Characteristics and genesis of iron formation in the Changlongshan Formation of Qingbaikou System in the Ming Tombs area,Beijing

Iron formation is widely distributed through the entire Precambrian geological record,however,there is no report on the outcropped Precambrian iron formation in the Changlongshan Formation of the Qingbaikou System in North China. Based on field investigation,XRD,SEM,ICP-MS and other methods,the distribution and mineral composition of iron formation and the source of ore-forming materials outcropped in the Ming Tombs area were studied in this paper to investigate the genesis of iron formation. The study showed that the main component of iron formation was hematite,which was associated with quartz,glauconite and other minerals. The iron formation of the Changlongshan Formation was divided into two types. Type I is associated with glauconite with relatively smaller thickness,and it is characterized by bands of glauconite sandstone interbeded with iron sandstone and near-spherical hematite microscopically. Type II is not accompanied by glauconite with relatively larger thickness,and it is represented by black iron-rich layers intercalated with quartz lens and acicular or snowflake hematite under the microscope. Tectonic evolution and trace element analysis showed that the iron source was mainly derived from the irony weathering crust at the top of the Xiamaling Formation. The type I iron formation is a mixture of chemical colloid and glauconite weathering,and the type II iron formation is mainly formed by colloidal chemical deposition and could undergo mylonitization in the later stage. The research results provide a sedimentary geological basis for distribution and prospecting of the Precambrian iron formation in North China,and it is also of great significance for enriching the metallogenic mechanism of the iron formation and revealing the tectonic evolution of Proterozoic of North China.