Origin of reedmergnerite in sodium carbonate successions and environmental implications in a Late Paleozoic alkaline saline lake, NW Junggar Basin, China

Abstract

The Fengcheng Formation originated from a Late Paleozoic alkaline saline lake, characterized by abundant evaporites and serving as the primary source rocks within this evaporitic basin. Boron accumulation in this set of evaporitic deposits is primarily in the form of reedmergnerite (NaBSi₃O₈). Although reedmergnerite can be found in each sub-environment of this alkaline saline lake, it may dominate in evaporite successions near the evapocenter. However, its distribution pattern and origins within the evaporite succession remain unclear. Through comprehensive analyses of a reedmergnerite-rich evaporite core section, we propose the following: (1) boron originates from volcanic ash and hot spring; (2) diagenetic processes within a hydrothermal environment involve replacement of sodium carbonates by loughlinite (Na₂Mg₃Si₆O₁₆▪8H₂O) fibers followed by crystallization of reedmernerite within loughlinite during contemporaneous diagenesis; (3) vertical distributions of reedmergnerite within evaporitic successions are controlled by episodic volcanic eruptions, hot spring ejections, and climatic cycles; and finally, (4) significant volumes of reedmergnerite occur within sodium carbonate successions due to multiple stages of volcanic eruptions and hot spring ejections. Therefore, the boron accumulations within the sodium carbonate successions in the Fengcheng Formation provide compelling evidence of deep sources contributing to the formation of chemical deposits in an alkaline saline lake environment. The accumulation of reedmergnerite can be regarded as a boron resource that formed specifically within this type of alkaline saline lake setting. Furthermore, the presence of laminated reedmergnerite and louglinite can serve as indicators for identifying episodic volcanic activities and hot spring occurrences, thereby providing a basis for assessing the environmental conditions during source rock formation in an evaporitic basin.