Geochemical Characteristics And Genesis Of Band Iron Formations From Iron Ore In Qian'an Area

The origin of Qian'An BIFs remains unclear, although plenty of efforts have been done in this area (especially on the Precambrian geology, petrology and geochemistry). In the early stage of"Exploring Mines Facing A Recourse Crisis"work, Large and high-grade iron ores were discovered, but their formations were still poorly understood. The protolith of magenetite-quartzite is basalt based on the diagrams of protolith restoration,the negative correlation of TFe2O3 and SiO2,the depletion of Nd,Ta andδ18O number, while the protoliths of wall rocks are sedimentary rocks or rhyolite, which indicates that there is little connection between the origin of magenetite-quartzite and wall rocks.The major and trace elements reported in this paper provide new insights on the origin and environment of Qian'An BIFs. The average bulk compositions of the BIFs are rich in total Fe and SiO2, but they have very low Al2O3 and TiO2 contents indicating that a minor terrigenous component input. They have very low trace element contents, including total REE contents. The PASS-normalized REE patterns strongly enriched in HREE, display La, Eu and Y positive anomalies. All of the geochemistry fingerprints, in combination with Y/Ho,δLa,LaN/YbN<1 ratios strongly suggest a combined origin of hydrothermal water and marine for Qian'An BIFs.There are no distinct differences between High-Grade ores and normal ores, demonstrating that they might have the same origins. The obvious sedimentary bands of ores, magnetite as the iron mineral,δ18O in High-Grade ores is lower than that in normal ores, all suggest that the high-grade ores were probably formed during the sedimentary period of volcanic-sedimentary formation. The tectonic deformation and metamorphism later contributed plastic flow of magnetite, which caused lenticular high-grade ores some areas.