Immiscible hydrous Fe–Ca–P melt and the origin of iron oxide-apatite ore deposits

• DOI: 10.1038/s41467-018-03761-4
• Publication Date: 2018-04-11T14:09:48Z
• Authors: Tong Hou; Bernard Charlier; François Holtz; Ilya Veksler; Zhaochong Zhang; Rainer Thomas; Olivier Namur
• Keywords: iron oxide; separation; oxidation; Science; economic geology; water; OXIDATION-STATE; temperature effect; 01 natural sciences; Article; iron; conjugate; PHASE-RELATIONS; fluorine; ore deposit; WATER; chemical composition; liquid; phosphorus; Dewey Decimal Classification::500 | Naturwissenschaften; 0105 earth and related environmental sciences; KIRUNA-TYPE; Science & Technology; calcium; silicate; Q; silicon; MAGMATIC ORIGIN; immiscibility; Multidisciplinary Sciences; EL-LACO; WESTERN TIANSHAN; Dewey Decimal Classification::500 | Naturwissenschaften::551 | Geologie, Hydrologie, Meteorologie; 13. Climate action; apatite; MAGNETITE DEPOSIT; fugacity; Science & Technology - Other Topics; SILICATE LIQUID IMMISCIBILITY; GEOCHEMISTRY; oxygen

AbstractThe origin of iron oxide-apatite deposits is controversial. Silicate liquid immiscibility and separation of an iron-rich melt has been invoked, but Fe–Ca–P-rich and Si-poor melts similar in composition to the ore have never been observed in natural or synthetic magmatic systems. Here we report experiments on intermediate magmas that develop liquid immiscibility at 100 MPa, 1000–1040 °C, and oxygen fugacity conditions (fO2) of ∆FMQ = 0.5–3.3 (FMQ = fayalite-magnetite-quartz equilibrium). Some of the immiscible melts are highly enriched in iron and phosphorous ± calcium, and strongly depleted in silicon (<5 wt.% SiO2). These Si-poor melts are in equilibrium with a rhyolitic conjugate and are produced under oxidized conditions (~FMQ + 3.3), high water activity (aH2O ≥ 0.7), and in fluorine-bearing systems (1 wt.%). Our results show that increasing aH2O and fO2 enlarges the two-liquid field thus allowing the Fe–Ca–P melt to separate easily from host silicic magma and produce iron oxide-apatite ores.