内蒙古白乃庙地区金矿成矿作用

MINERALIZATION OF GOLD DEPOSITS IN THE BAINAIMIAO AREA,INNER MONGOLIA

内蒙古白乃庙金矿床为石英脉型和石英脉+蚀变岩型金矿,赋存于海西期的岩浆岩和白乃庙组的绿片岩中。海西期的石英闪长岩和斜长花岗岩的金丰度值为9 0×10-9～15 0×10-9,矿石铅同位素与海西期斜长花岗岩的铅同位素组成相似,反映成矿母岩可能为矿区的海西期岩浆岩;硫化物铅同位素的Δγ和Δβ值及硫同位素δ34S值显示成矿物质来源于深部,与岩浆岩有关;石英流体包裹体的δD水值、计算得出的δ18O水值和方解石的δ13C值表明成矿流体起源于岩浆,演化过程中有大气降水的加入。成矿作用是在区域构造体制发生转换后,于伸展地球动力学环境下进行的,成矿期控矿构造均呈张性,具有多阶段活动的特点。Au在成矿热液体系中可能与Cl-、HS-结合形成络合物或与H3SiO-4结合形成AuH3SiO4并发生迁移而形成金矿。

Bainaimiao, located on the northern margin of North China Plate, is an important gold mineralization area. The gold deposits that occur in greenschist of the Bainaimiao Formation and the Hercynian igneous rocks, are classified two mineralization types: quartz vein and quartz vein +altered host rocks. The values of Co/Ni of pyrite in ores are from 125 to 204; the gold abundance of the Hercynian quartz diorite and plagiogranite varis from 906×10-9 to 150×10-9; the compositions of the lead isotopes from ores and the Hercynian plagiogranite share similar characteristics. Those data indicate that Hercynian magmatic rocks are probably the metallogenic host rocks of the deposits. The values of Δγ(1665 to 4251) and Δβ(1161 to 3027) of the lead isotope and δ34S(-426‰ to +28‰) of the sulfides in ores suggest that the oreforming substances are related to plutonic magma. According to the isotopes of hydrogen, oxygen and carbon(δDwater-17504‰ to -1089‰,δ18Owater-1098‰ to 002‰,δ13C-5695‰),the oreforming fluid was originally derived from the magma; and it mixed with meteoric water in its evolution. It is suggested that the regional tectonic regime changed from compression to extension in Hercynian period, and the gold mineralization occurred in the extensional dynamical environment in which the orecontrolling structures have undergone multiextended activities. It's considered that Au might move as AuCl-2, Au(HS)-2 or AuH3SiO4 in the oreforming fluid.