Zircon and titanite U-Pb dating of the Zhangjiawa iron skarn deposit, Luxi district, North China Craton: Implications for a craton-wide iron skarn mineralization

Abstract

The North China craton hosts numerous iron skarn deposits containing more than 2600 Mt of iron ores, mostly with an average grade of >45wt% Fe, which have been among the most important source of high-grade iron ores for the last three decades in China. These deposits typically form clusters and can be roughly divided into the western and eastern belts, which are located in the middle of Trans-North China orogen and to the west of the Tan-Lu fault zone in the eastern part of North China craton, respectively. The western belt mainly consists of the southern Taihang district, as well as the Linfen and Taiyuan ore fields, whereas the eastern belt comprises the Luxi and Xu-Huai districts. The Zhangjiawa deposit in the Luxi district has proven reserves of 290 Mt at an average of 46% Fe (up to >65%). The iron mineralization occurs mainly along contact zones between the Kuangshan dioritic intrusion and middle Ordovician marine carbonate rocks that host numerous evaporite intercalations. Titanite grains from the mineralized skarn are closely intergrown with magnetite and retrograde skarn minerals including chlorite, phlogopite and minor epidote, indicating a hydrothermal origin. The titanite grains have extremely low REE contents and low Th/U ratios, consistent with their precipitation directly from hydrothermal fluids responsible for the iron mineralization. Ten hydrothermal titanite grains yield a weighted mean 206Pb/238U age of 131.0±3.9Ma (MSWD=0.1, 1σ), which is in excellent agreement with a zircon U-Pb age (130±1Ma) of the ore-related diorite. This age consistency confirms that the iron skarn mineralization is temporally and likely genetically related to the Kuangshan intrusion. Results from this study, when combined with existing isotopic age data, suggest that iron skarn mineralization and associated magmatism throughout both the eastern and western belts took place coevally between 135 and 125Ma, with a peak at ca. 130Ma. As such, those deposits may represent the world's only major Phanerozoic iron skarn concentration hosted in Precambrian cratons. The magmatism and associated iron skarn mineralization coincide temporally with the culmination of lithospheric thinning and destruction of the North China craton, implying a causal link between the two.