南秦岭钡成矿带重晶石与毒重石成矿特征

在我国扬子地块北缘南秦岭一带的早古生代硅质岩建造中,存在一大批重晶石矿床和毒重石矿床,构成世界上极为罕见的大型钡成矿带。根据流体包裹体显微测温分析,重晶石流体包裹体均一温度峰值低于毒重石流体包裹体峰值。这些矿床中重晶石的3δ4S值比同期海水3δ4S值高(除去文峪矿区重晶石一个3δ4S值与同期海水3δ4S值接近),表明当时细菌对硫酸盐的还原作用引起了重S的富集。各钡矿床中的毒重石有各自的1δ3C和1δ8O值范围,毒重石中的C来自有机质事件反应。根据热力学计算表明,小于162.42℃的温度有利于形成重晶石矿石;在温度高于162.42℃时,海水中积累充足的CO32-(来自有机事件)有利于形成毒重石矿石。在重晶石矿床形成的晚期,如出现贫Ba2+热液,且海水中积累足够的CO32-和温度高于162.42℃,CO32-可以交代重晶石中的SO42-,形成交代成因的毒重石。

Metallogenic Characteristics of Barite and Witherite of the Barium Metallogenic Belt in Southern Qinling Mountains

More than 70 witherite and barite deposits(or points) are located in Southern Qinling Mountains,which is a unique large Ba metallogenic belt in the world.By microthermometry analysis,the peak of fluid inclusions homogeneous temperature has a increasing trend from barite to witherite.The δ34S values of barite of the deposits are higher than those of the same period water of ocean basin(except one δ34S value of barite of Wenyu deposit,which is close to those of the same period water of ocean basin),which shows that the 34S enrichment in barite could be due to the reduction from germs to sulfate.Witherite deposits of Southern Qinling exist different ranges of δ13C and δ18O value,and C of witherite comes from reaction of organic matter.According to thermodynamics data,temperatue below 162.42 ℃ is favourable to forming barite ore;when temperature is over 162.42 ℃,and there is enough CO2-3(from reaction of organic matter),it is favourable to forming witherite in sea water.In the late period of forming barite deposits,if Ba2+ appears deficient in hydrotherm,and CO2-3(from reaction of organic matter)is enough and temperature is over 162.42 ℃ in sea water,CO2-3 will metasomate the SO2-4 of barite to form metasomatic type witherite.