青海加吾金矿床花岗斑岩脉锆石U-Pb年龄、Hf同位素特征及其地质意义

CHARACTERISTICS AND GEOLOGICAL SIGNIFICANCES OF ZIRCON U-Pb AGES,Hf ISOTOPE OF GRANITE-PORPHYRY VEIN IN JIAWU GOLD DEPOSITS,QINGHAI PROVINCE

加吾金矿床大地构造上位于中央造山系"共和缺口"西缘,位于中央造山带多个块体交接转换的重要部位,其形成与区内印支期中酸性岩浆活动密切相关。对区内与金成矿关系密切的花岗斑岩脉中的锆石进行LA-ICP-MS U-Pb定年以及Lu-Hf同位素测试。测试结果显示,锆石206Pb/238 U加权平均年龄为233.4Ma±4.3Ma(MSWD=2.7),形成时代为中三叠世,与青海西秦岭地区其他印支中晚期中酸性岩浆岩形成时代相近;锆石εHf(t)值介于-12.35^-1.12之间,二阶段Hf模式年龄(tDM2)介于1.3Ga^2.0Ga之间,加吾金矿区内花岗斑岩的源区主要为早-中元古代地壳。相对较高的εHf(t)值的出现说明形成花岗斑岩脉的岩浆有一定幔源组分的加入。与加吾金矿成矿关系密切的花岗斑岩脉形成于陆-陆碰撞过程中挤压向伸展转换的地球动力学环境。由于俯冲陆壳板片断离,幔源岩浆上涌/底侵至下地壳底部,诱发增厚古老下地壳部分熔融产生壳-幔混合岩浆,在印支中期侵入到中下三叠统隆务河组地层中,为金矿的形成提供了热源和深部成矿流体。加吾金矿床成矿时代与花岗斑岩脉的成岩时间一致,即233.4 Ma±4.3 Ma。

The formation of Jiawu gold deposit is closely related to Indo-Sinian mid--acidic magmatism of the region. LA-ICP-MS U-Pb dating of zircons and Lu-Hf isotope of the granite porphyry veins indicate that it is closely related with gold mineralization in the region. It shows that zircon ^206pb/^238U weighted average age is 233.4 Ma±4. 3 Ma （MSWD=2.7）, the formation period is the Triassic era,same as the formation age of mid-late Indo-Sinian mid-acidic magmatism in West Qinling of Qinghai. Zircon εHf（t） values are between -- 12.35 and -- 1.12, Hf second stage model ages （tDM2） are between 1.3 Ga and 2.0 Ga,and the source region of granite porphyry in Jiawu gold deposit are mainly the Early-middle Proterozoic crust. The relatively high value of CHf （t） indicates that mantle component has been involved in the formation of parental magma of the granite porphyry veins. The granite porphyry veins that are closely related to gold mineralization in Jiawu gold deposits are formed under the geodynamic environment of the transformation from continent-continent collision compression to extension. Due to the subduction continental crust plate fragment,mantle-derived magma was upwelling and underplating to the bottom of the lower crust,causing partial melting of the thickened ancient lower crust and subsequent crust-mantle magma mixture. The magma mixture intruded into Longwuhe Formation in the middle-lower Triassic series,providing the heat source and mineralization-bearing fluids for the formation of the gold deposit. Therefore, the age of the mineralization of Jiawu gold deposit was the same as that of granite porphyry veins, that is, 233.4 Ma±4.3 Ma.