内蒙古赵井沟钽铌矿床成矿作用探讨--来自天河石化、钠长石化花岗岩年代学、岩石地球化学的证据

Discussion on metallogenesis of Zhaojinggou Ta-Nb deposit in Inner Mongolia:Evidence from amazonitization and albitization granite geochronology and geochemistry

内蒙古武川县赵井沟钽铌多金属矿床是近年来华北板块北缘新发现的大型稀有金属矿床,然而前人对该矿床的形成时代以及构造背景存在较大争议。本文以与成矿关系密切的天河石化花岗岩和钠长石化花岗岩为研究对象,开展了岩相学、岩石地球化学、岩浆和热液锆石LA-ICP-MS U-Pb年代学及Lu-Hf同位素的研究,探讨了岩浆源区性质和成岩成矿的构造环境。研究结果表明,钠长石化花岗岩中热液锆石加权平均年龄为118.9±1.8Ma;天河石化花岗岩岩浆锆石年龄为116±2Ma,热液锆石加权平均年龄结果为112.8±2.2Ma;热液锆石年龄略晚于岩浆锆石,说明热液锆石是在岩浆演化最后的熔体-流体相互作用阶段形成的。天河石化花岗岩和钠长石化花岗岩在主量元素上富硅、碱和铝,贫镁、钙、铁、钛和磷;在微量元素上均富集Rb、Th、U、Nb、Ta、Hf,强烈亏损Ba、Sr;其轻重稀土元素分异不明显,具有强烈的Eu负异常。岩体中发育以钠长石化,天河石化为主的蚀变,稀土元素呈现似M型"四分组"效应,Nb/Ta小于5,表明天河石化、钠长石化花岗岩经历了熔体和流体的相互作用。Lu-Hf同位素分析结果显示较负的εHf(t)值(天河石化花岗岩:-10.91~-8.17;钠长石化花岗岩:-10.98~-9.19),其对应的两阶段模式年龄tDM2分别为1689~1867Ma和1762~1871Ma,暗示富含Nb、Ta的中元古代渣尔泰山群、白云鄂博群为其岩浆源岩或提供了成岩、成矿物质。燕山晚期伸展的构造背景下,玄武质岩浆高位底侵富铌钽地壳造成减压部分熔融而形成的岩浆,岩浆在F、Cl等挥发组分的作用下发生结晶分异作用,造成铌、钽等成矿物质初步富集,并在岩浆晚期先后发生了以钠长石化、天河石化为代表的两期自交代作用及Nb、Ta的矿化作用,形成赵井沟钽铌多金属稀有矿床,该矿床为岩浆结晶分异-热液自交代作用混合成因。

The Zhaojinggou Ta-Nb polymetallic deposit,located in Wuchuan County,Inner Mongolia,is one of large rare metal deposits newly discovered in the northern margin of North China in recent years.However,intruding and mineralization ages for the deposit,as well as its metallogenic setting are still under controversial.In this paper,amazonitization granite and albitization granite,closely related to mineralization,are selected as the research objects,and their petrography,petrochemistry,geochronology and Lu-Hf isotope are studied,in aim to discuss the nature of magma origin region and tectonic environment of petrogenesis and mineralization.Magmatic and hydrothermal zircon LA-ICP-MS U-Pb dating results show that the weighted average age of hydrothermal zircons of albitization granite is 118.9±1.8 Ma,while the magmatic zircon age of amazonitization granite is 116±2 Ma,and the weighted average age of hydrothermal zircon is 112.8±2.2 Ma.The age of hydrothermal zircon is slightly later than that of magmatic zircon,which indicates that hydrothermal zircon crystallized in the fluid formed from the last stage of magmatic evolution.The bulk rock geochemical characteristics of amazonitization granite and albitization granite show that their major elements are rich in silicon,alkali and aluminum,but poor in magnesium,calcium,iron,titanium and phosphorus;their trace elements are riche in Rb,Th,U,Nb,Ta,Hf,and are strongly depleted in Ba,Sr;their rare earth elements(REEs)show unobvious light/height REEs fractionation,but strong negative Eu anomaly.The extensive albitization and amazonitization overprinting on the rock mass,obvious M-type REE"tetrad effect",lower Nb/Ta ratios(less than 5),indicate that these amazonitization granite and albitization granite had experienced extensive-fluid interaction.NegativeεHf(t)(amazonitization granite:-10.91~-8.17;albitization granite:-10.98~-9.19),as well as corresponding old two-stage model ages(1689~1867 Ma and 1762~1871 Ma,respectively),suggest that the Middle Proterozoic Zhaertai Group and Bayan Obo Group,which are rich in Nb and Ta,might be magma sources of the magmatism and mineralization.In Late Y anshanian,the northern margin North China was under an extensional tectonic background,the granitic magma was formed through partial melting of Nb-Ta-rich crust under reduced pressure induced by high position underplating of basaltic magma.These granitic magama had undergone high degree crystallization differentiation with high volatile components such as F,Cl,etc.,so that the ore-forming materials such as Nb and Ta can preliminarily be enriched.In the late magmatic stage,two periods of self-metasomatism,represented by albitization and amazonitization,and mineralization of Nb and Ta,formed the Zhaojinggou Ta-Nb polymetallic rare deposit,which is magmatic crystallization differentiation-hydrothermal self-metasomatism.