<p>The Tuguanzhai–Maliping ion-adsorption rare earth element (iREE) deposit, located in the high-altitude Tengchong Block of southwestern China, challenges the conventional view that highly complex terrains are unfavorable for iREE mineralization. This study presents a detailed investigation of weathering profile 3239 within this deposit, which exhibits significant fractionation between light rare earth elements (LREE) and heavy rare earth elements plus yttrium (HREY), characterized by LREE enrichment in the upper section and HREY enrichment in the lower section. Through systematic analyses, including ore microscopy, mineralogy, whole-rock geochemistry, sequential extraction of REE phases, and mass balance calculations using Zr as an immobile reference element, the fractionation and enrichment behaviors of REE during weathering are constrained. The mass balance coefficient τHREY in profile 3239 reaches a maximum value of 6.31, indicating intense supergene enrichment of HREY that contrasts with the LREE-enriched signature of the parent rock. Vertical migration of REE within the profile reveals that the differentiation between LREE and HREY is not gradual but rather marked by an abrupt enrichment of HREY in the lower section, accompanied by a sharp decline in the REE/HREY ratio. Correlation analysis shows that τHREY is positively correlated with τTFe₂O₃ and τMnO, and exhibits a significantly stronger correlation with the colloidal fraction of REE than with other fractions, suggesting that Fe–Mn oxides/hydroxides exert control over HREY enrichment. In addition, τHREY shows strong correlations with τAl₂O₃, τK₂O, and τNa₂O, reflecting the link between alkaline feldspar leaching and HREY migration–adsorption processes. The notably strong correlation between τHREY and τLOI further implies that soil solutions or groundwater-rich horizons play a critical role in the formation of HREY-enriched layers. In contrast, τHREY displays weak correlations with weathering indices such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), Na-feldspar index of alteration (NIA), and K-feldspar index of alteration (KIA), indicating that HREY migration and enrichment are largely insensitive to the overall degree of weathering. This detailed profile-based study suggests that, under specific high-altitude geochemical and supergene conditions, significant HREY enrichment can occur within granitic weathering crusts, leading to the formation of HREY-enriched ore bodies characterized by a distinctive “reverse” fractionation pattern. These findings expand the genetic model of iREE deposits and provide a theoretical basis for REE exploration in nontraditional, topographically complex regions, although this understanding requires further validation through additional profile studies.</p>