大兴安岭西坡甲乌拉铜银铅锌矿床富碱花岗斑岩的成因及其地质意义:锆石U-Pb定年和地球化学特征

Petrogenesis and Geological Significance of the Alkali-Rich Granite Porphyry in the Jiawula Cu-Ag-Pb-Zn Deposit in the Western Slope of the Great Xing'an Range:Zircon U-Pb Dating and Geochemical Characteristics

甲乌拉铜银铅锌矿床是大兴安岭西坡得尔布干铜(钼)-银铅锌成矿带内大型铅锌银矿床之一,长期以来被认为是与火山、次火山热液作用有关的浅成热液矿床;然而近期的研究显示,其成矿可能与深部细脉浸染型铜矿化有着密切的成因联系。为此,我们在野外调研的基础上,开展了与细脉浸染型铜矿化有关的富碱花岗斑岩的年代学和地球化学研究工作。实验结果揭示:花岗斑岩的w(SiO_2)为71.59%~72.36%,w(TiO_2)为0.46%~0.58%,w(Al2O3)为15.45%~15.92%,w(Fe_2O_3)为0.74%~0.90%,w(FeO)为1.67%~1.96%,w(MgO)为0.55%~0.89%,w(Na_2O+K_2O)为9.01%~9.91%,K2O/Na2O值为1.37~2.15,里特曼指数(σ)为2.77~3.43,碱度指数(AR)为3.13~3.96,指示该类岩石为钾玄岩系列、高钾质、过铝质的A型富碱花岗斑岩;稀土元素总量(∑REE)为(350.11~408.80)×10-6,轻重稀土比值(∑LREE/∑HREE)为11.25~11.44,δEu主要集中在0.38~0.44,具有较强的轻重稀土分馏和明显负铕异常、强烈亏损相容元素(Ni、Co、Cr、V、Yb、Eu等)、富集不相容元素(Cs、Rb、Pb、U、Th、Zr、Hf等)、相对亏损Li、Sr等不相容元素以及持有较高的Rb/Sr(0.93~1.31)、Zr/Hf(29.38~35.61)值等特征,揭示原始岩浆来源于以CO_2为主的流体交代下地壳部分重熔或以H2O为主的流体交代下地壳部分重熔,岩浆作用过程受到了强烈的碳酸盐混染,并具有提供铜银铅锌成矿流体的属性。鉴于所获该类岩石具有岩浆和热液锆石共伴生的特点,且岩浆锆石的206Pb/238 U年龄加权平均值为(146.4±1.6)Ma(MSWD=1.30,n=12)、热液锆石206Pb/238 U年龄加权平均值为(143.1±3.9)Ma(MSWD=0.56,n=5),结合前人的研究成果,得出甲乌拉矿床成矿发生在早白垩世初期,与成矿密切的岩浆作用发生在晚侏罗世末,并认证甲乌拉矿床为斑岩型铜-浅成热液铜银铅锌成矿系统;成岩成矿地球动力学背景适值古太平洋板块向欧亚板块俯冲的大陆内部挤压向伸展转换的构造环境,或蒙古—鄂霍茨克洋闭合后造山带碰撞伸展的环境。

The Jiawula deposit is one of the large-scale Pb-Zn-Ag deposits in Derbugan Cu（Mo）-Ag-PbZn metallogenic belt in the western slope of the Great Xing＇an Range,and has been still considered as an epithermal deposit related to volcanic-subvolcanic hydrothermal.Recent researches show that the mineralization has closely genetic association with the fine vein and disseminated Cu mineralization in the deep.The geochronology and geochemistry researches have been carried out on the ore-hosting granite porphyry associated with the fine vein and disseminated Cu mineralization.The experimental results show that the granite porphyry has the w（SiO2）content ranging from 71.59% to 72.36%,w（TiO2）from 0.46% to 0.58%,w（Al2O3）from 15.45% to 15.92%,w（Fe2O3）from 0.74% to 0.90%,w（FeO）from 1.67% to 1.96%,w（MgO）from 0.55%to 0.89%,w（Na2O＋K2O）from 9.01%to 9.91%,w（K2O/Na2O）from 1.37 to 2.15,rittmann index（σ）from 2.77 to 3.43,and alkalinity index（AR）from 3.13 to 3.96.The major element composition indicate that the rock belongs to shoshonite series,high potassium,peraluminous A type alkali-rich granite granite porphyry.The total amount of rare earth elements（∑REE）changes from 350.11×10^-6 to 408.80×10^-6,the ratio of light and heavy rare earth（∑LREE/∑HREE）varies from 11.25 to 11.44,and value ofδ Eu mainly ranges from 0.38 to 0.44.These granite porphyry samples also have such geochemical characteristics as stronger fractionation of light and heavy rare earth,obvious negative Eu anomaly,strong depletion of compatible elements（Ni,Co,Cr,V,Yb,Eu,etc.）,enrichment in incompatible elements（Cs,Rb,Pb,U,Th,Zr,Hf,etc.）,relative depletion of Li,Sr and other incompatible elements,higher ratio values of Rb/Sr（0.93 to 1.31）and Zr/Hf（29.38 to 35.61）.According to data mentioned above,it can be concluded that the original magma of the granite porphyry was derived from the partial remelting of lower crustal layer metasomatized by the fluid dominated with CO2 or H2O,and the magmatic process were suffered from strong hybridization with carbonate,which is favorable for ore-forming fluid of Cu-Ag-Pb-Zn mineralization.Considering the facts that there are both magmatic and hydrothermal zircon grains in the granite porphyry samples,and the weighted average of magmatic zircon ^206Pb/^238U age is（146.4±1.6）Ma（MSWD=1.30,n=12）and the weighted average of hydrothermal zircon ^206Pb/^238U age is（143.1±3.9）Ma（MSWD=0.56,n=5）,such conclusions can be drawn as followings.The first is that the Jiawula deposit formed at the initial stage of Early Cretaceous,and the magmatism closely related to the mineralization occurred at the end of Later Jurassic.The second is that the Jiawula deposit belongs to the porphyry Cu-epithermal Cu-Ag-Pb-Zn metallogenic system.The third is that the geodynamic background of diagenesis and metallogenesis was either conversion from extrusion to extension related to the subduction of the Paleo Pacific plate to the Eurasian plate,or extension stage after the collision associated with the closure of the Mongolia-Okhotsk ocean.