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

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-sized venis (Kalu’an No.803, No. 805, No.806, No.807), one medium-sized vein (Kelumute No.112), nine small-sized deposits (such as Jiamukai, Qunkuer, Azubai), with a total of 130,000 tons Li₂O 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 host rocks, including metasedimentary rocks and a granitic batholith, from 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 SiO₂ (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 41–4017 ppm and δ⁷Li values of −0.4-+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 δ⁷Li 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 δ⁷Li values, rather than high differentiation of Halong granite. The study highlights the importance of the anatexis of metasedimentary rocks in the formation of Li-rich pegmatites, which is key to mineral exploration for Li pegmatites in the Altai orogenic belt.