Apatite as a tracer for magmatic-hydrothermal transition and Li mineralization in the world-class Bailongshan Li pegmatite deposit, Northwest China

Lithium (Li)-rich pegmatites are the principal economic source of Li, a critical metal underpinning global net-zero CO2 emission targets and clean energy transitions. However, the timing and geochemical variations of the magmatic-hydrothermal transition governing Li hyperenrichment and world-class pegmatite formation remain poorly understood, impeding efficient exploration. Here, we report on our investigation of the world-class Bailongshan pegmatite Li-Be deposit of the Western Kunlun Orogenic Belt, Northwest China (>5.06 Mt Li2O resources), using integrated apatite geochemistry and geochronology. Apatite from mineralized pegmatites shows distinct core-rim or patchy zoning, while that from barren pegmatites is homogeneous. Micro-X-ray fluorescence and laser ablation−inductively coupled plasma−mass spectrometry maps reveal sharp chemical gradients: Magmatic apatite cores (Ap-1) are enriched in rare earth elements (REEs; Y, La, and Ce), and hydrothermally modified rims and patches (Ap-2) are Sr-enriched, which is consistent with optical cathodoluminescence (OCL; Ap-1 exhibits homogeneous yellow luminescence; Ap-2 exhibits irregular luminescence). LA-ICP-MS U-Pb dating yields consistent ca. 190 Ma lower-intercept ages for cores and rims across zoning types: core-rim zoned apatite (cores: 190.3 ± 2.1 Ma, mean square of weighted deviates [MSWD] = 1.7, n = 28; rims: 188.9 ± 8.3 Ma, MSWD = 1, n = 30) and patchy zoned apatite (cores: 192.7 ± 3.7 Ma, MSWD = 2.5, n = 25; rims: 192.5 ± 7.8 Ma, MSWD = 1.3, n = 25). While these ages cannot quantify transition duration, their tight clustering contradicts long-term cooling models. Coupled dissolution-precipitation textures (e.g., core-rim interface dissolution and heterogeneous elemental redistribution) indicate rapid fluid-melt interactions, which were critical for preventing Li loss and forming Bailongshan’s world-class resources. Apatite geochemistry distinguishes mineralized from barren pegmatites. Compared to barren counterparts, mineralized apatite exhibits lower CaO (52.3−54.6 wt% versus 55.1−57.0 wt%) and total REE (ΣREE; average [avg.] 74.0 ppm versus 3417 ppm), but higher MnO (avg. 1.7 wt% versus <0.26 wt%), Li (avg. 97.0 ppm versus 3.65 ppm), and Sr (avg. 323 ppm versus 202 ppm), and significantly higher Sr/ΣREE ratios (>20 versus 0.05−0.06). This study establishes apatite as a robust geochemical chronometer for decoding magmatic-hydrothermal transitions in Li-rich pegmatites. The transition at Bailongshan reveals that Li hyperenrichment is a geologically transient event driven by rapid fluid-melt immiscibility. Critically, apatite Sr-REE zoning, Li concentrations, and Sr/ΣREE ratios constitute quantifiable exploration tools for Li-rich pegmatites in the Western Kunlun−Songpan−Ganze Orogenic Belt and global orogenic settings, addressing the urgent demand for efficient Li discovery.