Lithium pegmatite formation in Kelumute-Jideke pegmatite field, Chinese Altai: Insight from geochronology, petrology and lithium isotope geochemistry

• Halong granitic batholith show four stages of magmatism, including 446±5 Ma, 403- 384 Ma, 345±19 Ma, 220- 209 Ma. • The granite in the mining area is S-type and poor in Li, while the metasedimentary rocks are weakly weathered, immature, and rich in Li. • Compared to Li-poor pegmatites, Li-rich pegmatites have lower δ 7 Li values, indicating limited fluid interaction. • Li-rich pegmatites originate from the partial melting of Li-rich metasedimentary rocks rather than from the crystallization differentiation of granite. The Kelumute-Jideke pegmatite field is an important lithium (Li)-rich pegmatite mineralization area within the Chinese Altai orogenic belt. There have been four large (Kalu’an No.803, No. 805, No.806, No.807 veins), one medium (Kelumute No.112 vein), nine small deposits (such as Jiamukai, Qunkuer, Azubai) with totally 130,000 tons Li 2 O found in this area. However, it remains enigmatic whether Li-rich pegmatite formed through fractional crystallization of coeval granite or direct anatexis of metasedimentary rocks. Here we selected pegmatites and their surrounding rocks of metasedimentary rocks and granitic batholith in this pegmatite field to conduct geochronology, whole major and trace element, and Li isotope analyses. The granite intrusions in this field exhibits multiple-stage emplacement ages, including 447.7 ± 4.2 Ma, 405.8 ± 0.0 – 397.4 ± 4.1 Ma, 358.4 ± 3.1–308.4 ± 4.6 Ma, 239.5 ± 1.6 – 213.7 ± 1.5 Ma. The youngest magmatism is coeval with the formation age of rare metal pegmatites. Geochemistry analyses of granites show a peraluminous S-type granite signature with high SiO 2 (71.21–75.01 wt%) contents and A/CNK (> 1) ratios. The metasedimentary rocks of Kulumuti Group in this field have experienced weak weathering and exhibit extremely high Li content (maximum of 1193 ppm), which is much higher than that of the Traissic granite (96.1 ppm). The average composition of metasedimentary rocks is used to calculate the partition coefficient of different mineral proportions during the modelled equilibrium melting of the metamorphic phase. This approach determines the mineral proportions and partition coefficients under various conditions. Subsequently, Rayleigh dehydration melting simulations were performed on the Li-rich and Li-poor metasedimentary rocks. The Rayleigh dehydration melting simulations show that melts produced from partial melting of Kulumuti Group display Li content of 80–3435 ppm and δ 7 Li values of 1.0–8.3 ‰, consistent with natural pegmatites. The low-degree partial melting of claystone-rich metasedimentary rocks (Li-rich) in the Kulumuti Group can produce a preliminary Li-rich melt. However, through Rayleigh fractional crystallization simulation of coeval granite, it is found that the calculated δ 7 Li value (+6‰) of pegmatites is much higher than that of natural Li-rich pegmatite (average + 3.0 ‰). Therefore, the formation of Li-rich pegmatites in the Kelumute-Jideke pegmatite field is predominantly attributed to the partial melting of Li-rich protoliths with low δ 7 Li values, rather than high differentiation of Halong granite. The study highlights the importance of anatexis of metasedimentary rocks in formation of Li-rich pegmatite, which is key to mineral exploration for Li pegmatite in Altai orogenic belt.