Located in the central-eastern section of the Yinshan Precambrian uplift in the western segment of the Inner Mongolian Axis of the Northern China craton,the newly discovered Zhaojinggou deposit is one of the largest Nb-Ta deposits in northern China.During the Hercynian orogeny,intense tectonic and igneous activities resultedin the large-scale granitoid magmatism in Zhaojinggou and its neighboring areas.The Zhaojinggou alkaline feldspar granitoid (AFG) intrusive complex was emplaced in the meta-sedimentary sequence of the Middle Carboniferous Shuanmazhuang Group,controlled by NE-,NNE-and NW-trending fault zones.The AGI complex has an outcrop area of 0.05 km2,and consists mainly of alkaline feldspar granite (AFG),AFG pegmatite and AFG aplite that have no difference in mineral assemblage.Rock-forming minerals are mainly quartz,amazonite and albite associated with minor biotite,muscovite and sericite.Accessory minerals are rutile,apatite,zircon,topaz,fluorite,allanite and columbite.The AFG intrusive complex is petrochemically characterized by relatively high Al2O3,SiO2,Na2O,Rb,Nb,Ta and Hf,obviously low CaO,Fe2O3,FeO,MgO,Sr,Ba,Zr and REE,belonging to silicon-high,sodium-rich rock of calc-alkaline series.Nb-and Tamineralization occurs entirely within the Zhaojinggou AFG intrusive complex as veinlets and disseminated blocks.Nb-and Ta-ores consist of parisite,columbotantalite,cassiterotantalite (ixiolite),monazite,bismuthinite,chevkinite,laplandite,rutile,topaz,apatite,amazonite,Li-Rb-bearing biotite,Li-Rb-bearing muscovite,epidote,fluorspar,hornblende,zircon,quarzt,barite,albite and calcite.As the most important economic Nb-and Taminerals,columbotantalite,cassiterotantalite (ixiolite) and parisite occur in the forms of impregnation,very fine grains,thin films,aggregated fine grains in the AFG intrusive complex.LA-ICP-MS U-Pb isotopic dating of the zircon separates from the AFG intrusive complex has given the ages varying from (277.14 ± 0.82) Ma to (276.6 ± 2.1) Ma.Hydrothermal alteration at Zhaojinggou shows gross zonation of silicification,amazonitization,Na-feldspathization,greisenization and propylitization from the core of the AFG intrusive complex outward.Of these alterations,amazonitization and Na-feldspathization are well developed,and have intimate relationship with Nb-and Tamineralization.The preliminary studies show that the Nb and Ta mineralization in the Zhaojinggou mineralized district and its adjacent area was apparently linked to a period of sustained granitoid magmatism within the craton,lasting from 276 to 135 Ma.This magmatism occurred in a transpressional to extensional setting and was produced from mantle-and crust-derived melts.Nb and Ta mineralized peraluminous AFG intrusive complex belongs to the low-phosphorous type of rare metal granites,indicating that they were generated from the granitoid magmas,most likely by extended fractionation of either crust derived or hybrid crust-mantle derived magmas.The multiple reworking and reactivation of the crust might have produced large volumes of Nb-and Ta-bearing granitoid melts and resulted in highly differentiated,rare-metal specialized plutons.The combined geochemical data,field geological observations and petrological evidence indicate that the Zhaojinggou deposit belongs to peraluminous granite type Nb and Ta deposit.It is believed to be a product of the Hercynian intraplate AFG magmatism.Meanwhile,the genetic model and mineral exploration criteria of the Zhaojinggou deposit can also be used as useful tools during the comprehensive evaluation of concealed Nb and Ta deposits on the northern margin of north China craton.%内蒙古武川县赵井沟矿床是近年来在内蒙古中部地区找到的一处大型铌钽矿床.铌钽氧化物储量为8000余吨(钽氧化物含量大于50％),其中,ω(Nb2O5)和ω(Ta2O5)的平均含量为0.011％和0.012％.铌钽矿化主要在早二叠世碱长花岗岩类侵入杂岩体内,呈浸染状和条带状产出,并且构成似层状、脉状和透镜状矿体.研究表明,碱长花岗岩、碱长花岗细晶岩和碱长花岗伟晶岩锆石U-Pb同位素年龄值分别为(277.14±0.82) Ma、(277.0±2.1) Ma和(276.6±2.1) Ma.鉴于铌钽矿化呈浸染状在含矿侵入杂岩体内产出,推测赵井沟矿床的成矿作用与海西期构造-岩浆活动有关.古大陆块体伸展构造条件下,富钠质钙-碱性岩浆作用为铌钽矿床的形成提供了动力和物质来源,而断裂构造为成矿物质沉淀聚集创造了空间条件.赵井沟矿床属富钠的过铝质花岗岩型铌钽矿床.
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