Evolution of the rare-metal mineralization system associated with collision-related pegmatites in the western Altyn Tagh Orogen, Tugeman, NW China

Abstract

On a regional scale, rare-metal pegmatite groups (pegmatites of common origin) display a zonal distribution with different chemical compositions. Whether the regional distribution of such pegmatites dike in a convergent zone is related to dehydration melting of different micas remains unclear. In recent years, the Altyn Tagh Orogen (ATO) in the northwest China has gained attention due to economically significant rare-metal mineralization. Our study focuses on the geochronology of columbite-group minerals (CGMs) and geochemistry of tourmaline from the Be-mineralized pegmatites in the Tugeman and Ayage Be deposits. CGM dating shows that these Be-mineralized pegmatites formed between 485 and 484 Ma. Subsequent hydrothermal activity, occurring around 453–450 Ma, is related to the development of a ductile shear zone. Evidence from tourmaline mineralogy and major elements reveals that the evolution of the Be-mineralized pegmatite is influenced by crystallization differentiation and subsequent fluid exsolution. Tourmalines from these pegmatites exhibit B isotope compositions ranging from –12.2 ‰ to –14.8 ‰, indicating a crustal metasediment source. Based on these findings, we propose a model for the rare-metal mineralization system in the Tugeman area: the sequential formation of Be to Li-mineralized pegmatites likely results from dehydration melting of muscovite and biotite during prolonged subduction and collision between the South Altyn Subduction–Collision Belt (SAB) and Central Altyn Block (CAB).