Contrasting Geochemistry of Apatite from Triassic Li-Mineralized and Barren Pegmatites in the Ke’eryin Field, Eastern Tibetan Plateau

Abstract Apatite is a common accessory mineral in igneous rocks and can be useful for examining the petrogenesis and mineralization potential of pegmatites. Triassic pegmatites in the eastern Tibetan Plateau have been the focus of numerous studies because of the discoveries of world-class Li deposits in several pegmatite fields. Among these, the Ke’eryin field hosts the Lijiagou and Dangba mineralized dikes and numerous barren dikes. These dikes intrude metasedimentary rocks of the Triassic Xikang Group and leucogranites of the Triassic Ke’eryin pluton. Apatite is an accessory mineral that occurs in both mineralized and barren dikes. Most apatite grains are subhedral-euhedral crystals (<200 μm) and are of magmatic origin. In the mineralized dikes, apatite coexists with lithiophilite [LiMn(PO4)], cheralite [CaTh(PO4)2], and spodumene [LiAlSi2O6], whereas apatite in the barren dikes is associated with monazite [(Ce, La, Nd)PO4] and xenotime [YPO4]. Pegmatites from both the mineralized and barren dikes all have initial ɛNd(t) values (-11.6 to -8.6) similar to leucogranites of the Ke’eryin pluton and metasedimentary rocks of the Xikang Group. It is likely that the pegmatites were fractionated products from the leucogranitic melts or from the anatectic melts of metasedimentary rocks. Apatite grains from the mineralized and barren dikes have distinctly different LREE and HREE concentrations, chondrite-normalized REE patterns, TE3 values, and (Sm/Nd)N, (La/Sm)N, and (La/Yb)N ratios. The REE patterns of the Lijiagou and Dangba dikes can be best reproduced by fractional crystallization of 40% plagioclase + 50% K-feldspar +7.77% biotite + 0.1% garnet + 0.03% monazite + 2% apatite + 0.1% zircon and 50% plagioclase + 40% K-feldspar + 7.57% biotite + 0.1% garnet + 0.03% monazite + 2% apatite + 0.3% zircon, respectively, from a leucogranitic melt with 15% of remaining melts. The REE patterns of the barren dikes can be best reproduced by fractional crystallization of 40% plagioclase + 50% K-feldspar + 9.95% biotite + 0.05% monazite from a leucogranitic melt with 25% of remaining melts. The results indicate that the melts from which the mineralized and barren dikes formed have different fractional crystallization trajectories. In addition, the melts from which the mineralized dikes formed are more evolved than the melts from which the barren dikes formed (F = 15% vs. 25%). The more evolved nature of melts from which the mineralized dikes formed agrees with the fact that apatite grains within them have higher MnO (2.54-5.75 wt.% vs. 0.25-1.89 wt.%) and Th (12-189 ppm vs. 0.29-27 ppm) than those from the barren dikes. Through modelling Li enrichment during partial melting and fractional crystallization processes, we conclude that a high degree of fractional crystallization is a key mechanism for Li-mineralization in pegmatites in the Ke’eryin field, although the involvement of Li-rich sedimentary sources may also be important. Together with Li concentrations of apatite from literature, our work demonstrates that apatite grains with high Li concentrations (>6ppm) can reflect the potential of Li-mineralization in limitedly exposed pegmatites. Our study of apatite offers valuable insights into the formation of highly evolved granitic pegmatites.