内蒙古白乃庙铜(钼)矿床辉钼矿Re-Os同位素年龄及其地质意义

内蒙古白乃庙铜(钼)矿床是华北板块北缘中段最重要的一个铜(钼)矿床, 矿床由南、北两矿带构成。南矿带矿体主要产于寒武系白乃庙组绿片岩中, 矿体产状与围岩基本一致;北矿带矿体大部分产于花岗闪长岩体中, 部分延进围岩。利用Re-Os同位素定年方法对南、北矿带主要矿段的5件辉钼矿样品进行了成矿时代研究, 结果表明, 辉钼矿样品的Re-Os同位素模式年龄为(435.8±6.0)~(443.0±6.1) Ma, Re-Os同位素等时线年龄为(440.5±4.4) Ma(2σ, MSWD=1.4), 属于早志留世, 与花岗闪长斑岩成岩时代(440.0 Ma)一致。结合区域地质背景和矿床特征, 认为白乃庙铜(钼)矿主成矿期为早志留世, 与早志留世花岗闪长斑岩(440.0 Ma)岩浆侵入关系密切, 又与早古生代古亚洲洋向华北板块强烈俯冲期(446.0~453.0 Ma)相吻合, 但晚于围岩白乃庙组。综合分析认为该成矿事件是早古生代华北板块北缘"沟、弧、盆"体系演化和陆缘增生的产物。

Re Os Isotopic Dating of Molybdenite from the Bainaimiao Cu(Mo) Deposit in Inner Mongolia and Its Geological Significance

The Bainaimiao Cu(Mo)deposit is the most important ones in the north margin of North China plate.It is composed of south belt and north belt.In south belt,ore bodies are concordant with ore-hosting green schist of the Cambrain Bainaimiao Formation,with near-bedded and lenticular shapes, the major bodies in north belt are located in granodiorite intrusion and partly extending into host rocks. The metallogenic age of this deposit has been disputed for many years.To dtermine the minerlization age,Re Os isotopic dating of five molybdenite samples,separated from both south and north ore belt, are completed,five molybdenite samples yield isotopic model ages of (435.8±6.0)Ma to (443.0±6.1) Ma,and an isochron age of (440.5±4.4)Ma (2σ,MSWD=1.4).These ages are well concordant with the emplacement age (440.0 Ma)of the granodiorite.Combining with regional geological background and deposit characteristics,authors think synthetically that the deposit formed in Early Silurian and has close relationship with granodioritic (440.0 Ma ) magmatic activities in Early Silurian. The mineralization age is also consist with the period (446.0 453.0 Ma)of the intensive southward subduction of the paleo-Asian oceanic plate beneath to the northern margin of the North China plate.By synthetical analysis,authors conclude that the major metallogenetic event were resulted from the evolution of the trech-basin-arc system and continent margin accretion in the North China plate during the Early Paleozoic epoch.