The role of fluid exsolution in the Li enrichment in granitic pegmatites: A case study from the Dahongliutan Li ore field in West Kunlun

Abstract

Pegmatite-host Li deposits provide substantial Li resource in the globe. However, the mechanism of Li enrichment in granitic pegmatites remain enigmatic. Here, we report age and whole-rock geochemical data for the garnet-tourmaline-bearing granites (GTGs) and tourmaline-bearing granites (TGs) from the Dahongliutan rare-metal pegmatite ore field located in the Western Kunlun orogen, NW China. The formation of Li-rich pegmatites in this ore field was proposed to have an intimate temporal-spatial association to the two-mica granites (TMGs). Zircon U–Pb dating for the GTGs and TGs yielded ages of 203.2 ± 2.3 and 203.4 ± 2.7 Ma, respectively. These ages are slightly younger than those of the TMGs (ca. 220–208 Ma), but overlapping with or older than those of the Li-rich pegmatites (ca. 214–190 Ma) reported in this area. The GTGs and TGs have middle to upper crust-like Sr–Nd isotope compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7159–0.7227; ε_Nd(t) =  − 10.7 to − 9.67), similar to those of the TMGs and Li-rich pegmatites. The decreasing CaO, MgO, TFe₂O₃, Sr, Ba and rare earth element contents, and increasing Na₂O, K₂O contents and Na₂O/K₂O ratios from the TMGs to GTGs, and to TGs, suggests fractionation of biotite, plagioclase and K-feldspar, monazite, muscovite, and garnet from the TMG magma. These evidence together with the field observations indicate that these granites and pegmatites in the Dahongliutan ore field represent a cogenetic evolutionary sequence. However, the contents of the incompatible element of Li decrease sharply from the TMGs to GTGs and TGs, which is likely resulted from magmatic fluid exsolution. Geochemical modeling for Li show that fluid saturation occurred at the early stage of magma evolution. In the following, the exsoluted fluids accumulation accompany by extraction of large amounts of fluid-soluble elements such as Li and Cs from the residual melts, and their removal and migration away from the granite system, may be pivotal in the generation of the Li-rich pegmatites.