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The Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin was developed in a volcanic-induced alkaline saline lacustrine environment, with deposition of thick-bedded sodium carbonates and high-quality source rocks, as well as the borosilicate reedmergnerite, which is a rare mineral globally but is very rich in the Fengcheng Formation. Based on petrography, fluid inclusion and boron isotope studies, the aim of this study was to discover the origin of the reedmergnerite which, in the Fengcheng Formation, often replaces carbonate minerals, including shortite, eitelite, northupite, calcite, trona, and even barytocalcite. The homogenization temperature of the primary inclusions in the reedmergnerite ranged from 100 to 116°C. Previous synthetic experimental studies have found that the δ11B values of reedmergnerite are 0.33-2.13‰. It is proposed here that, due to the limitations of temperature and pressure in surface or shallow burial processes, the source of boron in reedmergnerite was deep hydrothermal fluid in the original sedimentary environment, mainly in deep diagenetic environments (>3000 m). Therefore, the enrichment of reedmergnerite in the Fengcheng Formation is interpreted as the result of intense magmatic hydrothermal activity and a long diagenetic history. [ABSTRACT FROM AUTHOR]