Origin of Li brine-type deposits in the Cretaceous gypsum-bearing formation of the Jitai Basin, South China: Constraints from geochemistry and H, O, Li, B, and Sr isotopes

Abstract

Li brine-type deposits have recently been discovered in the Upper Cretaceous gypsum-bearing formation of the Jitai Basin, South China. The source and enrichment mechanism of Li in brines, however, are still unclear. To define the origin of lithium and evaluate the water-rock interaction process, this work presents chemical and multi-isotope data for Li-rich brines from the Jitai Basin. Seven brine samples collected from Meigang boreholes contain high salinities of 190–339 g/L, lithium concentrations of 60.6–106.0 mg/L, and low magnesium/lithium ratios of ∼11. The good correlation between δD and δ18O values suggests that the primary brine has been diluted by meteoric water, while contributions from halite dissolution or secondary brines having dissolved solid halite are also indicated by the high Na/Cl (0.58–0.63) and Cl/Br (average of ∼26987) mass ratios, the Na deficit and Ca excess, and the fact that high-salinity brines have lower δD, δ18O and δ34SSO4 values. Among the chemical geothermometers commonly used in the literature, only the SiO2-chalcedony, Na/K/Ca and K/Mg thermometric relationships give concordant temperature values of 120 ± 15 °C for the deep reservoir of these brines, suggesting that the latter are not in full chemical equilibrium with the reservoir rocks (excess of sodium due to halite dissolution), but this temperature can be considered as representative of their deep reservoir. Moreover, most of the chemical geothermometers yield close temperature values of 115 ± 15 °C for the deep reservoir of three neighboring dilute thermal waters. The homogeneous 87Sr/86Sr ratios (0.713865–0.713915) and δ11B (+1.8‰–+3.4‰) and δ7Li (+13.1‰–+14.2‰) values show a close affinity to the continental crust, possibly implying a fluid signature involving the interactions between clastic host rocks and brines. X-ray diffraction and scanning electron microscopy analyses and Li contents of reservoir rocks reveal the presence of dominant lithian muscovites of clastic origin, probably originating from weathering of Li-enriched igneous rocks, which contribute to most of the lithium input in the hydrologic system of the basin. Thus, it is proposed that lithium enrichment in brines from the Upper Cretaceous gypsum-bearing clastic aquifers of the Jitai Basin was generated by water-rock interactions involving lithian muscovites.