U-Pb GEOCHRONOLOGY OF HYDROTHERMAL ZIRCONS FROM THE EARLY CRETACEOUS IRON SKARN DEPOSITS IN THE HANDAN-XINGTAI DISTRICT, NORTH CHINA CRATON

The Handan-Xingtai district in the North China craton is one of the most important concentrations of iron skarn deposits in China, with proven reserves of 900 to 1,000 Mt at an average of 40 to 55 wt % Fe. The iron mineralization occurs predominantly along contact zones between Early Cretaceous intermediate-silicic intrusions and Middle Ordovician marine carbonates intercalated with numerous evaporite beds. In this paper, we present textural features and laser ablation ICP-MS U-Pb dating results of hydrothermal zircon from five major iron skarn deposits to place tight constraints on the timing and duration of the district-scale iron mineralization. Zircon grains from the mineralized skarns are anhedral to subhedral crystals and euhedral tetragonal bipyramids. They are closely intergrown or texturally associated with diopside, garnet, epidote, calcite, albite, and phlogopite. Other common minerals in the skarn assemblages include F-rich hornblende, wilkeite-F, F-apatite, and fluorite. Zircon grains typically contain abundant inclusions of skarn minerals and daughter mineral-rich (mostly magnetite, halite, and sylvite) fluid inclusions. Compositionally, these zircon grains have moderately to extremely high Th (518-7,477 ppm) and U (109-25,610 ppm) contents, with highly variable 'MX ratios ranging from 0.01 to 5.23. The morphological, textural, and geochemical features of the zircons confirm their hydrothermal origin and indicate that they most likely precipitated from high-temperature, F-rich, magmatic derived ore-forming fluids. The hydrothermal zircon grains yield well-defined concordant U-Pb ages for the five studied iron skarn deposits, with weighted mean Pb-206/U-238 dates ranging from 133.6 +/- 0.9 to 128.5 +/- 1.4 Ma (2 sigma). These ages are remarkably consistent with U-Pb ages (134.1 +/- 1.2 to 128.5 +/- 0.9 Ma; 2 sigma) of magmatic zircon grains from the ore-related intrusions in each deposit, demonstrating that iron skarn mineralization was genetically related to the coeval magmatism. Our new geochronological data, when combined with existing isotopic ages, indicate that iron mineralization and associated magmatism in the Handan-Xingtai district took place mainly at the ca. 137-133 and 131-128 Ma intervals. Iron skarn deposits of similar ages also occur widely in other parts of the eastern North China craton, forming the only known giant Mesozoic iron skarn province in a cratonic block on the Earth. The formation of these iron skarn deposits and associated intrusions coincided in time with litho spheric thinning or destruction of the North China craton, strongly suggesting a causal link between the two processes.