The role of fluid exsolution in controlling lithium mineralization in the Koktokay No. 3 pegmatite, Altay Orogenic Belt of China

Lithium-cesium-tantalum pegmatites, originating from highly evolved, volatile-rich felsic magmas, constitute the world’s primary source of lithium (Li). A key unresolved question is how these evolved rocks achieve extreme Li enrichment. Although fractional crystallization serves as a fundamental mechanism for Li concentration in residual melts, the characteristically high viscosity of silicic magmas severely impedes both the migration and coalescence of the Li-rich melts, thereby suppressing Li enrichment. Understanding the mobilization mechanisms and controlling parameters of these evolved Li-rich melts is consequently crucial for deciphering Li mineralization processes. In this study, we performed integrated major- and trace-element geochemical analyses, along with Li isotope measurements, on the Koktokay No. 3 pegmatite in the Altay Orogenic Belt of China, supplemented by a simulation model to quantify Li enrichment during pegmatite magma crystallization. The results show that fluid exsolution commenced at ∼50% crystallization, enabling residual melts to overcome rheological barriers and separate from the crystal matrix. This process continued progressively until ∼88% crystallization, facilitating the extraction and concentration of Li-enriched melts. As crystallization approached this threshold, Li saturation in the residual melt triggered large-scale precipitation of spodumene. We precisely constrain the timing of fluid exsolution to the formation of the cleavelandite-spodumene and quartz-spodumene zones, and emphasize that its essential role in Li enrichment and mineralization stems from enhanced melt-crystal separation. Our model thus establishes fluid exsolution as a critical mechanism for forming economic Li deposits and provides a fundamental framework for understanding pegmatite genesis.