Lithium (Li) concentrates extracted from the critical minerals spodumene and petalite account for over 50% of the global supply. Both minerals typically occupy the interior zones of granitic pegmatites which may not be exposed at the surface, presenting challenges for the discovery of new deposits. Given the continuously increasing global demand for Li, a better understanding of Li storage and transport in pegmatite-forming melts is required to guide exploration programs.
Experimental simulations using the piston-cylinder apparatus, cold-seal pressure vessels and the hydrothermal diamond-anvil cell were employed to monitor the crystallization of hydrous granitic melts at 400–625 °C and 100–665 MPa, focusing on the role of Li as a fluxing agent in the system. In the experiments, natural granitic melts doped with 3700–21000 ppm Li started crystallizing nominally Li-free minerals, such as alkali feldspars and quartz, leading to the accumulation of Li in the residual liquids (hydrous melt and/or aqueous fluid). Having exceeded saturation in the residual liquids, spodumene and petalite were readily formed. The addition of Li in the granitic melt decreased the liquidus and solidus temperatures, increased the crystallization interval, and enhanced the growth rates of alkali feldspars and quartz.
The experiments also revealed that alkali feldspars and quartz record the degree of Li enrichment of pegmatite-forming melts throughout the crystallization process. Linking the geochemical signatures of natural alkali feldspars and quartz with the initial Li concentration of the melt at magmatic temperatures and the presence of spodumene or petalite can be used as a powerful exploration tool for Li mineralization in pegmatites. Due to their ubiquitous nature in granitic pegmatites of all types, alkali feldspars and quartz offer a substantial advantage as Li indicators, particularly in cases of deposits with limited surface exposure.