Mineralogical characteristics and genesis mechanism of glauconite in the meso-neoproterozoic and cambrian strata in north China

Glauconite is a special mineral indicator of sedimentary facies and is widely developed in the Meso-Neoproterozoic and Cambrian strata in North China. A systematic study on the occurrence characteristics and genetic mechanism of glauconite is of great geological significance for the reconstruction of paleogeographic environments in North China. The results show that glauconites are well-developed in the Meso-Neoproterozoic Tieling, Xiamaling, Changlongshan, and Jingeryu formations and the Cambrian Xuzhuang Formation, with granular, cemented, fecal pellet, and halo - rimmed forms. The oxide components of glauconite mainly include K2O, Si2O, Al2O3, TFeO, MgO, etc. Moreover, the study area mainly developed low mature to mature glauconite based on the analysis of K2O content, color, SEM energy spectrum and XRD, and the maturity of glauconite in the Longshan and Jiguanshan areas is lower than that in the Jizhou area. The K, Fe, Mg, and Al ions source of glauconite is sufficient, and the parent rocks also provide sufficient substrate favorable for the formation of glauconite in the study area. Glauconites with different substrate components indicate different formation mechanisms. The glauconite with quartz and feldspar grains as the substrate is of a pseudomorphic replacement genesis, while the glauconite with biotite as the substrate is of a layer lattice theory genesis, and cemented glauconite is of an authigenic precipitation genesis. Furthermore, the study revealed that glauconites with fecal pellets and stromatolites as substrate are a combination of biogenesis and verdissement or authigenic precipitation, and halo - rimmed glauconite is of a combination of pseudomorphic replacement and authigenic precipitation. According to the findings, the evolution of K2O-TFeO in Precambrian glauconite is not only controlled by a specific substrate, but also by specific chemical conditions. Because of the differences in composition of solubility of the substrate, the pH conditions of the pore water required for glauconization are different. The sedimentary environment and substrate type affect the form and genetic mechanism of glauconite, but the varied pH conditions suitable for the substrate and the sub-oxide environment near the Fe oxidation-reduction interface are the necessary conditions for glauconization. As a result, glauconite can be used as a signature mineral for the palaeo-redox facies. The study provides insights into the formation mechanism of different glauconite in the process of sedimentation or diagenesis, and the scientific basis for the restoration of diagenesis conditions and the paleogeographic environment in North China.