Early Eocene paleoclimate and brine composition from fluid inclusion analysis of Bahadur Khel Salt, Upper Indus Basin, Pakistan

Fluid inclusions are crucial for reconstructing past climatic conditions because they provide information on evaporation rates and salinity variations over time. Fluid inclusion analysis of the Eocene Bahadur Khel Salt was carried out using thin sections, Raman spectroscopy, XRD, XRF, and microthermometry. Thin-section analysis indicated that the Bahadur Khel Salt predominantly comprises halite, with minor amounts of gypsum, anhydrite, and dolomite. Geochemical data indicate that halite primarily contains Na, Cl, O, Ca, Mg, Si, Al, Fe, Br, Sr, K, Zn, P, Ni, Pt, and S. Halite is not entirely pure, with an average content of 94.2 % in the primary crystals and 92.8 % in the secondary crystals. The presence of minor constituents such as calcite, ferrodolomite, ankerite, and anhydrite, as well as elements such as Mg, Ca, O, K, Si, and Al, suggests that the halite was formed from a combination of solutions with different compositions and was influenced by exposure to the atmosphere and meteoric water. Most primary crystals had a higher density than secondary crystals, indicating the presence of highly concentrated fluids. In the Bahadur Khel Salt, most inclusions are secondary and formed after deposition due to exposure to the atmosphere, which is influenced by tectonic activity and diagenetic processes. The homogenization temperature (Th) ranges from 5.8 °C to 24.3 °C, indicating varied seasonal transitions and hydrological processes, reflecting fluid entrapment under different paleoclimatic conditions. Lower Th values (5.8 °C) suggest cooler environments typical of winter or spring, indicating less evaporation and higher precipitation levels. In contrast, higher Th values (24.3 °C) reflect warmer summer conditions associated with increased evaporation and potentially drier periods.