鄂西北碱性火山岩型铌矿床榍石矿物学及对铌富集机理的指示  

Titanite Mineralogy and Its Implications for Nb Enrichment Mechanism of Alkaline Volcanic-Rock Hosted Nb Deposit in NW Hubei Province

作　　者：苌笙任 苏建辉 秦志军 赵新福[1] Chang Shengren;Su Jianhui;Qin Zhijun;Zhao Xinfu(School of Earth Resources,China University of Geosciences,Wuhan 430074,China;Hubei Geological Survey,Wuhan 430034,China)

机构地区：[1]中国地质大学资源学院,湖北武汉430074 [2]湖北省地质调查院,湖北武汉430034

出　　处：《地球科学》2022年第4期1316-1332,共17页Earth Science

基　　金：国家重点研发计划项目(No.2017YFC0602401);国家自然科学基金(No.41822203)。

摘　　要：位于南秦岭武当地区的天宝铌矿床是我国典型的碱性火山岩型铌矿床,但其岩浆成因、演化及成矿机理等方面的研究十分薄弱.天宝碱性火山岩主要分为粗面质岩石和碱性玄武岩两大类,两者在空间上紧密共生.在详细的岩(矿)相学及榍石显微结构研究的基础上,利用LA-ICP-MS分析技术对天宝粗面质火山岩中的榍石进行原位U-Pb定年,并利用EPMA对不同岩性中不同产状的榍石进行原位地球化学成分分析,从而反演岩浆演化过程及铌富集过程.粗面质岩石中榍石原位U-Pb定年的结果为432.4±4.4 Ma(n=30,MSWD=2.4),这与区域上其他粗面岩、基性岩、碳酸岩-碱性岩杂岩体形成年龄基本一致,说明研究区在早志留世发生过大规模的碱性岩浆活动.碱性玄武岩和粗面岩中榍石均可以分为岩浆榍石及捕获榍石.不同类型的榍石具有明显的产状和地球化学特征上的差异,其中岩浆榍石以自形、粒度大、振荡环带发育为主要特征,也有部分以微小粒状的形式产于基质或斑晶榍石周围,其通常具有贫Al和低Al/Fe的特点;而捕获榍石以破碎程度高、多产在岩屑内部及周围为典型特征,部分以晶屑的形式弥散状分布,且具有富Al及高Al/Fe的特点.早期形成的碱性玄武岩中的岩浆榍石具有相对较低Nb_(2)O_(5)(<0.47%)、Na_(2)O(<0.05%)和REE_(2)O_(3)(<0.87%)的特征,但Al_(2)O_(3)+Fe_(2)O_(3)的含量变化较大(0.80%~2.91%),说明早期玄武质岩浆的Nb含量相对较低.而粗面岩中的岩浆榍石明显富集Nb_(2)O_(5)(0.19%~1.50%)及REE_(2)O_(3)(0.02%~4.06%),结合镜下观察到微细铌铁矿、易解石等铌矿物发育在粗面质岩石中,指示了碱性岩浆从玄武质到粗面质演化的过程中,Nb的含量发生了明显的提高并最终在粗面岩中富集成矿.The Tianbao niobium deposit,located in the Wudang area of the South Qinling belt,is a typical alkaline volcanic-rock hosted Nb deposit in China.However,the research on the origin and evolution of alkaline magma as well as the enrichment mechanism of Nb is lacking.The alkaline volcanic rocks in Tianbao can be divided into trachyte series and alkaline basalt series,which are spatially associated.In this study,it presents detailed petrography observation combined with in-situ U-Pb dating of titanite by LA-ICP-MS,and in-situ element analyses of different types of titanite by EPMA,to reveal the magma evolution and the Nb enrichment processes.The trachyte rocks have titanite U-Pb age of 432.4±4.4 Ma(n=30,MSWD=2.4),which is consistent with the zircon U-Pb age of other trachytes,mafic dykes,and carbonatite-alkaline complexes in the South Qinling belt,indicating that the study area had large-scale alkaline magmatism activation in the Early Silurian.The titanite from both trachyte and alkaline basalt rock are of magmatic and detrital origins based on their occurrences and geochemical characteristics.Most magmatic titanite grains are euhedral and coarse,and have oscillating zoning,while some grains are present in the form of small particles around the titanite phenocrysts.They have lower Al contents and Al/Fe ratios compared to detrital titanites,which are typically fragmented.The magmatic titanites in the early basaltic rock have lower concentrations of Nb_(2)O_(5)(<0.47%),Na_(2)O(<0.05%)and REE_(2)O_(3)(<0.87%),but have a large variety of Al_(2)O_(3)+Fe_(2)O_(3)(0.80%-2.91%),indicating that the Nb content in the early basaltic magma is relatively low.The magmatic titanites in the trachyte are obviously enriched in Nb_(2)O_(5)(0.19%-1.50%)and REE_(2)O_(3)(0.02%-4.06%).Trachytes also contain fine-grained other Nb minerals such as pyrochlore,columbite,and aeschynite,indicating that the contents of Nb are significantly increased during magma fractionation from basalt to trachyte,and eventually lead to ore-grade mineralizatio

关 键 词：鄂西北 碱性火山岩铌矿床 榍石矿物学 成岩成矿过程 矿床 

分 类 号：P611.1[天文地球—矿床学]