内蒙古白乃庙铜矿床成矿时代的研究

The Metallogenic Epochs of Bainaimiao Copper Deposit in Inner Mongolia

内蒙古白乃庙铜矿床是华北地区知名的大型铜矿床之一,位于兴蒙造山系满都拉-霍林郭勒弧盆系白乃庙早古生代岛弧内。该矿床包括南、北两个矿带,南矿带矿体产于白乃庙组绿片岩中,北矿带矿体产于花岗闪长斑岩内。白乃庙组共划分为五个岩性段,第一、三、五岩段岩性主要为绿泥斜长片岩、阳起绿泥斜长片岩,是白乃庙铜矿的主要赋矿层位;第二、四岩段为长英片岩和变质流纹英安岩组合。白乃庙组地层的原岩为一套中基性-中酸性海相火山-沉积岩组合。对于该矿床及白乃庙组地层的形成时代争议较大,前人多认为二者均形成于中元古代。本文采用锆石SHRIMP U-Pb同位素定年获得了白乃庙组第三岩性段绿泥斜长片岩的形成年龄为439.8±6.7Ma,花岗闪长斑岩的形成年龄为433.9±2.2Ma、451.8±3.0Ma,采用辉钼矿Re-Os同位素分析获得辉钼矿的形成年龄为433.9±3.1Ma。结合前人成果,提出白乃庙组地层及花岗闪长斑岩的成岩时代和白乃庙铜矿的成矿时代均为晚奥陶-早志留世,它们同属早古生代白乃庙岛弧岩浆作用的产物。

As one of the famous large copper deposits in the North China area,the Bainaimiao copper deposit, Inner Mongolia, is located in early Paleozoic Bainaimiao island-arc belt of the Mandula-Huolinguole arc- basin system in Xing~ An-Mongolia Orogenic Belt. The deposit is comprised of the northern and the southern ore belts. The host rock of the southern ore belt is Bainaimiao greenschist formation,whereas the orebodies in the northern belt mainly occurs in the granodiorite porphyry. Bainaimiao formation consists of five members. The first,third and fifth members of Bainaimiao formation consisting of chlorite plagioclase schist and actinolite plagioclase schist are the chief ore-bearing country rock. Its protolith is a suite of mafic-intermediate-acidic marine volcanic rock. The second and fourth members are composed of felsie schist and metarhyolite suits. The protolith of Bainaimiao formation is a suite of mafic- intermediate-acidic marine volcanic-sedimentary rock. There is still controversy for quite a long time over the forming ages of the copper deposit and Bainaimiao formation, both of which were supposed of forming in Mesoproterozoic by the predecessors . In this paper,two of the SHRIMP U-Pb ages of zircons obtained from the chlororite- plagioschist-the third member of Bainaimiao Group and granodiorite porphyry, are 439.8±6.7Ma and 433.9±2.2Ma,451.8±3.0Ma respectively, and the Re-Os isotopic age of molybdenite is 433.9±3.1Ma. Combining with the isotopic ages obtained by predecessors, it is suggested that the ages of the rock-forming of Bainaimiao formation and granodiorite porphyry, and the ages of the ore-forming of Bainaimiao copper deposit formed in late Ordovician to early Silurian. Both of them were the products of early Paleozoic Bainaimiao arc magmatic activity.