Petrogenesis of granitic pegmatites in the Chinese Altay: Insights from apatite, garnet, and quartz geochemistry

Abstract

Granitic pegmatites, derived from highly evolved and volatile-rich felsic magmas, represent one of the world’s major sources of rare metals, including Li, Be, Nb, Ta, Zr, Hf, Rb, and Cs. Despite their economic importance, the petrogenesis of pegmatites remains a subject of considerable debate. In this study, we conducted in situ major- and trace-element analyses of apatite, garnet, and quartz from six pegmatite bodies (Xiaokalasu, Dakalasu, Yelaman, Jiamanhaba, Amulagong, and Tiemulete) in the Chinese Altay, using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). Apatite samples display elevated MnO contents (0.08–7.39wt%; mean = 1.5; N = 78), low Zr/Hf ratios (0.38–69.43; mean = 9.67), and high Y/Ho ratios (29.63–120.97; mean = 45.26). Garnet samples exhibit variable Zr/Hf ratios (4.91–239.59; mean = 31.67; N = 101) but consistently high Y/Ho ratios (27.29–240.84; mean = 50.94). Quartz samples are characterized by high Ge/Ti ratios (0.16–7.68; mean = 1.12; N = 172) and Al/Ti ratios (1.22–1952.20; mean = 40.12). These diagnostic geochemical signatures provide strong evidence for a hydrothermal origin, confirming that the magmas of these six pegmatites reached magmatic fluid saturation during crystallization, as predicted by the J-B model. Importantly, distinct ionic radii, charge, chemical reactivity, and crystal structure account for the varied geochemical behaviours of rare metals during hydrothermal processes: (1) Nb-Ta concentrations show only weak correlations with Y/Ho ratios, suggesting limited hydrothermal influence on their enrichment; (2) In contrast, Li concentrations exhibit strong positive correlations with both Al/Ti and Ge/Ti ratios, indicating significant hydrothermal mobilization and subsequent mineralization. These results provide new constraints on the hydrothermal processes controlling rare-element enrichment and mineralization, offering particularly valuable insights into the mechanisms of Li enrichment.