内蒙古甲乌拉银铅锌矿床成矿斑岩体锆石U-Pb年龄、地球化学特征及其成矿意义

Zircon U-Pb age,geochemical characteristics and metallogenic significance of ore-bearing porphyry of the Jiawula Ag-Pb-Zn deposit in Inner Mongolia

内蒙古甲乌拉是大兴安岭得尔布干铜(钼)-银铅锌成矿带内大型银铅锌矿床之一。对矿区内与成矿有关的花岗斑岩、石英斑岩开展了年代学和地球化学研究。其中花岗斑岩锆石206Pb/238U年龄为141.9±0.5 Ma,形成于早白垩世。岩石地球化学特征表明,成矿斑岩体花岗斑岩、石英斑岩具有准铝质-弱过铝质和高钾钙碱性-钾玄质的特征;富集大离子亲石元素Rb、K及亲流体的高场强元素U、Th、Pb等,亏损Sr、Ba等大离子亲石元素,强烈亏损Nb、Ta、Ti等高场强元素,其岩浆源区可能由地幔熔体和新生地壳熔体混合形成。成矿斑岩体形成于蒙古-鄂霍茨克洋闭合后,由同碰撞造山环境向非造山板内环境转换的碰撞后伸展环境内。目前认为,成矿的花岗斑岩、石英斑岩岩体更可能是矿床与提供成矿物质和热液的深部岩浆房之间的通道,深部岩浆房晚期分异的富水熔体相在浅部就位形成石英斑岩、花岗斑岩体,其不断出溶的流体在岩体的外围形成了甲乌拉矿床。

The Jiawula Ag-Pb-Zn deposit is one of the large-sized Ag-Pb-Zn deposits in the Deerbugan Cu(Mo)-Ag-Pb-Zn metallogenic belt of Da Hinggan Mountains.The authors carried out chronological and geochemical study of granitic porphyry and quartz porphyry related to mineralization.The zircon U-Pb age of granitic porphyry is 141.9±0.50 Ma(MSWD=1.2),suggesting Early Cretaceous.Petrogeochemical characteristics show that granitic porphyry and quartz porphyry belong to quasi-aluminous-weak peraluminous,high-K calc-alkaline series to shoshonite series,that the porphyry is enriched in large ion lithophile elements(LILE)Rb,K and incompatible high field strength elements(HFSE)U,Th,Pb,depleted in large ion lithophile elements(LILE)Sr,Ba and strongly depleted in high field strength elements(HFSE)Ti,Nb,Ta,which indicates that the porphyry is the product of magma with the mixing of mantle melt and new crustal melt.The ore-bearing porphyry in the Jiawula mining area might have been formed in the post-collisional extensional environment which was transformed from collisional orogenic environment to non-orogenic intraplate environment after the closure of the Mongolian-Okhotsk Ocean.The authors hold that the granitic porphyry and the quartz porphyry were more likely to be the channels between the deposit and the deep magma chamber which provided mineralizing materials and hydrothermal fluids.The water-rich melt facies which was differentiated from the deep magma chamber formed quartz porphyry and granite porphyry in the shallow part of the crust in the late stage,and fluids continuously dissolved from the melt facies to form the Jiawula deposit in the periphery of the porphyry.