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 ℃,whereas those from the witherite have two peaks of 165-175 ℃,and 215-225℃,respectively.Laser Raman analysis of fluid inclusions indicates that the vapor phase of fluid inclusions in barite is dominated by H_2O,although some contains N_2,H_2S,and CH_4.The compositions of the vapor and liquid phases of fluid inclusions in witherite can be divided into two end-members,one dominated by H_2O without other volatiles,and the other containing CH_4,C_2H_6,C_3H_8,C_2H_4,and C_6H_6 in addition to H_2O.CO_2,H_2S,and some CH_4 are interpreted as products of chemical reactions during mineralization.Organic gases（CH_4,C_2H_6,C_3H_8,C_2H_4,and C_6H_6） in the fluids were critical in the formation of barium sulfate versus carbonate.The δ^（34）S values of barite range from 38.26‰ to54.23‰（CDT）,the δ^（34）S values of sulfides coexisting with barium minerals vary from 22.44‰ to25.11‰（CDT）,and those in the wall rock from 11.60‰ to 19.06‰（CDT）.We propose that the SO_4^（2-）generally experienced bacterial sulfate reduction in seawater before mineralization,and some SO_4^（2-）also experienced thermochemical sulfate reduction in hydrothermal system during mineralization.The δ^（13）C values of witherite range from-27.30‰ to-11.80‰（PDB）,suggesting that carbon was sourced from organic substances（like CH_4,C_2H_4,and C_2H_6）.The formation of witherite was possibly associated with thermochemical sulfate reduction,which caused the consumption of the organic gases and SO_4^（2-） in the hydrothermal solutions,consequently inhibiting barite formation.The important conditions for forming
