Abstract:  Grain size analysis is a very important method in the field of sedimentation. It is widely used in the evaluation of oil and gas reservoirs and the identification of sedimentary environment for terrigenous clastic rocks, but is rarely applied to endogenous clastic rocks such as phosphorus rocks. In this paper, the sedimentary phosphorite of Yangchang phosphate mine in Zhenxiong County, Yunnan province, is taken as the research object. The grain size information is obtained by measuring thin sections under the microscope, and the grain size distribution histogram, frequency curve, cumulative frequency curve, and probability cumulative curve are drawn and analyzed. The sedimentary environment of the study area is analyzed by using Cent-Median diagrams and structural parameter scatter diagrams. The research results indicate that grain size parameters are closely related to the hydrodynamic conditions of sedimentary water bodies. High energy hydrodynamic environments are prone to the formation of phosphate rocks with larger average grain size, poor sorting, bimodal and flat kurtosis. Low energy stable environments often form phosphate rocks with smaller average grain size, good sorting, single peak and sharp kurtosis. Based on various parameters and diagrams as well as the structural and texture characteristics, the grain size parameters of the sample exhibit the characteristics of beach sand. It is concluded that the Zhenxiong Yangchang phosphate mine was deposited in a shallow marine environment with continuous sea level oscillations and strong hydrodynamic forces.

Key words: grain size analysis, phosphorite, sedimentation, Yangchang phosphorite mine, sedimentary environment, hydrodynamic force