Lithium partitioning between aqueous fluids and granitic melt and implications for ore genesis of pegmatite-type Li deposits

Abstract

The formation and evolution of pegmatite-related Li deposits usually undergo a magma-hydrothermal transition phase. Yet a crucial question remains unclear regarding the geochemical behavior of Li in a coexisting crystal–melt–fluid systems. Here, a method of synthetic fluid inclusions in silicate glass was used to determine the partitioning of Li between granitic melts and aqueous Cl-bearing fluid (s) at 800 °C, 150 MPa, and the fO₂ of the Ni-NiO buffer. Partition coefficients of Li ($D_{Li}^{fluid/melt}$) were calculated from compositions of coexisting fluid inclusions and quenched glasses determined by Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS). The experimental results show that $D_{Li}^{fluid/melt}$ values exhibit a strong dependence on the fluid compositions, and the values linearly increase from 0.11 ± 0.02 (1σ) to 3.11 ± 0.94 (1σ) with the concentration of NaCl from 0.17 to 3.48 (mol/kg H₂O) in the fluid phase. The addition of CO₂ into constant aqueous NaCl-bearing fluid results in a pronounced drop in $D_{Li}^{fluid/melt}$ values, dropping from1.83 ± 0.40 (1σ) to 0.42 ± 0.23 (1σ) as the mass fraction of CO₂ increase from 0.01 to 0.1. The F contents and aluminum saturation index (ASI) of melt have only a weak impact on $D_{Li}^{fluid/melt}$ values. Our data suggest that the Li partitions into the fluid phase as chloride complex (LiCl⁰). Numerical modeling indicates that Li in low-salinity (5–7 wt% NaCl, correspond to$D_{Li}^{fluid/melt}$ < 1) magmatic fluids is extracted by excessive fractional crystallization of melt. In relatively high salinity (5–15 wt% NaCl, correspond to$D_{Li}^{fluid/melt}$ ≥ 1) magmatic fluid systems, Li strongly partitioning into fluid phase in a coexisting crystal–melt–fluid system. This process allows the fluids to efficiently extract metal Li from magma and reach a sufficient enrichment, ultimately resulting in the formation of Li-mineralized pegmatites.