Ore Fluid Geochemistry And Metallogenic Mechanism Of The Large Barium Metallogenic Belt Of The South Qinling

Nearly a hundred of large,medium and small barium deposits locate at Early Paleozoic strata of the South Qinling.This metallogenic belt producing unique witherite deposits has the characteristic of symbiotic differentiation of barite and witherite.According to barite content,they can be divided into the barite ore belt and the witherite ore belt.Their wall rock are mainly composed of carbonaceous slate and chert.Acorrding to major elements,trace elements and REE geochemistry of ore-bearing rocks,the result shows that the chert which is composed of rock with miners formed in a continental margin environment,and mixing of hydrothermal and terrigenous supplying which caused saturated Si-rich seawater formed the thick ore-bearing siliceous rocks.The analyses of composition of barium deposits’sulfur,carbon and oxygen isotope indicate:the sulfur isotope values of barite ore are higher than those of the same period of seawater,which associated with bacterial sulfate reduction,in the barite ore belt;the sulfur isotope values of barite ore are close to or slightly higher than those of the same period of seawater,which was caused by thermochemical sulfate reduction,in witherite ore belt;the sulfur in barite ore was derived from sea water;the more negative carbon isotope characteristics of witherite and barytocalcite reflect the organic matter source of carbon.The analyses of trace element,REE and Pb isotope of ore demonstrate that ore-forming hydrothermal fluid intruded into organic-rich sedimentary layer and carried out hydrocarbons form it.The process of hydrothermal precipitation maight be caused by difference of redox condition,nature of the solution and end-menber mixing.Microscopic analyses of fluid inclusions of ore reveal that the homogenization temperature peak of barite fluid inclusion is 120～160℃and that of witherite fluid inclusion is 160～200℃,which shows that forming temperature of wetherite is higher than that of barite.Laser Raman analyses show that ore-forming fluid of witherite contains CH4、C2H4、C2H6、C3H8、C6H6 and other organic gases,combining with analyses of homogenization temperature of witherite,material composition and carbon isotope,which shows witherite is closely related to thermochemical sulfate reduction.According to thermodynamics,when temperatue is below 162.42℃,there will be favourable to forming barite ore;when temperature is over 162.42℃,there will be favourable to forming witherite ore;thermaochemical sulfate reduction can happen between hydrocarbons and solution containing Ba2+ and SO42-,and this reaction promoted witherite mineralization and obstructed barite mineralization;Higher temperature of fluid,containing some CO2,higher Ba2+ concentration,before thermochemical sulfate reduction(TSR) the fluid resolusion containing high HS- concentration,rapid diffusion of H2S and more alkaline environment are important conditions for forming witherite.