The barium deposits in Ankang and Xunyang counties, Shaanxi Province, China, occur in the northernmost part of the world-class barium metallogenic belt in south Qinling. The deposits are hosted by the Lower Silurian carbonaceous siliceous rocks, with a unique combination of barite and witherite. The homogenization temperatures of fluid inclusions in the barite are mainly concentrated between 135 and 155 °C, whereas those from the witherite have two peaks of 165–175 °C, and 215–225 °C, respectively. Laser Raman analysis of fluid inclusions indicates that the vapor phase of fluid inclusions in barite is dominated by H2O, although some contains N2, H2S, and CH4. The compositions of the vapor and liquid phases of fluid inclusions in witherite can be divided into two end-members, one dominated by H2O without other volatiles, and the other containing CH4, C2H6, C3H8, C2H4, and C6H6 in addition to H2O. CO2, H2S, and some CH4 are interpreted as products of chemical reactions during mineralization. Organic gases (CH4, C2H6, C3H8, C2H4, and C6H6) in the fluids were critical in the formation of barium sulfate versus carbonate. The δ34S values of barite range from 38.26‰ to 54.23‰ (CDT), the δ34S values of sulfides coexisting with barium minerals vary from 22.44‰ to 25.11‰ (CDT), and those in the wall rock from 11.60‰ to 19.06‰ (CDT). We propose that the SO42– generally experienced bacterial sulfate reduction in seawater before mineralization, and some SO42– also experienced thermochemical sulfate reduction in hydrothermal system during mineralization. The δ13C values of witherite range from ?27.30‰ to ?11.80‰ (PDB), suggesting that carbon was sourced from organic substances (like CH4, C2H4, and C2H6). The formation of witherite was possibly associated with thermochemical sulfate reduction, which caused the consumption of the organic gases and SO42– in the hydrothermal solutions, consequently inhibiting barite formation. The important conditions for forming witherite include high fluid temperatures, high Ba2+ concentrations, CO2 in the fluids, low HS– concentrations, and the subsequent rapid diffusion of H2S during thermochemical sulfate reduction of the fluids.