Factors controlling lithium accumulation in brines from Upper Cretaceous to Lower Eocene aquifers of a continental evaporite basin

Abstract

The origin of deep-seated Li-rich brines in a continental evaporite basin remains poorly understood. To clarify hydrogeochemical processes governing the Li enrichment, this study investigated chemical compositions and multi-isotopic (H–O–Li–Sr) features of a set of water samples from Upper Cretaceous to Lower Eocene sedimentary brines, saline groundwaters and oilfield brines, thermal waters and surficial river waters in the Jiangling Basin, central China. Waters from deep reservoirs evolve through multiple hydrogeochemical processes including evaporation, dilution, water-rock reactions and evaporite dissolution. These processes cause a range of Li and Sr contents (5.43–100 mg/L and 32.8–1307 mg/L) and isotope compositions (+8.6 to +19.3‰ and 0.706926–0.712480), indicating contributions from reservoir rock dissolution to δ7Li values and 87Sr/86Sr ratios. Chemical geothermometers applied to brines, saline waters and thermal waters give reservoir temperature values ranging between 70 and 200 °C. Temperature has a distinct effect on advancing host rock-water interactions, while high-temperature waters enhance leaching of Li in rocks compared to the low- and medium-temperature waters. The variable δ7Li values lower than river waters and high Cl/Br ratios in the Upper Cretaceous to Lower Eocene aquifers suggest a result of evaporite dissolution to different degrees, whilst waters in the Upper Cretaceous aquifers have the lowest δ7Li values similar to the thermal waters and the highest Cl/Br ratios suggest a stronger salt dissolution effect. Multi-isotope tracing highlights the role of temperature, reservoir rock types and different hydrogeochemical processes in defining the geochemical and isotopic water composition. Combination of these factors, together with high heat gradients associated with intense volcanic activity eventually results in Li accumulation in deep-seated brines from the Jiangling evaporite basin. Our results contribute to a further comprehensive understanding of hydrogeochemical processes that dominate deep brine formation and Li enrichment in an ancient continental evaporite basin.