The Zhaojinggou Nb-Ta polymetallic deposit in Inner Mongolia is a newly discovered large rare metal ore deposit.The reserve of Nb-Ta oxides is more than 8200 tons.Previous studyfocus on geological features of this deposit,but they the precise mineralization age remains unknown.In this paper,muscovite in amazonite granite pegmatite was used to constrain the timing of mineralization using 40Ar-39 Ar isotope method.Results show that Nb-Ta mineralization is mainly hosted in alkali-feldspar granite,alkali-feldspar aplite,greisen,and amazonite granite pegmatite.According to the geologicalrelationship between Nb-Ta minerals and muscovite,we can infer that Nb-Ta minerals and muscovite are formed at the same time.Thus,the muscovite plateau ages of 123.57 ± 0.66 Ma and 124.0 ± 2.0 Ma can represent the mineralization age.Results show that the Zhaojinggou Nb-Ta polymetallic deposit was formed at late Yanshanian (or at least Early Cretaceous),the 40Ar-39Ar age have bigger difference with the zircon age,which is of great importance for ore genesis.Combining with regional geology,the author thinks that an essential rare-metal mineralization happened in the middle part of inner Mongolian axis on the north margin of north china platform during the late Yanshan movement.%内蒙古赵井沟铌钽多金属矿是近年来取得较大找矿进展的大型稀有金属矿床,初步探明铌钽氧化物储量超过8200吨,前人对其矿床地质特征已有初步研究,但其成矿时代尚未精确厘定.本文以赵井沟铌钽多金属矿区内天河石花岗伟晶岩脉中的白云母为研究对象,利用40 Ar-39 Ar同位素年代学方法,以确定其成矿时代.研究发现矿区内铌钽矿化主要赋存在碱长花岗岩、碱长花岗细晶岩、云英岩和天河石花岗伟晶岩中,且铌钽矿物与白云母同时形成,有重要的共生关系,故本次研究获得的白云母坪年龄123.57±0.66 Ma 和等时线年龄124.0±2.0 Ma具有地质意义,可代表成矿年龄.这一成果表明赵井沟铌钽多金属矿床形成于燕山晚期(至少延续至早白垩世),与前人获得的锆石年龄有较大差异,这对分析其矿床成因有重要意义.同时,结合区域地质资料,本文认为华北地台北缘内蒙古地轴中部在燕山运动的晚期存在一次重要的稀有金属成矿事件.
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