Geochronological constraints on the genesis of high-grade iron ore in the Gongchangling BIFs from the Anshan-Benxi area, North China Craton

The Gongchangling iron deposit located in the northeast of the North China Craton is hosted in late Neoarchean Algoma-type BIFs. It is famous for the major production of high-grade iron ore in the Gongchangling No.2 mining area in China. With regard to the genesis of high-grade iron ore, more and more evidences indicate that it was related with hydrothermal enrichment of BIFs. However, the hydrothermal nature was argued for meteoric, metamorphic or migmatitic fluid. In this study, the trace element compositions of garnet from the altered wall-rock of high-grade iron ore obtained by LA-ICP-MS show compositional zoning, indicating it is a metamorphic origin. These garnets yield a Sm-Nd isochron age of 1888 +/- 77 Ma, interpreted as the time of metamorphism in this area. LA-ICP-MS U-Pb dating of zircon from the upper migmatite zone shows that the migmatitic granite was formed at 2478 +/- 36 Ma. Since the REE patterns of garnet suggest that it was formed in the nearly neutral fluid rather than the acid meteoric fluid in the Paleoproterozoic. Hence, combined the ages of metamorphism and migmatization with high-grade iron ore of 1840 +/- 7 or 1860 +/- 7 Ma obtained by previous studies, it indicates that the genesis of the high-grade iron ore derived from the enrichment of BIFs by metamorphic hydrothermal fluid.