Apatite chemistry as an indicator of pegmatitic magmatic evolution and rare-metal mineralization: Evidence from the Koktokay pegmatite field, Xinjiang, NW China

• Chondrite-normalized REE patterns at Koktokay define two apatite groups formed via feldspar accumulation and prolonged feldspar-monazite fractionation. • Apatite REE patterns demonstrate that the No. 3 pegmatite crystallized earlier than its comagmatic muscovite-albite granite at Koktokay. • Liquid immiscibility governs Li enrichment disparities between the cogenetic No. 3 pegmatite and muscovite-albite granite at Koktokay. • The formation of Koktokay pegmatite dikes originated from multiple magmatic events, with Li mineralization potential governed by source-region rare-metal abundance and partial melting processes. This study investigates apatite geochemistry and textures in the Koktokay pegmatites (No. 1, No. 2, No. 3) and associated muscovite-albite granite (NW China) to constrain magmatic evolution and controls on Li mineralization. The results demonstrate that lithium content in apatite robustly indicates mineralization potential. Pegmatite No. 3 exhibits the strongest Li enrichment (6.33–13.67 ppm; average: 10.68 ppm), followed by No. 1 pegmatite (4.05–7.56 ppm; avg. 5.75 ppm) and muscovite-albite granite (2.54–11.77 ppm; avg. 5.05 ppm). In contrast, the barren No. 2 pegmatite shows significantly lower values (0.28–5.91 ppm; avg. 2.08 ppm). Chondrite-normalized REE patterns delineate two distinct apatite groups (Group I and Group II), indicating derivation from highly differentiated melts through feldspar accumulation (Group I apatite: ∑REE + Y typically < 200 ppm with positive Eu anomalies) and prolonged feldspar-monazite fractionation (Group II apatite: ∑REE + Y typically > 200 ppm with negative Eu anomalies). Geochemical coherence (e.g., CaO, MnO, FeO, ∑REE + Y, HREE/LREE ratios of 2.2–2.4), nearly identical REE contents and distribution patterns in Group II apatite, concordant geochronological evidence, and analogous crystallization temperatures collectively support a cogenetic relationship between No. 3 pegmatite and the muscovite-albite granite. Notably, No. 3 pegmatite crystallized earlier, as evidenced by primitive Group I apatite. The divergent evolutionary paths revealed by apatite HREE/LREE fractionation diagram across the three pegmatites, when integrated with previously documented significantly distinct δ 7 Li values and mica Li vs. K/Rb trends, collectively confirm multiphase magmatism as the genesis of these pegmatites. Li enrichment in pegmatites from the Koktokay field is predominantly governed by source-region Li abundance and partial melting processes, rather than the extent of magmatic differentiation. This is evidenced by Li exhibiting no significant correlation with key indicators such as apatite MnO, FeO, ΣREE + Y, Y/Ho, or trace element indices (TE 1,3 ). The Li compositional contrast between the cogenetic No. 3 pegmatite and muscovite-albite granite originates from liquid immiscibility: Li, Mn, Fe, and Be preferentially partition into volatile-rich B-type melts. These melts form high-BSE-intensity, Mn-Fe-rich apatite – a critical phase for No. 3 pegmatite mineralization – whereas conjugate volatile-poor A-type melts generate the granite.