Rare earth element enrichment in sedimentary phosphorites formed during the Precambrian–Cambrian transition, Southwest China

Numerous sedimentary phosphorites in Southwest China were formed around the Precambrian–Cambrian transition (PC–C), including the upper Ediacaran Doushantuo Formation and lower Cambrian Gezhongwu Formation. The Gezhongwu phosphorites in Zhijin exhibit marked rare earth element (REE) enrichment (>1000 ppm), and may represent new REE resources. Although the main characteristics of the Gezhongwu phosphorites have been well constrained, the REE enrichment mechanisms remain unclear. We undertook a comparative study of three typical sedimentary phosphorites with variable REE contents formed at the PC–C transition in central Guizhou Province, Southwest China. These include sections A and B of the Doushantuo phosphorites (560 ±8 Ma) from the Weng’an area (i.e., WA-A and WA-B), and the Gezhongwu phosphorites (527 ±24 Ma) from the Zhijin area (ZJ). The phosphorites were investigated with state-of-the-art macroscale to nanoscale analytical techniques. In contrast to the extraordinary REE enrichment in the ZJ phosphorites (average ΣREE = 1157 ppm), the phosphorites in WA-A (average ΣREE = 234 ppm) and WA-B (average ΣREE = 114 ppm) are REE-poor. Elemental mapping by laser ablation–inductively coupled plasma–mass spectrometry, along with transmission electron microscopy analyses, showed the REEs in the studied phosphorites are hosted in nanoscale francolites. The ⁸⁷Sr/⁸⁶Sr and Y/Ho ratios of the francolite grains indicate that greater terrigenous input may have led to more REE enrichment in the WA-A than WA-B phosphorites, but this cannot explain the extraordinary REE enrichment in the ZJ phosphorites. The F/P₂O₅ values of the francolite grains in the ZJ phosphorites (~0.097) are higher than those in the WA-A (~0.084) and WA-B (~0.084) phosphorites, and the grain size of the francolite in the ZJ phosphorites (~89.9 nm) is larger than those in the WA-A (~56.6 nm) and WA-B (~57.4 nm) phosphorites, indicative of more intense reworking of the ZJ than WA phosphorites during early diagenesis. A plot of Nd concentration versus Ce/Ce* reveals that lower sedimentation rates characterized the ZJ phosphorites. Therefore, intense sedimentary reworking during early diagenesis resulted in more REEs being sequestered by the marine phosphates from seawater and pore waters at a lower sedimentation rate, which resulted in the extraordinary REE enrichment in the ZJ phosphorites. Our findings highlight the multiple factors that controlled formation of sedimentary phosphorites around the PC–C transition (especially the intense reworking and redox conditions of the overlying seawater), and provide further insights into REE enrichment in sedimentary phosphorites worldwide.