Contributions of arid climate and hydrothermal fluid flow on sedimentation in saline-alkaline lakes: Insight from the Ibar intramontane basin (Southern Serbia)

Saline-alkaline lakes are common in tectonically active, semi-arid regions, resulting from the interplay between tectonic, hydrothermal, surface processes and climate. This study investigates their contribution to the evolution of the saline-alkaline succession in the intramontane Ibar Basin (Southern Serbia). The succession is investigated using detailed sedimentological analysis, in situ U–Pb geochronology of carbonates acquired by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), major and trace element geochemistry and fluid inclusion analysis. Dominantly clastic sedimentation is represented by alluvial fan, flood plain, delta and marginal and profundal lake facies associations. Microbialite and littoral to sublittoral oil shales formed in the areas protected from clastic input. The stratigraphic succession shows a transition from alluvia to a hydrologically open and closed lacustrine environment. U–Pb geochronology of microbialite of a close lake phase (~17 Ma) suggests basin evolution during the Early to Middle Miocene age. The hydrologically closed lake phase is marked by borate-rich facies, which comprise mainly colemanite overgrown by secondary ulexite within profundal lake facies associations. The textural features suggest that colemanite initially precipitated at or below the sediment/water interface. The subsequent growth, as observed from the primary fluid inclusions in colemanite and calcite, indicates precipitation from the evolving bittern brine under evaporitic, redox conditions, which during diagenesis reached temperatures of 200–220°C. High Ca²⁺/Na⁺ ratios in the brine favour colemanite precipitation. Gradual Ca²⁺ depletion and clay breakdown (dewatering) lead to ulexite and borax precipitates. The results imply that B, Ca and Na were mobilised and transported into the basin by hydrothermal fluids that previously leached bedrock and/or by streams carrying products of the volcanic rocks' weathering. However, brine saturation and syndepositional precipitation were initiated by an arid climate through evaporation, while further growth was driven by reflux of fluids during diagenesis. This study highlights the importance of the tectonics of the collisional orogens and associated processes, arid climate and basin hydrological regime on the deposition of B, Ca-rich deposits in saline-alkaline lakes.