Skyrocketing worldwide demand for heavy rare earth element (HREE) resources has driven the exploration of ion adsorption−type rare earth element (REE) deposits beyond China. The contribution of REE deposits in Southeast Asia to global REE output recently exceeded 20%. However, a comprehensive understanding of the characteristics and genesis of these tropical region deposits remains insufficient, in contrast to the extensively investigated REE deposits in subtropical regions of South China. The newly discovered Menghai REE deposits of Southwest China are situated in the same tropical region. This study investigated the influence of hydrodynamic zonation, climate, and topography on the geochemical and mineralogical characteristics of the regolith, and analyzed the primary variables governing the activation, migration, enrichment, and differentiation of REEs.

Compared to the REE deposits in the Jiangxi area, the Menghai REE deposits are characterized by thicker and more intensely weathered regolith, and more of the REE resources are derived from the weathering-resistant REE minerals. The development of thick regolith results from substantial precipitation and diminished physical erosion due to abundant vegetation cover. Topographic conditions influence the combination of regolith horizons by regulating physical erosion. This, along with active microbial and plant processes in tropical regions, facilitate the chemical weathering of primary and secondary minerals, particularly those resistant to weathering in the humic soil (A horizon). The hydrodynamic zonation in the Menghai regolith significantly influenced REE mineralization. The REE ions released migrate vertically in the high-velocity vadose zone and are deposited in the low-velocity capillary fringe. The distinctive redox environment led to positive Ce anomalies above the capillary fringe and negative Ce anomalies within it. The vertical differentiation of REEs is primarily controlled by the diffusion process of REE complexions after they enter the capillary fringe. This process forms an upper light REE−enriched ore body overlying a lower HREE-enriched ore body within the regolith.