Lithium isotope evidence for deep magmatic fluid control on Li mineralization in South China A-type granites

Although most Li-bearing pegmatite deposits are genetically linked to S-type granites, large-scale Li mineralization also occurs within A-type composite granitic plutons in South China. However, Li isotopic behavior and enrichment in such A-type systems remain poorly constrained. The Jiuyishan composite pluton, a representative A-type granite in South China, hosts Li-Rb-Cs rare-metal mineralization. It consists of shallow-level granodiorite, porphyritic biotite granite, and albite granite, as well as deep-seated fine-grained granite. Notably, large-scale Li mineralization is hosted in greisen formed by alteration of the porphyritic biotite granite, rather than in the more evolved albite granite. This suite provides an ideal opportunity to investigate Li isotopic behavior and enrichment mechanisms within A-type systems. To address these issues, we present whole-rock Li and zircon O isotopic data from the Jiuyishan A-type pluton. The homogeneous zircon δ18O values (8‰–9‰) preclude significant meteoric water contamination and suggest that Li isotopic fractionation and enrichment result from internal magmatic evolution. During the early magmatic evolution of shallow granitic melts, the coupled increase in Li contents and δ7Li values is consistent with fractional crystallization dominated by biotite and feldspar. In contrast, the albite granites exhibit high Li contents (289–570 ppm) but systematically lower δ7Li values (+1.00‰ to +1.57‰) than the porphyritic biotite granite. This 7Li depletion is attributed to the preferential partitioning of Li into the exsolved fluid phase, implying that the albite granite experienced intense magmatic fluid exsolution. Calculations based on crystals–melt–fluid partition coefficients indicate that Li was not entirely extracted into the fluid; instead, a substantial proportion remained in the residual melt. Consequently, the Li concentration in the exsolved fluids is <1000 ppm. The deep-seated fine-grained granite has higher Li contents (1134–1954 ppm) but consistently lower δ7Li values (−2.23‰ to −1.86‰). To constrain the isotopic variations in this lithology, we applied Rayleigh fractionation modeling. Simple crystal-melt fractionation alone cannot fully reproduce the observed Li systematics. Instead, a two-stage Rayleigh fractionation model involving coexisting crystal, melt, and fluid phases reproduces both the Li concentration and isotopic variations. Our calculations indicate that fluids exsolved from the deep-seated fine-grained granite could reach Li concentrations of up to 10,339 ppm. Compared with the porphyritic biotite granite, the greisen has higher Li contents and lower δ7Li values, indicating that the ore-forming fluids were Li-enriched and characterized by low δ7Li signatures. We conducted water-rock interaction modeling with unaltered porphyritic biotite granite as the initial wall-rock composition and fluids exsolved from the shallow albite granite and deep-seated fine-grained granite as endmembers. The modeling results demonstrate that all greisen samples plot within the compositional field defined by deep fluid-rock reactions, indicating that fluids released from shallow, highly evolved felsic melts are incapable of forming giant Li mineralization. Therefore, we propose that the ore-forming fluids were primarily derived from this deep unit. Upon ascent to shallow levels, these fluids reacted with the porphyritic biotite granite, triggering extensive greisenization and promoting the precipitation of Li-bearing minerals. Our model elucidates the critical magmatic-fluid processes controlling the formation of large-scale Li deposits in South China.