Modified magnetite and hydrothermal apatite in banded iron-formations and implications for high-grade Fe mineralization during retrogressive metamorphism

Abstract

Modified magnetite and hydrothermal apatite in banded iron-formations (BIFs) are ideal minerals for studying hydrothermal and metamorphic processes, and are applied to linking with high-grade Fe mineralisation and metamorphism in iron deposits hosted by BIFs. This study investigates the geochemical composition of modified magnetite and hydrothermal apatite, and in situ U-Pb geochronology on apatite from the Huogezhuang BIF-hosted Fe deposit in the northeastern China. The magnetite in metamorphosed BIF is modified, locally fragmented and forms mm-to μm-scale bands. The apatite is present surrounding or intergrowing with magnetite, and has corroded surfaces and contains irregularly impurities and fluid inclusions, indicating that it has been partly hydrothermal altered. Original element compositions (e.g., Fe, Al, Ti, K, Mg and Mn) of magnetite in BIFs have been modified during high-grade Fe mineralisation and retrogressive metamorphism with the temperature reduction and acids. The hydrothermally altered apatite has been relatively reduced in Ca, P, F, La, Ce, Nd, δCe, δEu, and total REEs contents compared to non-altered apatite. The magnetite and apatite in low-grade BIFs are poorer in FeO T than those of from the high-grade Fe ores, indicating that Fe is remobilised during the transition from BIFs to high-grade Fe ores. The magnetite and apatite in high-grade Fe ores are overgrown by greenschistfacies minerals formed during retrograde metamorphism, suggesting that the high-grade Fe mineralisation may be related to retrogressive metamorphism.In situ U-Pb geochronology of apatite intergrown with magnetite and zircon LA-ICP-MS U-Pb dating at Huogezhuang deposit reveal that the BIF-hosted magnetite was altered and remobilised at ca. 1950-1900 Ma, and deposition This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.