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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 witherite include high fluid temperatures,high Ba~(2+) concentrations,CO_2 in the fluids,low HS~- concentrations,and the subsequent rapid diffusion of H_2S during thermochemical sulfate reduction of the fluids.
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, but those from the witherite have two peaks of 165-175 ° C, and 215-225 ° C, respectively. Laser Raman analysis of fluid inclusions that that the vapor phase of fluid inclusions in barite is dominated by H 2 O, although some contains N 2, H 2 S, 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 2 H 6, C 3 H 8, C 2 H 4, and C 6 H 6) in the formation of barium sulfate versus carbonate.The δ ~ (34) S values of barite range from 38.26 ‰ to 54.23 ‰ (CDT) , the δ ~ (34) S 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 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 associated with thermochemical sulfate reduction, which caused the consumption of the organic gases and SO_4 ~ (2-) in the hydrothermal solutions,
witherite include high fluid temperatures, high Ba 2+ concentration, CO_2 in the fluids, low HS ~ - concentrations, and subsequent subsequent rapid diffusion of H_2S during thermochemical sulfate reduction of the fluids.
witherite include high fluid temperatures, high Ba 2+ concentration, CO_2 in the fluids, low HS ~ - concentrations, and subsequent subsequent rapid diffusion of H_2S during thermochemical sulfate reduction of the fluids.