Biogenic mineralization controls exceptional REY enrichment in Early Cambrian phosphorites from South China

Abstract

The Early Cambrian represents a globally significant phosphorus-formation period, particularly noted for producing REY-rich phosphorites in South China. However, substantial differences exist in the REY enrichment levels among these phosphorites. Particularly, the REY contents of the Early Cambrian Meishucun Stage phosphorites from Guizhou and Yunnan exhibit pronounced disparities, with the controlling factors behind these variations remaining a subject of debate. To address this issue, this study conducted comparative analyses of representative REY-bearing phosphorites from Cambrian Meishucun Stage in Bailongtan, Yunnan province, and Zhijin, Guizhou province, China. Advanced analytic techniques, including XRF, ICP-MS, and ICP-OES, were employed to determine major, trace elements and REY concentrations in different phosphorite types. And complementary methodologies including OM, SM, SEM-EDS, and EPMA were utilized to examine the mineralogical and textural features. The results indicated that biofossils preserved in the phosphorites of both regions are small shelly fossils (SSFs), with carbonate fluorapatite as the principal component. Zhijin phosphorites exhibit significantly higher REY enrichment compared with Bailongtan phosphorites, and REY contents positively correlate with the abundance of SSFs. In-situ analyses revealed that REYs are primarily concentrated within SSFs, which were better preserved in oxic and open environments with relatively slow sedimentation rates. The multilayered nested structures of SSFs facilitate the absorption of REYs from seawater and porewater, which corresponding to the biogenic structures of bioapatite observed in modern deep-sea REY-rich sediments. These findings suggested that SSFs play a crucial role in REY enrichment, with Zhijinites potentially serving as the optimal host phase for REY enrichment. Conclusively, biota play an important part in the REY-rich mineralization of phosphorites, offering new insights into the genesis of global sedimentary REY-bearing phosphorites. This understanding also opens new considerations for optimizing REY extraction processes from such phosphorites.