Geochemical characteristics and reservoir model of Middle Triassic Li-rich brine in the Lower Yangtze

Objective The origin of lithium−rich brines has long been a focal point in salt lake resource research, with ongoing debates primarily centered on the genesis of such brines and the source of lithium under arid and hot evaporative conditions. During the Middle Triassic, extensive and thick anhydrite−dolomite sequences were deposited in the Ma'anshan−Wuhu (Ma−Wu) region of the Lower Yangtze area, hosting lithium− and boron−enriched brines that reach industrial grades. However, the metallogenic mechanism underlying these lithium−rich brines remains poorly understood.

Methods This study analyzes 19 brine samples collected from the Middle Triassic gypsum layer in the WWY1 borehole, supplemented by 10 samples from thermal springs and surface waters in the outcrop area, to investigate hydrochemical characteristics and hydrogen−oxygen isotopic compositions.

Results Hydrochemical analysis reveals that the WWY1 brine exhibits a Total Dissolved Solids (TDS) content ranging from 66.24 g/L to 77.41 g/L, classifying it as a sulfate−sodium type water enriched in Li⁺, B³⁺, Br⁻, and I⁻. Characteristic geochemical coefficients—including the sodium−chloride coefficient, bromine−chloride coefficient, r(Ca²⁺+Mg²⁺)/r(HCO₃⁻+SO₄²⁻) ratio, and calcium−magnesium coefficient—indicate a predominantly marine sedimentary origin, with evidence of meteoric water input. Hydrogen−oxygen isotope data show a positive shift in δ¹⁸O values, suggesting a mixed genetic signature involving deep−circulating meteoric water (via thermal springs) and residual marine sedimentary brine. Reservoir lithology and imaging log analyses demonstrate that the brine is primarily hosted within fractures and pores of anhydrite−bearing dolomite.

Conclusions The integrated geochemical and reservoir characteristics support a multi−stage genetic model for the formation of lithium−rich brines. Under an arid climate triggered by the Middle Triassic collisional orogeny between the Yangtze and North China blocks, syn−sedimentary marine brines underwent intense evaporation and concentration. Subsequent water−rock interactions further enriched lithium and modified the brine composition. This process was augmented by additional lithium input from deep geothermal fluids driven by circulating meteoric water. The resulting lithium−enriched fluids eventually migrated and accumulated in porous and fractured dolomite layers interbedded with gypsum strata, forming a viable lithium−rich brine deposit. The proposed genetic model can be summarized as "anhydrite basin controlling brine occurrence, compound anticline facilitating brine concentration, and porous−fractured dolomite serving as the primary reservoir."