Lithium partitioning between aqueous fluids and granitic melt and implications for ore genesis of pegmatite-type Li deposits
Yi An, Hui Zhang, Yong Tang, Zheng‐Hang Lv, Zhanlong Ma
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 f O 2 of the Ni-NiO buffer. Partition coefficients of Li ( D L i f l u i d / m e l t ) 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 L i f l u i d / m e l t 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 H 2 O) in the fluid phase. The addition of CO 2 into constant aqueous NaCl-bearing fluid results in a pronounced drop in D L i f l u i d / m e l t values, dropping from1.83 ± 0.40 (1σ) to 0.42 ± 0.23 (1σ) as the mass fraction of CO 2 increase from 0.01 to 0.1. The F contents and aluminum saturation index (ASI) of melt have only a weak impact on D L i f l u i d / m e l t values. Our data suggest that the Li partitions into the fluid phase as chloride complex (LiCl 0 ). Numerical modeling indicates that Li in low-salinity (5–7 wt% NaCl, correspond to D L i f l u i d / m e l t < 1) magmatic fluids is extracted by excessive fractional crystallization of melt. In relatively high salinity (5–15 wt% NaCl, correspond to D L i f l u i d / m e l t ≥ 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.