Factors Controlling Precipitation of Barite and Witherite and Genesis of the Ankang–Xunyang Barium Deposits, Shaanxi, China

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.

Factors Controlling Precipitation of Barite and Witherite and Genesis of the Ankang-Xunyang Barium Deposits,Shaanxi, China

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 SO4^2– generally experienced bacterial sulfate reduction in seawater before mineralization, and some SO₄^2– 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 SO₄^2– 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.