Genesis of the Guodian Iron Skarn Deposit in the Luxi Block: Insights from Geochronology, Fluid Inclusions, and H-O Isotopes

The Qihe-Yucheng district (Luxi Block， eastern North China Craton) hosts multiple iron skarn deposits characterized by high-grade ores (TFe>50%)， significant thicknesses， and a clustered distribution. This district provides an excellent opportunity to study the formation of high-grade iron mineralizations. Despite their economic importance， the timing of the mineralizations and the characteristics of the ore-forming fluids remain poorly understood， hindering the development of genetic models and effective exploration strategies in the region. This study focuses on the Guodian deposit， a representative high-grade skarn iron deposit within the Qihe-Yucheng district. We conducted zircon and garnet LA-ICP-MS U-Pb geochronology， as well as H-O isotope and fluid inclusion analyses of skarn minerals， to constrain the genesis of the Guodian deposit. Zircon U-Pb dating indicates diorite crystallization at ～132 Ma， while U-Pb dating of garnet from the early skarn stage (I) yields 131.8 ± 3.5 Ma， demonstrating mineralization during the Early Cretaceous. Fluid inclusions of the skarn stages (Ⅰ) and (Ⅱ) contain daughter crystals and are rich in liquid phase; they have similar homogenization temperatures (376-460 °C) but variable salinities and homogenization behaviors. Fluid inclusions of the oxide stage (Ⅲ) are predominantly vapor-rich or liquid-rich， and their homogenization temperatures and salinities are concentrated at 310-390 °C and 6-10 wt% NaCl eq. The carbonate stage (Ⅴ) is characterized by liquid-rich fluid inclusions with homogenization temperatures and salinities of 150-200 °C and 3 wt% NaCl eq， respectively. H-O isotope data primarily point to magmatic-hydrothermal sources of the ore-forming fluids. We propose that the Guodian iron skarn deposit formed during an Early Cretaceous extensional tectonic setting triggered by the rollback of the Paleo-Pacific slab. This tectonic regime facilitated the emplacement of intermediately acidic intrusions (dominantly dioritic)， which drove metasomatism of carbonate wall rocks and formed high-grade iron skarn mineralization.