Geochronology and multiphase magmatic-mineralization of rare-metal pegmatites at the Tashidaban and Kumusayi Li deposits, Central Altyn Tagh Orogen, NW China

Recent research indicates that the spatiotemporal evolution of pegmatites within orogenic belts and their associated rare-metal deposits was closely linked to the peaks of magmatic activity across orogenic cycles. In recent years, many rare-metal granitic pegmatite deposits have been newly discovered in the Altyn Tagh Orogen of northwestern China, exhibiting clear evidence of multiphase pegmatite emplacement. To constrain the temporal framework and tectonic setting of rare-metal pegmatites in this region, we conduct LA-ICP-MS U-Pb dating of zircon, columbite–tantalite, and monazite from the Tashidaban and Kumusayi pegmatite-hosted Li deposits. Zircon and columbite-tantalite U-Pb ages from Tashidaban reveal two stages of Li mineralization in the Early Ordovician (448.3 – 440.2 Ma) and the Silurian (424.6 – 415.6 Ma). A minor Late Triassic magmatic phase was also identified (215.4 ± 1.5 Ma). U-Pb ages on columbite-tantalite (216.7 ± 1.8 Ma) and monazite (205.3 ± 0.8 Ma) from Kumusayi constrain the ore-bearing pegmatite emplacement to 216.7 – 205.3 Ma.

Integrating published studies, we propose a four-phase ore-forming model for rare-metal pegmatites in the Altyn Tagh Orogen: (1)486 – 454 Ma: syn-collisional compression among intra-orogenic microblocks (Central Altyn Tagh, Northern Altyn Tagh, and Qaidam) led to crustal thickening and high-pressure metamorphism. Dehydration-triggered partial melting of lower crustal metasedimentary sequences, which generated highly differentiated granitic melts with initial Li-Cs-Ta enrichment; (2)453 – 425 Ma: the final amalgamation of the Central Altyn Tagh, Northern Altyn Tagh, and Qaidam blocks may have caused crustal thickening and shearing, which induced large-scale granitic magmatism. This likely generated rare-metal fertile melts in the Central Altyn region, providing favorable conditions for rare-metal pegmatite formation and main-phase mineralization; (3)425 – 385 Ma: Transition to post-collisional extension. Lithospheric gravitational collapse occurred, and mantle-derived underplating likely induced further partial melting in the lower crust. This likely sustained magma ascent and fractionation. Although the melt volume may have decreased, Li enrichment likely persisted; (4)240 – 205 Ma: Reactivation of the Altyn Tagh fault system may have formed lithosphere-scale strike-slip and shear zones, which provided pathways for magma ascent and crustal melting. This further emphasizes the key role of major shear zones in rare-metal mineralization.

Collectively, this study reveals the episodic nature of rare-metal mineralization in the Altyn Tagh Orogen and proposes a four-stage metallogenic model, including a newly identified Triassic spodumene pegmatite emplacement event. These findings provide new insights into the tectono-magmatic controls on Li-Cs-Ta (LCT)-type pegmatite formation and highlight the potential for rare-metal exploration in long-lived orogenic systems.