The Origin of Middle Triassic Thick Anhydrite in the Lower Yangtze Region, South China: Implications for the Evolution of Lithium-Rich Brine

Lithium-(Li)-rich brines represent an increasingly important strategic resource and are commonly associated with evaporites. Their formation has traditionally been mainly attributed to evaporation-driven concentration under arid paleoclimatic conditions. This study investigates the genetic mechanism of the thick anhydrite deposits and associated lithium-rich brines in the Middle Triassic Zhouchongcun Formation of the Lower Yangtze region. Through integrating petrographic, elemental, and isotopic geochemistry and fluid inclusion analyses, combined with reconstruction of the stratigraphic burial-thermal history, we demonstrate that the formation and evolution of these evaporates are closely linked to multiple tectonic-thermal events. Petrographic characteristics and carbon-oxygen isotopic data indicate that the anhydrite initially formed in a restricted evaporative environment during the Middle Triassic, with δ13C ranging from -2.9 to -0.4‰ and δ18O values from -8.1 to -5.6‰, showing a significant positive correlation. Fluid inclusion data reveals subsequent complex hydrothermal overprinting. Primary inclusions record homogenization temperatures of 181.3-210.2 °C, corresponding to the peak paleo-temperature reached during early deep burial. In contrast, secondary inclusions yield lower homogenization temperatures of 135.8-174.3 °C, consistent with hydrothermal activity associated with the Yanshanian tectonic-thermal event. Burial-thermal history reconstruction indicates significant heating during the Middle Jurassic, with maximum temperature reached between approximately 150-170 Ma, coinciding with regional magmatic-tectonic activity. These results indicate that the anhydrite underwent intense reworking by medium- to high- temperature hydrothermal fluids during the Yanshanian period, superimposed upon its primary evaporative sedimentary origin. Geochemical data further shows that both fluid inclusions within anhydrite and the formation brines belong to a mixed sodium sulfate-sodium chloride hydrochemical type, supporting a common fluid source The lithium content in secondary inclusions is more than 4.8 times higher than that in primary inclusions, which the lithium content in regional intrusive rocks is 9.4 times higher than background levels, indicating a dominant contribution from Yanshanian magmatic-hydrothermal fluids. Micro-Raman spectroscopy combined with brine ion composition analysis jointly provides additional evidence for substantial hydrothermal input of lithium. Based on these observations, a three-stage genetic model is proposed: (1) sedimentary foundation, characterized by anhydrite precipitation and initial brine formation in a closed evaporative basin during the Middle Triassic; (2) lithium addition via hydrothermal reworking, whereby lithium-rich magmatic-hydrothermal fluids ascended during the Yanshanian event (∼150 Ma), modifying pre-existing anhydrite and enriching brines at temperatures of 150 °C; and (3) structural adjustment and accumulation, involving lithium migration and concentration within anhydrite-dolostone anticline traps during post-Late Cretaceous uplift and cooling under extensional tectonic conditions. This model highlights the key controlling role of medium-low temperature hydrothermal events in the formation of evaporite-type lithium resources and provides a new genetic framework and exploration guide for lithium-rich brines in evaporite basins.