西秦岭安房坝金矿床矿化花岗闪长岩脉锆石U-Pb年龄、微量元素及Hf同位素特征

Zircon U-Pb ages,trace elements and Hf isotopes of the mineralized granodiorite dikes in the Anfangba gold deposit,West Qinling

安房坝金矿床位于"甘肃崖湾-大桥地区金锑矿"整装勘查区,是近十年来找矿工作中新探获的一处中型规模石英脉型金矿床。矿区内花岗闪长岩脉发育,空间上与石英脉型金矿体密切伴生,局部有金矿化。通过对两类不同矿化蚀变程度脉岩样品中的锆石CL图像特征及LA-ICP-MS微量元素分析,本文识别出了岩浆锆石与热液锆石两类不同成因的锆石。其中热液锆石通常作为岩浆锆石增生边出现,具有典型的暗色增生边结构,更高的普通铅含量及Th、U含量,比原生岩浆锆石更加富集轻稀土且Ce正异常特征不明显。通过对花岗闪长岩样品AFB.21中的岩浆锆石U-Pb定年获得安房坝金矿区花岗闪长岩脉体的侵位年龄约为219.6±2.8Ma,安房坝金矿床金成矿作用应晚于该时间。AFB.20、AFB.21两件样品中热液锆石U-Pb定年工作表明安房坝金矿区存在224~217Ma的岩浆期后热液活动。两类锆石具有相似的Hf同位素组成,表明两者是同一岩浆作用过程演化的结果,εHf(t)值为-2.5~-15.8,tDM2=1917~3097Ma,指示其可能来自于太古代-古元古代的古老地壳物质重熔。本次研究支持大桥-崖湾-安房坝一带存在隐伏岩体的剩余重力异常解译结果。蚀变花岗闪长岩脉体与隐伏岩体,以及该脉体中热液锆石与矿区内的金矿化的关系值得进一步研究与探讨。

The Anfangba gold deposit is located in the Daqiao-Yawan Au-Sb integrated exploration area,Gansu.It is a newly discovered medium-sized quartz vein type gold deposit in the past decade.The widely spread granodiorite dikes are spatially associated with the Au ore bodies.Zircon CL images and LA-ICP-MS trace element features of the two kinds of dikes has been studied.It indicates that the zircons have two distinctly different geneses,including magmatic and hydrothermal.Hydrothermal zircons have a typical non-luminescent mantle,higher 206 Pb,Th,U content,and are more enriched in LREE with no positive Ce anomaly.The U-Pb dating of the magmatic zircons indicate that the rock-forming age of the granodiorite dike is 219.6±2.BMa.The U-Pb dating of the magmatic hydrothermal zircons indicate that the period of the post-magmatic hydrothermal activity is 224~217 Ma.Similar Hf isotopic characteristics of the hydrothermal and magmatic zircons indicate a comagmatic evolution process.εHf(t)=-2.5~-15.8,tDM2=1917~3097 Ma,indicate that they may derive from the remelting of the Archean-Paleoproterozoic ancient crust.This study supports the interpretation results of the residual gravity anomalies,which indicate the existence of a concealed intrusive in the Daqiao-Y awan area.Relations between the altered granodiorite dikes with the concealed intrusion,and the hydrothermal zircons of the dikes with the gold mineralization in the Anfangba gold deposit need to be further studied.