Research subject. Fluid inclusions in quartz of three vein systems associated with tectonic cracks occurred during the development of thrust and shear deformations of the Western Transbaikalia. Vein systems localized in volcanic, terrigenous and granite rocks. Methods. The composition of fluid inclusions was studied by microthermometry, Raman spectroscopy and gas chromatography-mass spectrometry. Results. The quartz under study was formed by high-density medium-temperature NaCl-KCl and a low-

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... d medium-salinity water-carbon dioxide fluid (2-5 wt % NaCl eq.). A distinctive feature of quartz of different vein systems was found to be the gas phase composition of inclusions associated with the composition of host rocks. The minimum temperatures of vein quartz formation were determined to range from 180 to 450°C under the minimum fluid pressure values of 0.7-2.9 kbar. Conclusions. Complex vein systems of the junction zone of the Baikal-Muiskaya and Barguzino-Vitimskay SFZ, confined to different host rocks and having different structural positions, are characterized by a similar fluid composition and a common thermodynamic history reflecting the change of deformation aggregates. The metamorphogenic-hydrothermal nature of the fluid involved in the formation of quartz vein systems was established. The development of the hydrothermal system was accompanied by its influence on the host rocks, which was manifested in the variations of fluid composition. The evolution of a single hydrothermal system is associated with tectonic deformation processes, which can be described by three dynamic stages corresponding to three quartz generations.