The Phosphorus Sources And Its Enrichment Mechanisms Of The Cambrian Xinji Formation In Baofeng,Western Henan

The Ediacaran phosphate enrichment is influenced by hydrothermal activity and tectonics,after the Cambrian phosphate was enriched by upwelling.However the phosphate enrichment mechanism of the early Cambrian is still unclear.Xinji Formation is the earliest Cambrian deposit on the North China Craton southern margin,characterized by marine phosphorus-bearing clastic rocks,whose phosphate source and enrichment mechanism need to be further studied.In this paper,we selected the middle of Xinji Formation that a typical phosphorus-bearing in Chunshuping,Baofeng,Pingdingshan,western Henan for the study.It made sedimentological,petrological and geochemical methods to investigate the phosphorus source and enrichment mechanism of phosphorus nodules and obtained the following conclusions.（1）The middle of the Cambrian Xinji Formation from study area is phosphorus-rich sandstone and siltstone,with simple sedimentary structures and mainly blocky laminations.The middle of Xinji Formation was formed in the coastal-shallow marine transition zone.（2）The phosphate nodules of the middle of Xinji Formation in the study area are mainly composed of phosphate minerals and quartz.The phosphates minerals all retain characteristics of seawater primary deposits and are produced as incomplete apatite and colloidal phosphorites.According to micropetrological characteristics of the colloidal phosphate ores,those can classify into cryptocrystalline rounded colloidal phosphate ores and microcrystalline oolitic colloidal phosphate ores.The phosphate crystalline form is unstable and precipitates mainly as phosphate ultramicroscopic berry groups.（3）The phosphorus-bearing sandstones and nodules from the moddle of Xinji Formation in the study area have the highest P₂O₅ content of 15.63%,the highest Ca O content of 22.33%and the lowest Mg O content of 0.07%.While the pro-biotic elements Ni and Zn,which represent the degree of organic matter enrichment,are as high as2860×10^-6μg/g and 3345×10^-6μg/g,respectively.The NASC standardised partitioning pattern of rare earth elements for sandstones and phosphorus nodules indicates the involvement of biogenic effects in the formation of phosphorus nodules.（4）Phosphorus from the Xinji Formation in the study area is mainly derived from deep-sea phosphorus-rich water.The deep-sea phosphorus-rich water reaches the continental slope and accumulates there under the action of upwelling transport.Small crustal organisms on the surface of the shallow sea interact with the phosphate,increasing the phosphate concentration in the surrounding water column and changing the water column medium.At the same time,due to the upwelling of deep seawater,the temperature increases,the pressure decreases and CO₂ escapes causing a rise in the p H of the water column,leading to a decrease in phosphate solubility.Under changing hydrodynamic conditions,supersaturated phosphate in oxygen-rich waters begins to precipitate due to chemical and bio-chemical interactions and is deposited by organic matter adsorption.This study provides richer evidence for recovery of the early Cambrian phosphate enrichment mechanism at the North China Craton southern margin,while facilitating the establishment of a cross-sectional comparison of phosphate deposition patterns on a global scale.This paper includes 22 pieces of figures,8 pieces of tables and 115 pieces of references.