<p>With the global vigorous development of high-tech industries, the market demand for rare earth elements and Yttrium (REY) have experienced explosive growth. In recent years, phosphate associated REY resources (up to 18,000 ppm) found in modern deep-sea muds have attracted significant attentions. It is believed that their REY originated directly from porewater in deep-sea sediments, with a process of detrital dissolution releasing REY into the porewater. However, it remains unclear whether this model could be applied to REY-rich phosphorites from geological history, which contrasts sharply with the widely accepted Fe-oxide redox pump model at the seawater-sediment interface. Here, we present the mineralogy and REY geochemistry of clay minerals, and whole-rock Si-Fe isotopic compositions (δ³⁰Si and δ⁵⁶Fe) from early Cambrian REY-rich Zhijin phosphorite (ΣREY, ~2000 ppm) and REY-poor Meishucun phosphorite (ΣREY, &lt;400 ppm) in South China. In the Zhijin samples, illite formed around the edges of muscovite, exhibiting seawater-like REY patterns. In contrast, muscovite and orthoclase in the Meishucun samples show no contact relationship and display REY patterns typical of granite-forming minerals. The REY contents (reach 310 ppm) of muscovites from Zhijin samples are much higher than that of detrital muscovite (REY, 0.09–5.86 ppm) and orthoclase (REY, 2.69–6.45 ppm) from Meishucun samples. Furthermore, the data can be classified into two categories based on the correlation between δ³⁰Si values and phosphate-hosted REY enrichment in Zhijin and Meishucun samples. One is characterized by higher δ³⁰Si values (average 0.6‰) and Y/Ho ratios (average 56), and lower SiO₂ and REY contents (average 8.10% and 1076 ppm, respectively). The other group exhibits lower δ³⁰Si values (average 0.2‰) and Y/Ho ratios (average 52), and higher SiO₂ and REY contents (average 17.45% and 2085 ppm, respectively). In addition, compared to the Meishucun phosphorites formed in well oxidized marine environment (δ⁵⁶Fe, ~0‰), the Zhijin phosphorites developed under fluctuating oxic-suboxic marine condition exhibit a significant negative correlation between Fe and Si isotopic compositions. These findings indicate Zhijin muscovite underwent REY exchange with seawater under fluctuating redox marine conditions, leading to REY release into the porewater and the formation of authigenic illite. This process did not occur in the Meishucun samples. It’s well known that continental weathering transformed feldspar into REY-rich muscovite or illite. These REY-rich clays, transported to the seawater-sediment interface, were dissolved and released REY into the porewater within a fluctuating oxic-suboxic marine environment. We proposed for the first time that fluctuating marine redox conditions acted as a trigger for REY enrichment in ancient phosphorites, with the primary REY source being terrigenous REY-rich clay. This perspective not only provides an insight into the mechanism of REY enrichment in phosphorites, but could also account for the lack of anomalous REY enrichment in phosphorites exhibiting the precipitation of extensive pyrites if only Fe-oxide redox processes are considered.</p>