华北克拉通北缘张家口地区古生代-中生代岩浆岩锆石氧逸度对金成矿的制约

Restrictions on gold mineralization by zircon oxygen fugacity from the Paleozoic-Mesozoic magmatic rocks in the Zhangjiakou district,northern margin of the North China Craton

华北克拉通北缘张家口地区是我国重要的金矿集区,区内金矿床普遍富碲,金成矿与岩浆岩具有密切成因联系。本文通过对张家口地区古生代-中生代岩浆岩锆石氧逸度进行分析,评价各岩浆岩成矿潜力,从而对金和碲的物质来源及矿床成因进行约束。锆石Ce^(4+)/Ce^(3+)、Ce-U-Ti、Ce/Nd和Eu_(N)/Eu_(N)^(*)等氧逸度计算方法获得的区内岩浆岩氧逸度总体趋势一致,结合地质特征及各氧逸度计适用性,本文认为锆石Ce-U-Ti氧逸度计更为可靠。张家口地区岩浆岩氧逸度从高到低依次为青羊沟花岗岩(平均ΔFMQ=+4.4)、张家口组流纹岩(平均ΔFMQ=+3.2)、水泉沟碱长正长岩(平均ΔFMQ=+2.6)、伟晶岩脉(平均ΔFMQ=+2.5)、上水泉花岗岩(平均ΔFMQ=+1.5)、响水沟花岗岩(平均ΔFMQ=+1.4)、象山花岗闪长岩(平均ΔFMQ=+1.2)。中高氧逸度(硫化物-硫酸盐转变阶段)有利于Au、Te的迁移富集。水泉沟碱长正长岩、伟晶岩脉及青羊沟花岗岩等岩体与东坪和大白阳金矿床成矿流体的氧逸度范围一致,其出溶的岩浆流体氧逸度高,携带迁移Au、Te等元素的能力强,成矿潜力较大。水泉沟碱性岩锆石微量元素具有Eu_(N)/Eu_(N)^(*)>0.3、10000×(Eu_(N)/Eu_(N)^(*))/Y>1、(Ce/Nd)/Y>0.01、Dy/Yb<0.3及(Eu_(N)/Eu_(N)^(*))/Y与(Ce/Nd)/Y呈正相关的特点,具有成矿岩体的特征。锆石氧逸度及微量元素组成显示水泉沟碱性岩具有很好的Au-Te成矿潜力,而上水泉花岗岩的成矿潜力较差,张家口地区富碲金矿床应与水泉沟碱性岩具有密切成因联系。

The Zhangjiakou district,located at the northern margin of the North China Craton,is an important goldfield in China.Gold deposits in this district are commonly enriched in tellurium,and gold mineralization is closely associated with magmatic rocks.In this paper,we provide zircon oxygen fugacity of the Paleozoic-Mesozoic magmatic rocks from this district,with the aim to evaluate their mineralization potential and to constrain their gold and tellurium sources and genesis.The results of different zircon oxybarometers,including Ce^(4+)/Ce^(3+),Ce-U-Ti,Ce/Nd and Eu_(N)/Eu_(N)^(*),give a relative consistent trend of magmatic oxygen fugacity.Combined with geological characteristics and other oxybarometers,we think the Ce-U-Ti oxybarometer is more reliable.The order of the oxygen fugacity values for magmatic rocks from the Zhangjiakou district is Qingyanggou granite(averageΔFMQ=+4.4),Zhangjiakou Formation rhyolite(averageΔFMQ=+3.2),Shuiquangou syenite(averageΔFMQ=+2.6),pegmatite vein(averageΔFMQ=+2.5),Shangshiuquan granite(averageΔFMQ=+1.5),Xiangshuigou granite(averageΔFMQ=+1.4),Xiangshan granodiorite(averageΔFMQ=+1.2).Medium-high oxygen fugacity(around sulfide-sulfate transition)is efficient for migration and enrichment of Au and Te.The Shuiquangou syenite,pegmatite vein and Qingyanggou granite exhibit similar oxygen fugacity levels to the Dongping and Dabaiyang gold deposits.The exsolution magmatic fluids associated with these rocks possess elevated oxygen fugacity,a strong capacity to transport Au,Te and other elements,and a high potential for ore formation.Zircon trace element compositions from the Shuiquangou alkaline rocks indicates several characteristics of fertility magmatic suite,including Eu_(N)/Eu_(N)^(*)>0.3,10000×(Eu_(N)/Eu_(N)^(*))/Y>1,(Ce/Nd)/Y>0.01,Dy/Yb<0.3,and a positive correlation between(Eu_(N)/Eu_(N)^(*))/Y and(Ce/Nd)/Y.The oxygen fugacity and trace element composition of zircons reveal that the Shangshuiquan alkaline rocks possess a favorable potential for Au-Te mineralization,whereas the mineralization potential of the Shangshuiquan granite is limited.The tellurium-rich gold deposits in the Zhangjiakou district have close genetic relationships with the Shuiquangou alkaline rocks.