Ion adsorption rare earth elements deposits are the principal global source of heavy rare earth elements (HREEs). The current perception is that HREE deposits are generated from HREE-enriched granitoids, whereas LREE deposits originate from LREE-enriched intermediate-acidic magmatic and metamorphic rocks. However, contrary to the prevailing view that HREE-rich ion-adsorption mineralization requires HREE-enriched parent rocks, this study demonstrates that intensive weathering of LREE- dominated granodiorite (LREE/HREE = 3.1) can yield an HREE ore body. To elucidate this infrequent ore-forming process, geochemical, mineralogical, and REE speciation analyses were conducted on the weathering profile samples from the Meng Khun (MK) deposit, Laos. The results show that the weathering-resistant monazite is strongly enriched in LREEs (LREE/HREE = 11.4), contributes 66.2% of the total REE content and controls the LREE-dominated signature of the parent rock. In contrast, titanite and apatite are relatively enriched in HREEs (LREE/HREE = 0.45 and 0.54, respectively) and more susceptible to weathering, and account for 25.2% of the REE content. Their decomposition provided the source for the formation of the HREE ore body in a LREE-enriched regolith. In addition, Ce³⁺ was oxidized and immobilized as cerianite (CeO₂) through interaction with Fe-Mn oxides and atmospheric oxygen, or selectively complexed by organic matter to form stable complexes, further enhancing HREE proportions—particularly in the ion- exchangeable fraction. HREEs exhibit stronger mobility than LREEs and tend to migrate deeper, resulting in greater accumulations of HREEs at the base of the ore body (LREE/HREE = 0.34 in ion-exchangeable fraction). Simultaneously, carbonate- selective complexation fixed HREE in the deeper layers of the weathering crust rather than their migration outward. This particular mineralization process is collectively controlled by the differential weathering of REE-bearing accessory minerals, Ce immobilization, varied REE mobility, and selective complexation with (bi) carbonates. This study constructs the genetic model of this unconventional HREE-rich ion adsorption ore and broadens the prospecting perspectives of HREE resources.
Xie et al. (Mon,) studied this question.