陕西秋树坪金矿似斑状奥长花岗岩脉锆石U-Pb年龄、地球化学特征及地质意义

ZIRCON U-Pb AGES,GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL SIGNIFICANCE OF PORPHYRITIC TRONDHJEMITE VEINS IN QIUSHUPING GOLD DEPOSIT,SHAANXI PROVINCE

陕西秋树坪金矿床发育花岗岩脉,其形成年代及岩石成因背景尚不清楚。本次研究表明,其岩石类型为似斑状奥长花岗岩脉,锆石LA-ICP-MS U-Pb定年研究揭示其成岩年龄为881.9±9.6 Ma。奥长花岗岩脉w(SiO_(2))为72.91%~74.40%,w(K_(2)O)为0.38%~1.18%。稀土元素含量为51.45×10^(-6)~104.16×10^(-6),稀土元素配分曲线右倾,表现出LREE富集,HREE亏损的特点。(La/Yb)_(N)的值为8.06~27.76,平均值为21.06,轻重稀土分馏程度较高。根据主微量元素特征,认为似斑状奥长花岗岩脉形成于火山弧环境。岩脉具有高Sr含量,高Sr/Y比值、贫Yb和Y以及Eu正异常,推测其源区残留相可能为石榴子石和金红石,无斜长石,部分熔融程度相对较低,且板片流体影响较小。脉体内可见浸染状黄铁矿化、褐铁矿化,为金矿化体,暗示研究区新元古代有形成斑岩型金矿的可能。

Geochronological and geochemical analyses are carried out to unravel the petrogenetic background and formation age of granitic dikes that occurred in Qiusuping gold deposit,Shaanxi Province.The petrological study shows that the granitic dikes in the area are porphyritic trondhjemite.Zircon LA-ICP-MS U-Pb dating for the dike reveals that its rock-forming age is 881.9±9.6 Ma.Geochemical analysis of the rock shows that the contents of w(SiO_(2))and w(K_(2)O)are 72.91%-74.40%and 0.38%-1.18%respectively.The REE content is 51.45×10^(-6) to 104.16×10^(-6),and the REE distribution curve is right inclined,showing LREE enrichment and HREE depletion.The value of(La/Yb)_(N) ranges from 8.06 to 27.76,with an average value of 21.06.According to the characteristics of major elements and trace elements,it is considered that these porphyritic trondhjemite dikes were formed in volcanic arc environment.In addition,the porphyritic trondhjemite dikes are characterized by high Sr content,high Sr/Y ratio,poor Yb and Y,and positive Eu anomalies,suggesting that the residual facies in the source area may be garnet and rutile,without plagioclase,and with relatively low partial melting degree and little influence of plate fluid.Disseminated pyritization and limonization occur in the vein body,indicating the possibility of forming porphyry gold deposits in Neoproterozoic in the study area.