西藏琼嘉岗伟晶岩型锂矿矿物包裹体形成过程及其对熔-流体特征的指示

The formation of mineral inclusions in accessory minerals from Qongjiagang pegmatite-type Li deposit and the implications to melt-fluid characteristics

西藏琼嘉岗伟晶岩型锂矿是喜马拉雅地区发现的首例具有工业价值的伟晶岩型锂矿床,该矿床的发现证实了喜马拉雅地区具有成为我国稀有金属战略基地的潜力,也为喜马拉雅地区寻找伟晶岩型锂矿床提供了指示意义。本研究主要针对琼嘉岗锂矿花岗岩和伟晶岩中三种主要副矿物独居石、磷灰石和锆石中的矿物包裹体进行扫描电镜分析,确定矿物包裹体种类、频率以及产状(与裂隙关系),结合磷灰石中长石包裹体电子探针分析,综合指示琼嘉岗锂矿熔-流体性质及演化过程。研究表明:(1)琼嘉岗锂矿独居石、磷灰石和锆石中主要发育硅酸盐、氧化物、磷酸盐以及少量硫化物包裹体,其中填充裂隙和穿切裂隙的富稀土矿物独居石和磷灰石包裹体均由热液蚀变形成,而锆石放射性损伤强烈的区域中发育的远离裂隙的晶质铀矿和方钍石包裹体的形成也和流体作用相关;(2)电气石白云母花岗岩的磷灰石中发育钶钽铁矿以及烧绿石包裹体表明早期花岗岩岩浆富集铌和钽,稀有金属包裹体的数量以及类型也对高演化的花岗岩和伟晶岩是否具有稀有金属成矿潜力以及矿化类型具有一定指示意义;(3)锂辉石伟晶岩磷灰石中发育异常高且变化大的An值的斜长石包裹体,它们分别记录了磷灰石在结晶时对早期富钙熔体的捕获以及熔体的分异过程。本文的研究结果还表明副矿物中矿物包裹体的种类以及元素组成可以为高演化花岗岩-伟晶岩体系的熔-流体特征及演化提供指示意义。

Qongjiagang pegmatite-type Li deposit in Tibet is the first discovered pegmatite-type deposit with economic value in the Himalayan region,which confirms that the Himalayan region has the potential to become a strategic base of rare metal in China,and provide indications to find pegmatite-type Li deposit in the Himalayan region.In this study,we use SEM to identify the type,frequency and occurrence(relationship with cracks)of mineral inclusions in the three main accessory minerals,monazite,apatite and zircon from granite and pegmatite of Qongjiagang Li deposit,combining with the EPMA analysis of feldspar inclusions in apatite to comprehensively trace the property and evolution of the melts and fluids.Our study indicates that:(1)the main mineral inclusions in monazite,apatite and zircon from Qongjiagang Li deposit include silicate,oxide,phosphates and a small amount of sulfide,not only the REE-rich monazite and apatite filling or intersecting cracks are formed by hydrothermal alteration,but also the uraninite and thorianite isolated from cracks occur in the self-irradiation region of zircon are related to fluids;(2)the appearances of columbite and pyrochlore inclusions in the apatite from tourmaline-muscovite granite demonstrate that the initial melt is enriched in Nb and Ta,the amount and type of rare metal mineral inclusions can be used as an indicator for rare metal mineralization in highly evolved granite and pegmatite;(3)the plagioclase inclusions with high and a large range of An values in apatite from spodumene pegmatite represent the capture of less-differentiated melt and continuously fractional crystallization.Our results indicate that the types and compositions of mineral inclusions in accessory minerals can be good tracers for the characteristics and evolution of melts and fluids in the highly evolved granite-pegmatite system.