Constraints on metallogenic mechanism of ion-adsorption type HREE deposit from hydrothermal alteration and evolution features of minerals in Lincang granite, Yunnan

Heavy rare earth elements (HREE) are strategic metals in China, primarily sourced from ion-adsorption type heavy rare earth deposits (iHREE). Granite constitutes an important parent rock for iHREE. Generally, rare earth elements (REE) undergo pre-concentration during the process of rock formation, which is crucial for the mineralization of HREE. However, the mechanisms of migration and concentration of HREE during the magmatic-hydrothermal (alteration) processes remain unclear. The typical iHREE in Lincang, Yunnan Province, exhibits multiple episodes of granite intrusion, and the partial enrichment of HREE in the ore bodies offers a natural research subject. The authors selected the granites in the area as the research subjects. Specifically, the granodiorite (YNlc2-j3) has a zircon shrimp U-Pb age of 214.7 Ma, and the medium-coarse-grained biotite granite (MCGB, YNlc4-j1) has a zircon shrimp U-Pb age of 217.7 Ma. Analysis using back-scattered electron (BSE) imaging, Electron Probe X-ray Micro-Analyzer (EPMA), and TESCAN integrated mineral analyzer (TIMA) revealed that during the magmatic-hydrothermal (alteration) process: (1) the shape of monazite and zircon changes from authomorphic to anhedral. (2) At the magmatic stage, monazite and zircon exist as independent minerals; however, at the hydrothermal (alteration) stage, monazite associates with apatite, and zircon associates with xenotime. (3) HREE-bearing fluorocarbonate minerals from nonexistence to pass into existence. These results indicate that HREE-bearing minerals undergo complex hydrothermal alterations, with a preferential accumulation of HREE in HREE-bearing fluorocarbonate minerals, which are shown to play significant roles in the formation of iHREE.