内蒙古赵井沟过铝质花岗岩浆演化与铌钽等元素富集机制

Nb and Ta are the critical raw metals of strategic emerging industries. The Zhaojinggou Nb-Ta deposit, occurring in the peraluminous alkali-feldspar granite in Wuchuan county, Inner Mongolia, is a newly explored large scale deposit. Previous studies revealed that the mineralization of Nb, Ta and other rare elements were plausibly associated with the magmatic evolution process. However, the characteristics of the primary magma and the feature of the source region(s) of the Zhaojinggou magma remain largely undefined. Moreover, the degree of the enrichment of Nb, Ta, as well as other rare metals during the different stages in the process of magmatic evolution and the crucial geological/geochemical controls which are responsible for the mineralization of rare metals are also unclear. These scientific issues could be identified by the comprehensive study including silicate melt inclusion, mineral major and trace element composition and zircon Hf isotope analyses in present research. The metallogenetic model for the Zhaojinggou Nb-Ta deposit will be also proposed by our study. Furthermore, general rules for the enrichment of Nb, Ta as well as other rare elements in peraluminous magmas could be summarized. Finally, we try to evaluate the metallogenic potential of K-feldspar granite, granite and aplite dikes which are located in the southern part of Zhaojinggou deposit.

铌和钽是战略性新兴产业发展必需的重要金属原料。位于内蒙古武川县的赵井沟是一例新近取得突破的、赋存于过铝质碱长花岗岩体中的大型铌钽矿床。研究表明，岩浆的分异演化很可能是导致铌钽等元素富集的主要因素。然而，目前赵井沟岩体原始岩浆的性质和源区特征尚不清楚；铌钽等成矿物质的来源和在岩浆演化各阶段的富集程度亦未明确；控制该岩体成矿的关键性地质和地球化学因素亟待总结。本项目拟通过硅酸盐熔体包裹体测试、矿物化学成分分析和锆石Hf同位素研究等手段查明上述问题。在此基础上，建立赵井沟矿床的成岩成矿模型，并总结过铝质岩浆体系中铌钽等元素富集与矿化的一般规律，最终评价矿区南部钾长花岗岩及细晶/花岗岩脉的成矿潜力。

赵井沟铌钽矿是内蒙古地区发现的一处大型铌钽矿。铌钽矿体主要赋存于含天河石钠长花岗岩中，其次为花岗细晶岩、云英岩及天河石花岗伟晶岩。主要稀有金属矿物为铌钽铁矿、钽锡石、天河石、独居石、磷钇矿、氟碳铈矿、含锂铷白云母、含锂铷黑云母、锂云母及锆石等。钠长花岗岩LA-MC-ICP-MS独居石U-Pb定年获得下交点年龄分别为（124±2）Ma、（124±3）Ma、（121±1）Ma、（124±2）Ma；获得LA-MC-ICP-MS锆石U-Pb年龄为（125±1）Ma和（125±0.9）Ma。含钾矿物40Ar-39Ar定年获得黑云母40Ar-39Ar坪年龄为（133.84 ±0.79）Ma，等时线年龄为（134.55 ±0.79）Ma；白云母坪年龄为（129.23±0.52）Ma，等时线年龄为（128.52 ±1.08）Ma；天河石坪年龄为（130.12±0.57）Ma，等时线年龄为（128.80 ±1.30）Ma。这些年龄限定赵井沟铌钽矿是燕山早期构造岩浆活动的产物。花岗岩具有高SiO2、Na2O+K2O的特征，属高钾钙碱性花岗岩。稀土元素富集LREE，具有明显的负Eu异常（δEu=0.01~0.79），微量元素富集Rb、Th、U、Pb等元素，亏损Ba、Sr、P、Ti等元素。εHf(t)介于-18.54~-8.18之间，tDM2介于1.68~2.32Ga之间；εNd(t)值变化于−10.9~−7.4之间，TDM2介于1.52~1.81Ga；铅同位素组成均一，(206Pb/204Pb)i、(207Pb/204Pb)i和 (208Pb/204Pb)i值分别为16.865~17.023、15.351~15.384和37.105~37.309。这些特征限定赵井沟铌钽矿花岗岩是A1亚类的A型花岗岩，是燕山期板内伸展体制下构造岩浆活动的产物，其源区为华北克拉通元古代变质岩系。流体包裹体测温显示成矿流体特征为中温（189~388℃）、低盐度（1.03~9.84 % NaCl）流体，成矿流体的δ18OV-SMOW=0.57~3.67‰，δDH2O‰=-116.8~-99.8，指示成矿流体为岩浆水与大气水组成的混合热液。综合研究认为赵井沟铌钽成矿不仅受控于岩浆的分异演化，后期的流体活动同样是铌钽成矿的关键控制因素。在野外调查和室内研究的基础上，建立了赵井沟矿床的成矿模型，并总结了找矿标志。