several magma replenishment events. The formation model can explain the observed compositional cyclicity by several magma replenishment events in which a plagioclase-laden magma is injected into the Bushveld magma chamber. Based on trace elements in titanomagnetite and clinopyroxene, this study revealed two major compositional shifts correlating with the stratigraphic position of two magma injections identified. These shifts towards more evolved compositions are related to injections from different compartments of the sub-Bushveld staging chamber, arguing for a sub-compartmentalized magma chamber as a source of the magma injections. Estimated oxygen fugacities are around FMQ-2 and show only minor variations within the stratigraphy, suggesting constant prevailing fO2 conditions during crystallization of the Upper Zone. Consequently, it can be concluded that the injected magma has similar oxygen fugacity conditions as the resident magma. The late stage evolution of Upper Zone with apatite saturated cycles is marked by the onset of silicate liquid immiscibilty producing an Fe- and a Si-rich silicate liquid recorded in apatite hosted melt inclusions. The Fe-rich melt is also enriched in P and percolates downwards within the crystal mush. Cumulates of these segregated Fe-P-rich liquids then produced the nelsonite layers observed in the Upper Zone.
Publications
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(2016). Fe-Ti-V-(P) resources in the upper zone of the Bushveld complex, South Africa. In Papers and Proceedings of the Royal Society of Tasmania (Vol. 150, No. 1, pp. 15-22)
Fischer, L.A., & Yuan, Q.
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(2016). Immiscible iron-and silica-rich liquids in the Upper Zone of the Bushveld Complex. Earth and Planetary Science Letters, 443, 108-117
Fischer, L.A., Wang, M., Charlier, B., Namur, O., Roberts, R.J., Veksler, I.V., Cawthorn, R.G., Holtz, F.
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(2016): Melt inclusions reveal immiscibility in the Upper Zone of the Bushveld Complex. EMC meeting, Rimini, Italy
Fischer, L.A., Charlier, B., Namur, O., Roberts, J., Holtz, F.
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(2017). Pulses of plagioclase-laden magmas and stratigraphic evolution in the Upper Zone of the Bushveld Complex, South Africa. Journal of Petrology, 58(8), 1619-1643
Yuan, Q., Namur, O., Fischer, L.A., Roberts, R.J., Lü, X., Charlier, B.
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(2017): Silicate liquid immiscibility in the Upper Zone of the Bushveld Complex. SEG meeting, Beijing, China
Fischer, L.A., Namur O., Wang, M., Holtz F., Charlier B.
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(2017): Vanadium distribution in the Upper Zone of the Bushveld Complex: Constrains on fO2. GeoBremen, Bremen Germany
Fischer, L.A., Namur O., Horn, I., Holtz F., Charlier B.
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(2018): Constrains on the parental magma of the Upper and Upper Main Zone of the Bushveld Complex. GeoBonn, Bonn Germany
Fischer, L.A., Charlier, B., Namur, O., Holtz, F.
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(2018): Effect of starting material and temperature path on phase equilibria. EMPG conference, Clermont-Ferrand, France
Fischer, L.A., Spijkerboer, A., Charlier, B., Namur, O., Holtz, F.
