In-situ texture and geochemistry of apatite from the Jinling and Zhangjiawa iron skarn deposits, eastern North China Craton: Implications for ore-forming processes and formation of high-grade ores

•Origins and ore-forming processes of Fe mineralization were revealed by apatite.•The ore-forming fluids were high-fluorine fluids.•The fluorine promoted iron leaching from wallrocks.•Fluid metasomatism contributed to elevate and lower Fe grade, respectively.