Character and age of ore forming processes of the Kumusayi pegmatite-type lithium deposit in Altun (NW China) revealed by in-situ geochronology (Rb-Sr and U-Pb dating) and mica geochemistry

Pegmatite-type lithium (Li) deposits are a crucial source for modern batteries, yet their formation remains contentious due to limited understanding of their timing and processes. The existence of Indosinian lithium mineralization in the Altun region and the mechanisms of lithium migration and enrichment during magmatic-hydrothermal processes are particularly debated. The Kumusayi Li deposit, situated in biotite schist with muscovitization and greisenization halos around ore veins, provides significant insights. U-Pb dating of cassiterite (219.7 ± 5.2 Ma) from Kumusayi spodumene-bearing pegmatites confirms an Indosinian mineralization age. In-situ Rb-Sr isotopic analysis of micas reveals two metallogenic episodes: recorded by magmatic-hydrothermal muscovite P1 (206.8 ± 1.9 Ma) and hydrothermal muscovite H1 (196.2 ± 7.1 Ma). Microanalytical data of mica generations reveal a systematic geochemical evolution: Nb-W-Sn-Ta contents increase from P1 to H1-H3, while Li-Rb-Co-Ni decrease, with peak concentrations in H1 (Li 2 O = 1.8–2.3%). Altered biotite schist shows significant lithium depletion (primary biotite: Li 2 O = 0.18–0.25% vs. Re-equilibrated biotite: 0.03–0.06%), indicating substantial lithium mobilization during hydrothermal alteration. The metallogenic process comprises three stages: early magmatic crystallization of spodumene depletes lithium; late magmatic formation of lower-Li muscovite P1 (Li 2 O = 0.83–1.33%); and hydrothermal overprinting, where Li-enriched fluids precipitate high-Li muscovite H1, followed by progressive lithium depletion through H2-H4 (Li 2 O = 0.08–0.15%). This magmatic-hydrothermal mechanism is appears to be essential for Li enrichment leading to Li mineralization. Recognizing Indosinian lithium mineralization at Kumusayi and understanding lithium recycling between magmatic and hydrothermal systems offers new genetic models and exploration strategies for rare-metal deposits in the Altun belt. Notably, lithium replenishment from country rocks during hydrothermal alteration provides a novel perspective for regional prospecting.