Hydrothermal Zircon U-Pb Dating for Early Cretaceous Iron Skarn Mineralization in the Handan-Xingtai District, North China Craton

Abstract:

The Handan-Xingtai district in the North China craton (NCC) is one of the most important concentrations of iron skarn deposits in China. The iron mineralization occurs predominantly along contact zones between early Cretaceous intrusions and Middle Ordovician marine carbonates. We present textural features and LA-ICP-MS U-Pb dating results of hydrothermal zircon to constrain on the time of iron mineralization. Zircon grains from mineralized skarns are subhedral crystals and euhedral tetragonal bipyramids, which are closely intergrown with skarn minerals. They contain abundant inclusions of skarn minerals and daughter mineral-rich (mostly magnetite, halite, and sylvite) fluid inclusions, and have variable Th (518-17477 ppm) and U (109-25610 ppm) contents, with Th/U ratios of 0.01-5.23. These features indicate their hydrothermal origin.

The hydrothermal zircon grains from 5 deposits yield concordant U-Pb ages ranging from 133.6 ± 0.9 to 128.5 ± 1.4 Ma (2σ). These ages are consistent with U-Pb ages (134.1 ± 1.2 to 128.5 ± 0.9 Ma; 2σ) of magmatic zircon grains, confirming the reliability of hydrothermal zircon U-Pb ages. Our new geochronological data indicate that iron mineralization and associated magmatism in the Handan-Xingtai district occurred mainly at the ca. 137-133 and 131-128 Ma intervals. These iron skarn deposits represent as the only known giant Mesozoic iron skarn province in a cratonic block on the Earth. Their formation time coincided with lithospheric thinning or destruction of the NCC, strongly suggesting a causal link between the two processes. Our work provides a valuable approach to date iron skarn mineralization by using U-Pb geochronology of hydrothermal zircon and highlights its potential for similar studies.

Keywords: Hydrothermal zircon; LA-ICPMS; U-Pb geochronology, iron skarn deposits; North China craton