Petrogenetic Controls on Ion‐Adsorption Type REE Mineralisation in Western Yunnan, China: Insights From Contrasting Palaeogene Granites in the Banggunjianshan and Polunshan Plutons

ABSTRACT

Ion‐adsorption type rare earth element (iREE) deposits form by intense lateritic weathering of granitoids and constitute a critical global REE resource. Recently, Cenozoic granite‐related iREE prospects have been identified within the Tengchong Block, Southwest China. However, the specific granitic compositions responsible for generating these deposits in the Tengchong Block remain poorly constrained, hindering effective exploration. We present petrographic, geochronological and whole‐rock geochemical data for the Banggunjianshan (BGJS) and Polunshan (PLS) plutons and their associated regolith. Zircon U‒Pb dating indicates ∼50 Ma emplacement of the BGJS monzogranite, consistent with residual zircon ages from the overlying regolith‐hosted deposits. They host abundant readily weatherable REE‐bearing minerals (allanite and titanite) and exhibit elevated whole‐rock ΣREE contents (381–906 ppm). Geochemically, this intrusion is metaluminous to slightly peraluminous and enriched in Th, U, Zr and Hf. These characteristics indicate a strongly differentiated nature with a transitional I‐A affinity, conducive to REE enrichment. In contrast, the PLS granite (∼62 Ma) is characterised by lower ΣREE abundances (218–290 ppm) and sparse allanite and titanite. It is high‐K calc‐alkaline and depleted in HFSEs, consistent with weakly fractionated I‐type granite. These results demonstrate that Eocene monzogranites constitute the primary exploration target for iREE deposits within the Tengchong Block.