Pegmatite evolution and mineralization: insights from tourmaline geochemistry and boron isotopes

• Tourmaline’s major and trace elements decipher its crystallization environment. • Tourmaline in altered country rock has heavier δ 11 B values than that in the pegmatite. • Variations in δ 11 B across tourmaline zones indicate open or closed magmatic system. • No direct correlation was observed between tourmaline’s Li content and mineralization. • The closure of the magmatic system is a critical control for Li mineralization. While the magmatic source and evolution significantly influence rare metal mineralization in pegmatites, a critical unresolved question remains: within the same mining district, some spatially close pegmatites are mineralized while others are not. This highlights the existence of other crucial factors governing pegmatite mineralization. Tourmaline, a common borosilicate mineral in pegmatite systems, serves as a key indicator for deciphering mineralization processes. We studied the tourmaline from the Murong Li deposit (Asia’s largest single-vein Li deposit) using geochemistry, boron isotopes, and mineralogy. Three types were identified: BP-Tur (tourmaline in barren pegmatite), FP-Tur (tourmaline in fertile pegmatite), and AC-Tur (tourmaline in altered country rock). BP-Tur and FP-Tur exhibit pronounced zoning textures, indicating magmatic-hydrothermal formation. BP-Tur crystallized in a low-salinity, high-ƒO 2 , fluid-rich setting. In contrast, FP-Tur formed under initially low ƒO 2 and low salinity, with both parameters increasing during later crystallization. AC-Tur is fine-grained, non-zoned and formed in a low-temperature, high-pressure, high-salinity, and high-ƒO 2 environment. Its heavier δ 11 B values stem from 11 B-enriched magmatic fluids. Systematic boron isotope variations across growth zones in BP-Tur and FP-Tur record magmatic evolution under open and closed conditions, respectively. This interpretation is corroborated by higher estimated pressures for FP-Tur relative to BP-Tur, consistent with fluid loss and pressure decrease in open systems. Unlike previous studies, we find no direct correlation between the contents of Li, Sn, Nb, or Ta in tourmaline and mineralization. Instead, the closure of the magmatic system is a critical control on Li mineralization.