黑龙江金厂金矿花岗斑岩锆石地球化学、U-Pb年代学及地质意义

为确定与金厂金矿成矿有关的岩浆类型、活动时限和构造背景,采用LA-ICP-MS技术对研究区花岗斑岩开展了锆石U-Pb年龄及原位微区微量元素测定。结果表明:锆石环带发育,wTh/wU值>0.4,具有岩浆锆石特征;锆石年龄分布于220Ma和103～123Ma 2个区间,代表了2期岩浆事件;对2种锆石分别命名为捕获岩浆锆石和新生岩浆锆石;锆石的地球化学和年龄信息显示捕获岩浆锆石的原岩为早三叠世花岗岩;锆石微量元素信息暗示花岗斑岩是早三叠世花岗岩高度熔融结晶分异、侵位于浅部氧化环境而形成,这一过程导致新生岩浆锆石负Eu异常程度降低。新生锆石加权平均年龄为(113.5±3.8)Ma,与成矿年龄一致,据此认为早白垩世的岩浆事件是金厂金矿成矿事件的直接原因,成矿背景为太平洋板块俯冲后的岩石圈伸展。

Geochemistry and U-Pb geochrony of zircon from granite porphyry of Jinchang gold deposit in Heilongjiang, China and its geological significance

The Jinchang gold deposit,located in the east segment of the Central Asian orogenic belt,is a large hydrothermal deposit associated with the Mesozoic magmatic activities.In order to determine the type,age and tectonic setting of the metallogenic magmatic rock,the authors measure the U-Pb dating and in situ trace elements of the zircons from granitic porphyry by cathodoluminescence(CL) images and LA-ICP-MS.All zircon grains have fine oscillatory zonings in CL images,and the ratio of Th/U is >0.4,indicating their magmatic origin.Zircon U-Pb dating is 220 Ma and 103~123 Ma,representing the two-phase magmatic events.The zircon with age of 220 Ma belongs to the captured magmatic zircon.Correspondingly,zircons with ages of 103~123 Ma are derived from the new magma.Geochemistry and U-Pb dating of the zircons suggest that the captured zircon was derived from the Early Triassic granite porphyry.The trace elements of the zircons indicate that the granite porphyry originated from the highly melting and crystallization differentiation and the emplacing in the oxidizing environment in the shallow crust of the Early Triassic granite.The values of negative Eu anomalies in zircons from the new magma decreased in the oxidizing environment.The weighted average U-Pb age(113.5±3.8 Ma) of zircons from new magma is identical with the metallogenic age.Therefore,the Early Cretaceous magmatic event directly resulted in the gold mineralization in the Jinchang deposit.The mineralization occurred in lithospheric extensional setting related to the subduction of the Pacific Plate.