Geochemical Characteristics and Formation Mechanism of Phosphorite of Lower Cambrian Maidiping Formation in Huangjiaping Area of Mabian County, Southern Sichuan

During the Meishucun period of the Early Cambrian, a large-scale phosphorus formation event occurred on the Upper Yangtze platform. As the product of this event, the sedimentary environment and formation mechanism of phosphorite remains elusive. In order to better understand the sedimentary environment and formation mechanism of phosphorite enrichment, we investigated the geochemical characteristics of phosphorite in Maidiping Formation of Lower Cambrian in Huangjiaping area of Mabian County. The results show that: ΣREE has significant positive correlation with P₂O₅, and (La/Yb)_N and (La/Sm)_N ratio are 0.98 ~ 1.61 and 0.93 ~ 1.39, indicating that REE enrichment is affected by early diagenetic adsorption, and the phosphorite retains the original fractionation characteristics of REE. Obvious negative Ce anomalies (average Ce/Ce* is 0.44) and the absence of framboid pyrite in the phosphorite indicating that the phosphorite were deposited under oxic/dysoxic conditions. The high Y/Ho ratio (mean 65) is consistent with the Y/Ho ratio of modern oxygen-containing seawater, indicating that phosphorite is derived from the original oxic seawater. The weak negative Eu anomaly (Eu/Eu* mean value is 0.9) indicates that the formation of phosphorite is not affected by hydrothermal activity. The REE distribution pattern is “hat type”, indicating that the formation of phosphorite is controlled by Fe-redox pump at sea-sediment interface. In combination with the above findings and combined with the highly stratified redox ocean structure and the intensive upwelling in the early Cambrian, the phosphate released during the redox of FeOOH was transported into the shallow water. In the shallow underground burial, phosphate is enriched by biodegradation and reductive release of FeOOH, and finally phosphate and Ca²⁺ are combined to form apatite, which is continuously enriched and ore-forming.