Progressive enrichment of rare earth elements (REE) between parent granites and weathering crust contributed to the generation of regolith-hosted REE deposits in South China

Rare earth elements (REE) concentrations and patterns in granites controlled the generation of regolith‑hosted REE deposits, but the REE occurrence and enrichment mechanisms in granites with few primary REE-rich minerals, such as allanite and titanite, are not well studied. In this contribution, we conducted detailed whole-rock and in-situ mineral geochemical studies on the Xunwu biotite monzogranite and muscovite syenogranite, where some samples from the monzogranite have total REE contents higher than 1000 ppm. The monzogranite and syenogranite were generated in the Late Cretaceous with similar zircon U-Pb ages (95.3±0.3 and 96.4±0.3 Ma, respectively). REE-phosphates and Ti-Fe oxides (magnetite and ilmenite) are the dominating REE-bearing minerals in the monzogranite and syenogranite. The REE-phosphates are distributed radially in fractures of feldspar or occur around and replace the primary apatite, indicating generation due to interaction between primary apatite and REE-rich fluid. The Ti-Fe oxides generally show high REE contents, which is also caused by interaction with the REE-rich fluid. The higher P and Fe contents in the monzogranite make it more effective at fixing the REE during fluid-rock interaction and thus display higher REE contents than those of the syenogranite. The occurrence of negative Ce anomalies in altered minerals from both the monzogranite and syenogranite indicates that the fluids had high oxygen fugacity with high REE but low Ce abundance. This kind of fluid is consistent with the circulating meteoric water carrying REE ions from the weathering crust. This study highlights the progressive enrichment of REE between parent granites and weathering crust as follows: (1) Granites intrude and the upper parts are weathered to generate the REE-bearing weathered crust; (2) Circulating REE-bearing meteoric fluids replace apatite with REE phosphate and enrich Ti-Fe oxides in granite with REE; followed by (3) Continued upgrading of REE enrichment lower in the developing weathered granite profile over time by downward circulating meteoric fluids which leach REEs from previously formed phosphate and oxide minerals and likely their weathered equivalents in the upper parts of the granite regolith. These processes favor the generation of regolith‑hosted REE deposits in the weathering profile for those granites with the absence of primary REE-rich minerals.