Fluid-Inclusion Constraints on Hydrothermal Fluid Circulation in Precambrian–Lower Paleozoic Rocks of the Sakmara Thrust–Accretionary Zone, Mugodzhar, Southern Urals, Kazakhstan

The Sakmara thrust–accretionary zone of the Mugodzhar segment, Southern Urals, preserves Precambrian–Lower Paleozoic volcanic and metasedimentary complexes whose hydrothermal evolution remains poorly constrained. This study compares aqueous fluid inclusions in quartz from the Lushnikov and Akai formations and amygdale-filling calcite from the Sugralin Formation. Homogenization temperatures (Th), final ice-melting temperatures, and NaCl-equivalent salinities were used to characterize contrasting fluid populations. Lushnikov quartz records Th of 95–119 °C. Akai sample E14 contains two populations at 165–178 °C and 240–280 °C, while samples E15 and E17 yield 190–200 °C and 235–250 °C, respectively. Sugralin calcite contains populations at 231–240 °C and 340–357 °C. Salinities range overall from 3.4 to 13.4 wt.% NaCl eq. The populations are consistent with repeated hydrothermal circulation influenced by volcanic heat supply, permeability, fracture reactivation, and later tectonic modification. Their chronology and trapping depths remain independently unconstrained. High Th in Sugralin calcite is compatible with submarine hydrothermal circulation, although natural post-entrapment re-equilibration cannot be excluded. The data provide a basis for evaluating polyphase thermal-fluid evolution in the Sakmara zone.