新疆大红柳滩伟晶岩型锂矿深部结构与区域成矿模型解释

Deep structure and interpretation of regional metallogenic model of the Dahongliutan pegmatite lithium ore deposit in Xinjiang,western China

为揭示花岗岩-伟晶岩型锂等稀有金属矿成矿系统的深部结构,对西昆仑造山带大红柳滩伟晶岩型锂矿集区开展了大地电磁测深法(MT)探测。通过MT三维反演电阻率模型,探测到两个0~20 km深度范围的高阻体,反映了出露于地表的大红柳滩复式花岗岩基和半隐伏的大红柳滩东花岗岩基;20~80 km深度范围内发现的大范围高导异常,则反映了深达上地幔的地壳重熔形成的大规模长英质岩浆储库。可见,成矿母岩大红柳滩花岗岩基是有根的,而且是规模巨大深达上地幔的岩浆储库,它们为超大型大红柳滩伟晶岩型锂矿的形成提供了物源和热源。与松潘-甘孜甲基卡超大型伟晶岩型锂矿集区对比,尽管川西甲基卡地区地表出露的花岗岩有限,但MT三维反演电阻率模型显示,其也存在深达上地幔的大范围高导异常,同样反映了大规模长英质岩浆储库的存在,只是剥蚀深度浅,上侵的花岗岩未被剥蚀出来而已。从而,深剥蚀的大红柳滩地区表现为大面积花岗岩出露的“热隆”特征,而浅剥蚀的甲基卡地区则表现为花岗岩围岩“片麻岩穹隆”热变质构造特征。西昆仑-松潘-甘孜伟晶岩型锂等稀有金属巨型成矿带两端的晚三叠世超大型矿床是大规模地壳重熔长英质岩浆作用中心的产物,由于锂等稀有金属的喜水性,H2O的饱和度是造就伟晶岩型锂超常富集的关键,并在长英质岩浆储库、上侵花岗岩和伟晶岩不同分异演化阶段,锂的“预富集”为大规模伟晶岩型锂矿成矿奠定了重要基础。

To reveal the deep structure of the granitic-pegmatitic type lithium and other rare metal mineral systems,amagnetotelluric(MT)survey was conducted in the Dahongliutan area of the West Kunlun orogenic belt.Through 3D resistivity inversion of the MT data,two high-resistivity bodies were detected within the depth range of 0~20 km,reflecting the large-scale felsic magma and the partially concealed eastern granite basement of Dahongliutan.Additionally,a widespread high-conductivity anomaly observed at depths of 20~80 km reflects the large-scale felsic magma reservoir formed by crustal remelting extending deep into the upper mantle.The mineralization host rocks,the granite basement of Dahongliutan,are shown to be rooted and represent a massive magma reservoir extending deep into the upper mantle,providing the source materials and heat for the formation of the giant Dahongliutan graniticpegmatitic type lithium deposit.In comparison with the Songpan-Ganzi Jiajika super-large graniticpegmatitic type lithium deposit,although the exposed granite in Jiajika is limited,the 3D resistivity inversion model of the MT data also reveals a widespread high-conductivity anomaly extending into the upper mantle,indicating the presence of a large-scale felsic magma reservoir.The difference lies in the shallower erosion depth in Jiajika,where the overlying granite intrusion has not been eroded.Therefore,Dahongliutan exhibits a“hot dome”feature with extensive granite outcrops resulting from deep erosional processes,while Jiajika represents a“gneiss dome”feature with the country rocks surrounding the granite subjected to thermal metamorphism.The giant ore belt of rare metal mineralization,including lithium,in Western Kunlun-Songpan-Ganzi,which hosts two super-large deposits of late Triassic age at its ends,is the product of large-scale crustal remelting and felsic magmatism.The saturation of H2O is a key factor in the exceptional enrichment of lithium and other rare metals in the granitic-pegmatitic type lithium deposits.Moreover,the“pre-enrichment”of lithium during different stages of differentiation and evolution in the felsic magma reservoir,intrusive granites,and pegmatites lays an essential foundation for the formation of massive granitic-pegmatitic type lithium deposits.