A genetic linkage between refertilized mantle and REE enrichment in an alkaline-silicate system: A case of nepheline syenite in the Gejiu district, SW China

Alkaline-silicate rocks represent an important worldwide reservoir of rare earth elements (REE). The weathering of these alkaline-silicate rocks has the potential to create regolith-hosted REE deposits. The factors contributing to the enrichment of REE in these alkaline-silicate rocks remain unclear. A giant regolith-hosted REE deposit, found in the Gejiu district on the western edge of the Youjiang Basin in Southwest China is linked to nepheline syenites. This study presents new geochemical and petrographic data for the Puxiong and Baiyunshan nepheline syenites, providing insights into the processes responsible for the enrichment of REE in the Gejiu alkaline-silicate system. The Gejiu nepheline syenites originated in the late Cretaceous period (84-80 Ma) and exhibit an enrichment of large-ion lithophile elements (LILEs), a depletion of high field strength elements (HFSEs), elevated (87Sr/86Sr)i and Rb/Sr ratios, low epsilon Nd(t) values, and elevated Ba/Rb ratios. The geochemical features of the Gejiu nepheline syenites suggest that they were formed from a metasomatic mantle with a high abundance of phlogopite. The zircon Hf-O isotopes, characterized by epsilon Hf(t) values ranging from -8.8 to -1.4 and delta 18O values between +7.0 %o and + 8.9 %o, in conjunction with elevated Ba and Rb contents, indicate that the origin of the alkaline rocks involved metasomatism by volatile-rich materials released from subducted marine sediments. The REE minerals present in these alkaline rocks are mainly britholite-(Ce) and tritomite-(Ce), indicating their magmatic and hydrothermal origins. The observed petrographic features indicate a higher concentration of REE minerals in the Gejiu alkaline rocks as a result of a self-hydrothermal process. The REEs and volatile phases present in these alkaline rocks are inferred to have originated primarily from the source, suggesting that the refertilization of REEs within the mantle enriches the Gejiu alkaline-silicate rocks with REEs. Moreover, the weathering crust derived from these alkaline-silicate rocks also demonstrates considerable potential as a suitable environment for REE deposits originating from regolith.