Constraints of sedimentary environment on phosphorization of phosphorus-bearing rock series in the Xinhua Gezhongwu Formation, Zhijin County, Guizhou Province

Abstract

Objective The Early Cambrian superlarge rare earth-rich phosphate deposit in Xinhua, Zhijin County, Guizhou Province, is a typical marine sedimentary deposit. There have long been controversial on the mineralisation environment and mineralisation mechanism. Methods In this paper, the sedimentary environment and formation mechanism of the phosphorus-bearing rock series in the Gezhongwu Formation are verified, through the study of the microscopic characteristics of phosphorus-bearing rock series rocks or ores and the evidence of elemental geochemistry. Results This study reveal the regular changes in the structure of phosphorite from the bottom up of the phosphorous-bearing rock series: the grain sizes decreasing; the interstitial materials between grains vary from bright dolomite cement to micritic matrix, such as siliceous and collophanite, followed by bright dolomite cement and finally micrite collophanite and siliceous. The cementation mode has mainly converted from pore cementation to pore-based cementation, and the supports change from grain to grain-matrix supports. Moreover, the single layers of the rocks become thinner and darker, and the development of staggered bedding was decreased. Conclusion These characteristics reflect that the sedimentary environment of the phosphorus-bearing rock series is a subtidal zone with strong hydrodynamic forces, and of the upper section has sedimentary depths higher and hydrodynamic conditions slightly weaker than those of the lower section. The significant negative Ce anomalies (δCe between 0.32-0.39) and low Ni/Co (0.98-6.07) and V/Cr ratios (0.57-12.50) of the phosphorus-bearing rock series show that the sedimentary water had oxidation characteristics similar to those of a modern marine environment. The lower 103·Sr/Ca(2.00-3.38) and 1/Σ(Al2O3+TiO2) ratios (0.57-3.45) indicate that the ancient water depth was generally shallow, but the relative depth changed frequently. The distribution characteristics of Fe, Cu and Ba contents indicate that the palaeoproductivity of carbonate in the lower section is higher than that in the upper section. In the palaeoenvironment where various conditions are coupled, the phosphorite was finally enriched and formed by precipitation-stirring-subdivision-cementation-solidification.

Keywords

• phosphorus-bearing rock series
• sedimentary structure
• geochemistry
• zhijin county, guizhou province