摘要
白乃庙矿床是中亚造山带东南缘的一个大型铜金矿床,由南、北两个矿带组成,矿体主要赋存在花岗闪长斑岩和白乃庙组绿片岩中。本文分析了南、北矿带含矿花岗闪长斑岩的岩石地球化学组成、锆石U-Pb年龄及其微量元素组成,结果表明,南、北矿带含矿花岗闪长斑岩均富集大离子亲石元素(LILE,如U,K,La等)、亏损高场强元素(HFSEs,如Nb,Ta,Ti等),稀土配分模式均呈现明显的右倾型特点;此外,北矿带斑岩较南矿带斑岩更亏损Ba,Sr,P等元素,显示其在形成过程中经历了更高的分离结晶作用。这些岩石地球化学特征,结合区域上出露有前寒武纪基底,表明含矿花岗闪长斑岩形成于陆缘弧环境。白乃庙含矿斑岩具有较低的Mg#(0.37~0.49),较高的Th/Ta值(15.4~58.7)和较低的Nb/U(1.2~4.63),指示了原始岩浆侵位过程中受到了地壳物质的混染。南、北矿带含矿斑岩锆石微量元素均表现出明显富集重稀土的特征,LREE/HREE平均值分别为0.068和0.064,同时具有中度的负铕异常(δEu平均值分别为0.81和0.64)和强烈的正铈异常(Ce4+/Ce3+平均值分别为31和181),锆石结晶温度平均值分别为807℃和729℃。由于北矿带花岗闪长斑岩具有更高的氧逸度条件,并且更接近斑岩矿化中心,指示其为白乃庙矿床的成矿岩体,而南矿带花岗闪长斑岩是含矿岩体。南、北矿带含矿斑岩锆石LA-ICP-MS U-Pb年龄分别为437.3±2.9 Ma和444.8±3.4 Ma,且与前人获得的辉钼矿Re-Os年龄437Ma和445Ma较为一致,指示白乃庙矿床斑岩矿化的时间约为445~438Ma。矿床的叠加矿化发生在区域变质作用之后,以发育大量未变形的石英-硫化物脉型矿石为特征。白乃庙铜金矿床属斑岩-叠加改造型矿床。
The Bainaimiao deposit, located in the southeast margin of the Central Asian Orogenic Belt, is a large copper-gold deposit. The ore body, which consists of two ore belts, is mainly hosted in the granodioriteporphyry and the Bainaimiao Formation greenschist. The geochemistry compositions, zircon U-Pb ages and trace element compositions of the ore-bearing granodiorite porphyries from the south and north ore belts are analyzed in this study. The trace element geochemistry of the granodiorite porphyry from both ore belts are characterized by significantly enriched large ion lithophile elements(LILEs, e. g., U, K, La.), depleted of high field strength elements(HFSEs, e. g., Nb, Ta, Ti.), and the chondrite normalized REE patterns of the granodiorite porphyry show obvious characteristics of light rare earth enrichment. Compared to the granodiorite porphyry in the south ore belt, the granodiorite porphyry in the north ore belt is more depleted in Ba, Sr and P, indicating that the granodiorite porphyry in the north ore belt experienced a greater degree of fractionation and crystallization process than the granodiorite porphyry in the south ore belt. These geochemical features, combined with the Precambrian basement in this region, indicate that the ore-bearing granodiorite porphyry at Bainaimiao was formed in a continental arc. The ore-bearing granodiorite porphyry at Bainaimiao shows a low Mg#(0.37~0.49), low Nb/U(15.4~58.7) and high Th/Ta(1.2~4.63) values, suggesting that the magma had been contaminated by the continental crust during magma emplacement. The zircon trace elements of ore-bearing granodiorite porphyry from both belts exhibit significant enrichment of heavy rare earths, with LREE/HREE average values of 0.068 and 0.064, respectively, as well as moderate Eu negative anomalies(the average values of δEu are 0.81 and 0.64 respectively) and strong Ce positive anomalies(the averages of Ce4+/Ce3+ are 31 and 181 respectively). Their average zircon crystallization temperatures are 807 ℃ and 729 ℃, respectively. The granodiorite porphyry in the north ore belt has higher oxygen fugacity and is closer to the porphyry mineralization center than the granodiorite porphyry in the south ore belt, indicating that it is the productive porphyry of the Bainaimiao deposit, while the granodiorite porphyry in the south ore belt is the ore-bearing porphyry. The Concorde ages of granodiorite porphyry in the south and north ore belts are 437.3±2.9 Ma and 444.8 ± 3.4 Ma, respectively, which are consistent with the predecessors’ Re-Os ages of 438 Ma and 445 Ma, indicating that the porphyry mineralization at Bainaimiao is 445~438 Ma. The overlapping mineralization is after the transformation of regional metamorphism and was characterized by a large number of undeformed quartz-sulfide vein ores. The Bainaimiao Cu-Au deposit is a porphyry-overprinted type deposit.
作者
马阁
申萍
潘鸿迪
曹冲
冯浩轩
周满红
MA Ge;SHEN Ping;PAN Hongdi;CAO Chong;FENG Haoxuan;ZHOU Manhong(Key Laboratory of Mineral Resource,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,100029;Institute of Earth Science,Chinese Academy of Sciences,Beijing,100029;University of Chinese Academy of Sciences,Beijing,100049;College of Earth Sciences,Changyan University,Xi'an,710054;North China University of Science and Technology,Tangshan,Hebei,063000;Bainaimiao Copper Co.,Ltd.,Ulanqab,Inner Mongolia,518000)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2019年第12期3144-3165,共22页
Acta Geologica Sinica
基金
国家重点研发计划“深地资源勘查开采”重点专项（任务编号：2017YFC0601306）资助成果
