Abstract:

The barite deposits in which Ba presents as BaSO4 are widespread in the world, whereas the witherite deposits in the presence of BaCO3 are rarely reported. Numerous layered witherite and barite deposits occur within the Early Paleozoic silicate formations of southern Qinling Mountains. These two types of deposits, representing a largescale particular metallogenic belt in the world, exhibit paragenetic and separated distribution characteristics. This paper focuses on the fluid inclusion features of witherite, barytocalcite, barite, and quartz in the barium metallogenic belt. The results indicate that the homogenization temperature of fluid inclusions of each mineral ranges from 90 to 310 ℃, mainly concentrating at the range of 120220 ℃. For all that, the peak of forming temperature of witherite and barytocalcite is higher than that of barite about 40 ℃, and quartz forming temperature distributes more homogeneously, and does not appear peak. The salinity of fluid inclusions of most minerals ranges from 1 to 15 wt% NaCleqv, but the salinity of fluid inclusions of witherite, barytocalcite, and quartz generally is more than 5 wt% NaCleqv, while for a considerable proportion of barite, the salinity of fluid inclusions is less than 5 wt% NaCleqv. The fluid inclusions of the barite mainly are of NaClH2O type, whereas the fluid inclusions of witherite are rich in aqueous solution with complicated components like N2CO2H2SCH4 etc., which shows that there is a large difference between the metallogenic environments of witherite and barite. Based on these evidences, we suggest that the thermochemical sulfate reduction (TSR) of the solution was the main formation mechanism of witherite deposits.

Key words: witherite deposit, fluid inclusions, thermochemical sulfate reduction, southern Qinling