The Gongchangling BIFs from the Anshan–Benxi area, NE China: Petrological–geochemical characteristics and genesis of high-grade iron ores

• The garnet exhibits the growth zoning produced during prograde metamorphism.
• The amphibolite recorded the metamorphic temperature of 567 ± 25 °C.
• High-temperature hydrothermal fluid supplied the ore-forming material.
• High-grade iron ore is a reformed product of BIFs by metamorphic hydrothermal fluid.

The Gongchangling BIFs are located in the Anshan–Benxi area, northeastern part of the North China Craton, of which the No. 2 mining area is the largest and most typical high-grade iron ore producing area in China. In this study, the mineralogy, petrology and chemistry of iron ores (BIFs and high-grade iron ores) and their wall-rocks (amphibolite and garnet–chlorite schist) from the Gongchangling No. 2 mining area were examined to constrain the genesis of high-grade iron ore.The BIFs are composed essentially of magnetite and quartz, in addition to actinolite in a few samples; while the high-grade iron ores are mainly composed of magnetite and the quartz is relatively rare. The low contents of Al2O3, TiO2 and HFSE in the BIFs and high-grade iron ores indicate that they are marine chemical precipitates with little input of terrigenous clastic sediments. The magnetite and quartz from BIFs and high-grade iron ores exhibit the similar chemical composition. The REY patterns of BIFs and high-grade iron ores suggest their precipitation after mixing of hydrothermal component with seawater in certain proportions under relatively low oxygen level, and the obvious positive Eu anomalies indicate that high-temperature hydrothermal fluids which percolate and dissolve the submarine volcanic rock may supply the ore-forming material to the Gongchangling iron deposit.The amphibolite as interlayer of BIFs recorded the metamorphic temperature of 567 ± 25 °C, moreover, the garnet from wall-rock of high-grade iron ore exhibits the growth zoning produced during prograde metamorphism, indicating that metamorphic hydrothermal fluids played an important role in the genesis of high-grade iron ore. It was proposed that the Gongchangling iron deposit was formed in an arc-related tectonic setting at the late Neoarchean, then it underwent lower amphibolite facies metamorphism, and the high-grade iron ore is the reformed product of BIFs by metamorphic hydrothermal fluids.