Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic FeTi deposits: A case study of the Damiao titanomagnetite ore deposit,North China Craton |
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| Institution: | 1. GeoZentrum Nordbayern, Friedrich-Alexander-Universität Erlangen-Nürnberg, Schlossgarten 5a, 91054 Erlangen, Germany;2. China Minmetals Corporation, 100010 Beijing, China;3. Institut für Mineralogie, Leibniz Universität Hannover, Callinstr. 3, 30167 Hannover, Germany;3. Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou 510640, China;1. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;2. School of Earth and Environmental Sciences, James Cook University, Townsville 4811, Australia;3. Faculty of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China;4. Laboratory of Isotope Geology, MLR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China |
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| Abstract: | Geochemical and iron isotopic compositions of magnetite, ilmenite and pyrite separates from the Fe Ti oxide ores hosted in the Damiao anorthosite-type FeTi ore deposit were analyzed to investigate sub-solidus cooling history of the titanomagnetite. The FeTi oxides form two series of solid solutions, namely, ulvöspinel-magnetite (Usp-Mtss) and hematite-ilmenite (Hem-Ilmss) solid solutions. The magnetite separates have 14–27 mol% ulvöspinel, while the ilmenite separates have 5–8 mol% hematite. Major element compositions of the mineral separates suggest that the ilmenites were mainly exsolved from the Usp-Mtss by oxidation of ulvöspinel in the temperature range of ~820–600 °C and experienced inter-oxide re-equilibration with the magnetites. Associated with the exsolution is the substantial inter-mineral iron isotope fractionation. The magnetite separates are characterized by high δ57Fe (+0.27 ? +0.65‰), whereas the ilmenite separates have lower δ57Fe (?0.65 to ?0.28‰). Two types of pyrite are petrographically observed, each of which has a distinctive iron isotope fingerprint. Type I pyrite (pyriteI) with higher δ57Fe (δ57Fe = +0.63 ? +0.95‰) is consistent with magmatic origin, and type II pyrite (pyriteII) with lower δ57Fe (δ57Fe = ?0.90 to ?0.11‰) was likely to have precipitated from fluids. Iron isotopic fingerprints of the pyriteI probably indicate variations of oxygen fugacity, whereas those of the pyriteII may result from fluid activities. The iron isotopic fractionation between the magnetite and ilmenite is the net result of sub-solidus processes (including ulvöspinel oxidation and inter-oxide re-equilibration) without needing varying oxygen fugacity albeit its presence. Although varying composition of magnetite-ilmenite pairs reflects variations of oxygen fugacity, inter-oxide iron isotopic fractionation does not. |
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