Experimental study of liquid immiscibility in the Kiruna-type Vergenoeg iron–fluorine deposit, South Africa

Abstract

In this study we experimentally assess whether the bulk composition of the Kiruna-type iron–fluorine Vergenoeg deposit, South Africa (17wt.% SiO2 and 55wt.% FeOtot) could correspond to an immiscible Fe-rich melt paired with its host rhyolite. Synthetic powder of the host rhyolite was mixed with mafic end-members (ore rocks) in variable proportions. Experimental conditions were 1–2kbar and 1010°C, with a range of H2O and F contents in the starting compositions. Pairs of distinct immiscible liquids occur in experiments saturated with fluorite, under relatively dry conditions, and at oxygen fugacity conditions corresponding to FMQ−1.4 to FMQ+1.8 (FMQ=fayalite-magnetite-quartz solid buffer). The Si-rich immiscible liquids contain 60.9–73.0wt.% SiO2, 9.1–12.5wt.% FeOtot, 2.4–4.2wt.% F, and are enriched in Na2O, K2O and Al2O3. The paired Fe-rich immiscible melts have 41.0–49.5wt.% SiO2, 20.6–36.1wt.% FeOtot and 4.5–6.0wt.% F, and are enriched in MgO, CaO and TiO2. Immiscibility does not develop in experiments performed under water-rich (aH2O>0.2; a=activity) and/or oxidized (>FMQ+1.8) conditions. In all experiments, solid phases are magnetite, ±fayalite, fluorite and tridymite. Our results indicate that the rocks from the Vergenoeg pipe crystallized in a magma chamber hosting two immiscible silicate melts. Crystallization of the pipe from the Fe-rich melt explains its extreme enrichment in Ca, F and Fe compared to the host rhyolitic rocks. However, its low bulk silica content compared to experimental Fe-rich melts indicates that the pipe formed by remobilization of a mafic crystal mush dominated by magnetite and fayalite. Segregation of evolved residual liquids as well as the conjugate immiscible Si-rich melt produced the host rhyolite. The huge amount of fluorine in Vergenoeg ores (∼12wt.% F) can hardly be explained by simple crystallization of fluorite from the Fe-rich silicate melt (up to 6wt.% F at fluorite saturation). Instead, we confirm a previous hypothesis that the fluorite enrichment is, in part, due to the migration of hydrothermal fluids.