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Factors Controlling Precipitation of Barite and Witherite and Genesis of the Ankang–Xunyang Barium Deposits, Shaanxi, China
Author(s) -
Shenghua WU,
Jiajun LIU,
Degao ZHAI
Publication year - 2015
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.12482
Subject(s) - barium , mineralogy , barium carbonate , mineralization (soil science) , fluid inclusions , raman spectroscopy , analytical chemistry (journal) , chemistry , geology , materials science , quartz , metallurgy , inorganic chemistry , environmental chemistry , nitrogen , raw material , physics , organic chemistry , optics
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 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 2 O without other volatiles, and the other containing CH 4 , C 2 H 6 , C 3 H 8 , C 2 H 4 , and C 6 H 6 in addition to H 2 O. CO 2 , H 2 S, 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 fluids were critical 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 SO4 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 2 H 4 , and C 2 H 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 2 S during thermochemical sulfate reduction of the fluids.