Key factors controlling the pre-enrichment of heavy rare earth elements in the Shitouping ion-adsorption type deposit, southern Jiangxi: Evidence from Ba isotopes

The pre-enrichment process of parent rocks in ion-adsorption-type heavy rare earth element (HREE) deposits is crucial for the formation of ore bodies. To investigate the two main pre-enrichment mechanisms-mineral fractional crystallization and fluid metasomatism, we conducted mineralogical, whole-rock major and trace element, rare earth element (REE), and Ba isotope studies on the Shitouping complex pluton (SCP), the parent rock of the Shitouping HREE deposit in southern Jiangxi. The SCP comprises medium- to coarse-grained biotite monzogranite (MGBG), medium- to coarse-grained biotite syenogranite (MGSG), and fine-grained alkali feldspar granite (FGAG). From MGBG to MGSG and FGAG, the SCP exhibits significantly high SiO 2 (73.27–79.59 wt%) and low P 2 O 5 (<0.02 wt%) contents, as well as δEu values (0.02–0.28), indicating a highly fractionated complex pluton formed through extensive mineral fractional crystallization. Compared to monzogranite, syenogranite and alkali feldspar granite have higher incompatible element contents (e.g., Nb, Rb), Rb/Sr ratios, and lower TFe 2 O 3 , TiO 2 , Al 2 O 3 , P 2 O 5 , MgO contents, Nb/Ta and K/Rb ratios, and δEu values, suggesting they represent more evolved lithological units. Fluid metasomatism in the SCP led to the development of REE minerals (e.g., bastnäsite-(Ce) and synchysite-(Y)) within biotite and fluorite while preserving their original crystal morphology. Fluorite and zircon also exhibit distinct core-rim textures due to fluid alteration. The Ba isotope compositions of the SCP range from − 0.28‰ to 0.21‰. MGBG has higher Ba content (avg. 219.78 ppm) and δ 138 / 134 Ba values (avg. − 0.01‰) compared to MGSG and FGAG (avg. 28.68 ppm and − 0.13‰, respectively). The more evolved MGSG and FGAG, which have lower Ba isotopic compositions, indicate that fractional crystallization is not the primary controlling factor of their Ba isotopic compositions. Instead, the low δ 138 / 134 Ba values reflect modification by deep-sourced magmatic exsolved fluids. The lack of correlation between REY (rare earth elements and yttrium), HREY, LREE/HREY ratios, and δ 138 / 134 Ba suggests that the enrichment of (heavy) REEs in the SCP are primarily controlled by fractional crystallization of major rock-forming and REE-rich minerals rather than fluid metasomatism. Fluid alteration redistributed light and heavy REEs in early-crystallized minerals (e.g., fluorite, apatite, and biotite), forming weathering-prone HREE minerals (e.g., bastnäsite-(Y) and synchysite-(Y)), which serve as critical sources for in ion-adsorption type HREE deposits (IAHDs).