Unlocking the secrets of pegmatite-type lithium deposits: control of melt-fluid mass ratio

The search for alternatives to traditional fossil fuels has become a global priority, with lithium-ion batteries emerging as a primary substitute for conventional energy sources. The importance of lithium has increased significantly due to the rapid development of new energy vehicles, with spodumene (LiAlSi ₂ O ₆ ) being the principal economic mineral for lithium extraction. Spodumene is predominantly found in pegmatites, despite that not all pegmatites are enriched in spodumene. Spodumene-bearing pegmatites are believed to form mainly during magmatic-hydrothermal processes, especially through the differentiation of highly evolved granites. Although fluids can significantly influence the enrichment of lithium, the role is contentious. Magmatic fluids can transport mobile elements (e.g., Li, F, B) and lead to their precipitation and concentration under favorable mineralization conditions. Conversely, fluid interactions may alter and redeposit already crystallized minerals, potentially decreasing the lithium grade. A quantitative assessment of the role of fluids in lithium ore formation is essential for resolving this debate, but the role of fluids is poorly understood. Here, we selected spodumene-bearing pegmatites from two pegmatite fields with well characterized zoned lithologies, Renli-Chuanziyuan and Ke’eryin, to elucidate the key controls on spodumene formation in pegmatites. Through detailed mineralogical observations, geochemistry, and simulation calculations, we propose that spodumene forms after the granitic melt reaches fluid saturation. As fluid mass in the system increases, the amount of spodumene formed initially rises and then declines. Therefore, the melt-fluid mass ratio is critical in controlling spodumene formation. We demonstrate that a melt-fluid mass ratio of ~ 2 is optimal for producing economically significant pegmatite-type lithium deposits, providing important guidance for lithium exploration and prospecting efforts.