Sources of rare earth elements and yttrium in the early Cambrian phosphorites in Zhijin, southwest China

Abstract

The early Cambrian Zhijin phosphorite in Guizhou province, China, has high rare earth elements and yttrium (REY) contents of up to 2500 ppm, with heavy REY (HREY) accounting for ∼30 % of the total REY. However, their REY sources are not well-determined. Herein, elemental geochemistry and Sr–Nd isotopes of phosphorites in four sections from the Zhijin phosphorite ore area were investigated, primarily to constrain the contributions of seawater and terrestrial REY sources to the REY enrichment in these phosphorites. Zhijin phosphorites have high Y/Ho (45–63) and seawater signature’s 87Sr/86Sr(i) ratios (0.7085–0.7092), suggesting a partial contribution from seawater-sourced REY. On the other hand, Zhijin phosphorites have εNd(t) values of −8.6 – −6.8, indicative of the great influence from continental weathering. The Y/Ho ratios exhibit negative correlations with their Zr contents and SmN/YbN ratios; meanwhile the 87Sr/86Sr(i) ratios exhibit positive correlations with their Sr contents and REY contents, indicating that terrestrial source contributions played an important role in REY enrichment in Zhijin phosphorites. Furthermore, the provenance of Zhijin shales indicates the terrigenous granite weathering, during which the regolith-hosted REY could be transported to Zhijin shallow seawater, providing abundant REY to Zhijin phosphorites. We proposed that the REY enrichment in Zhijin phosphorites could be associated with high REY seawater induced by granite weathering. This study provides a new perspective on the relationship between granite weathering and marine REY deposition, which may be a common phenomenon in geological history but remains to be fully investigated in future.