Source, enrichment mechanism, and mineralization age of REY-rich phosphorites: A case study of the Early Cambrian Kunyang and Dongchuan phosphorite deposits, SW China

Abstract

Sedimentary phosphorites have been recently found to have substantial concentrations of rare earth elements (REE) and yttrium (i.e., REY) which are considered strategic metals for advancing new energy and high-tech industries. This study investigates the REY enrichment mechanisms and mineralization age of REY-rich phosphorites in the Kunyang and Dongchuan phosphorite deposits using in-situ analysis of collophanite. Based on hand specimens and microscopic observations, the studied phosphorites are categorized into three types: clastic phosphorite (Type I), massive phosphorite (Type II), and striped phosphorite (Type III). The U-Pb dating of Early Cambrian phosphorites from the Kunyang deposit (Yunnan Province, SW China) yielded ages of 529.2 ± 14.8 Ma for the upper phosphorite layer and 535.1 ± 25.6 Ma for the tuff interlayer. The REY indices (e.g., La_N/Yb_N, Ce/Ce*, Eu/Eu*, and Y/Ho) and in-situ REY mapping indicate that diagenetic is the primary REY enrichment mechanism in both deposits. Additionally, the sub-oxidized depositional environment and reworking processes impacted REY enrichment. Hydrothermal alteration in the Kunyang phosphorites minimally affected the REY content but altered the REY pattern, whereas in the Dongchuan phosphorites, hydrothermal activity enhanced REY enrichment without altering the REY pattern. The phosphorites likely formed in a marine environment, with REY primarily sourced from seawater, while terrigenous clasts further contributed to the REY composition.