Formation of HREE-enriched ion-adsorption ore from LREE-rich granodiorite: a case study from the Meng Khun deposit, Laos

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.