湖北庙垭碱性杂岩体中铌成矿作用:来自金红石矿物学和年代学的制约  

Niobium Mineralization in the Miaoya Alkaline Complex,Hubei Province:Constraints from Rutile Mineralogy and Geochronology

作　　者：应元灿 陈唯[2] 柳加俊 杨帆 蒋少涌[2] YING Yuancan;CHEN Wei;LIU Jiajun;YANG Fan;JIANG Shaoyong(Faculty of Land and Resource Engineering,Kunming University of Science and Technology,Kunming 650093,China;State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China)

机构地区：[1]昆明理工大学国土资源工程学院,昆明650093 [2]中国地质大学地质过程与矿产资源国家重点实验室,武汉430074

出　　处：《高校地质学报》2024年第3期345-361,共17页Geological Journal of China Universities

基　　金：国家自然科学基金项目(42321001,42302090,42030811);科技部国家重点研发计划(2019YFA0708400)联合资助。

摘　　要：庙垭碱性杂岩体主要由富Nb和REE的碳酸岩和正长岩组成,已探明Nb_(2)O_(5)储量93万吨,平均品位0.12%,是国内仅次于白云鄂博的碳酸岩型Nb-REE矿床。前人对碳酸岩成因及稀土成矿作用开展了大量的研究,但对正长岩中铌的赋存状态和富集机制尚不清楚。文章在野外地质调查的基础上,结合TIMA、电子探针和LA-ICP-MS分析技术对正长岩中含铌矿物开展了详细的岩相学、矿物化学和U-Pb年代学研究。结果显示庙垭正长岩中主要含铌矿物为金红石,根据矿物结构和化学组成可划分为岩浆金红石和热液金红石两类。其中岩浆金红石少见,而热液金红石在各类正长岩中广泛发育,多呈浸染状沿原生金红石边缘或裂隙分布,或与黑云母和钛铁矿等其它矿物紧密共生。在化学组成上,原生金红石显示低Nb2O5(1.43%~2.56%)、FeO(0.74%~1.01%)和微量元素(如Ta、Cr、V、W、Mo、Sb)含量特征;而热液金红石具有不同程度的Nb_(2)O_(5)(3.48%~20.68%)、FeO(1.18%~6.92%)和微量元素富集特征。原位微区U-Pb定年显示两类金红石形成于不同的时代,原生金红石形成时代为446±21 Ma,而热液金红石形成时代为240±19 Ma,表明正长岩中铌矿化经历了早古生代岩浆阶段的初次富集和三叠纪热液交代阶段的二次富集。综上,我们认为庙垭杂岩体中铌富集成矿受控于岩浆和热液过程。在早古生代岩浆阶段,正长岩中Nb主要以晶格置换的方式进入早期金红石、黑云母等含钛矿物中;在三叠纪热液交代阶段,热液流体可分解黑云母形成次生富铌金红石和铌铁矿,亦或交代早期金红石和钛铁矿最终形成富铌金红石。The Miaoya alkaline complex is mainly composed of syenite and carbonatite enriched in Nb and REE,with a verified Nb_(2)O_(5) reserve of 0.93 Mt@0.12%.It is the second largest carbonatite-hosted Nb-REE deposit in China,second only to Bayan Obo.Previous studies have focused on the genesis of carbonatite and the process of REE enrichment,but the occurrence and enrichment mechanism of Nb in syenite are still unclear.Based on field geological surveys,detailed petrography,mineral chemistry,and U-Pb dating of the Nb-bearing minerals in syenite were obtained using TIMA,EMPA and LA-ICP-MS analyses.The results show that the main Nb-bearing mineral in the Miaoya syenite is rutile,which can be divided into magmatic and hydrothermal generations based on texture and chemical composition.Magmatic rutile is rare,while hydrothermal rutile is common in various syenites,mostly disseminated along the edges or fissures of primary rutile or closely associated with biotite and ilmenite.Magmatic rutile shows low contents of Nb_(2)O_(5)(1.43%~2.56%),FeO(0.74%~1.01%),and other trace elements(e.g.,Ta,Cr,V,W,Mo,Sb);while hydrothermal rutile has variable enrichments of Nb2O5(3.48%~20.68%),FeO(1.18%~6.92%),and other trace elements.In-situ U-Pb dating of rutile illustrates that the formation age of magmatic rutile is 446±21 Ma,while the formation age of hydrothermal rutile is 240±19 Ma,indicating that Nb mineralization in syenite experienced initial enrichment during the early Paleozoic magmatic stage and secondary enrichment during the Triassic hydrothermal metasomatic stage.In summary,we believe that Nb enrichment and mineralization in the Miaoyao complex are controlled by both magmatic and hydrothermal processes.During the early Paleozoic magmatic stage,niobium within syenite is mainly hosted by rutile,biotite and Ti-bearing minerals(e.g.,ilmenite);in the Triassic hydrothermal metasomatic stage,hydrothermal fluids can decompose biotite to form secondary Nb-rich rutile and columbite,or metasomatize primary rutile and ilmenite to generate h

关 键 词：碱性正长岩 铌矿床 金红石 成矿作用 庙垭 

分 类 号：P57[天文地球—矿物学]