阿尔泰可可托海3号脉伟晶岩型稀有金属矿床云母和长石的矿物学研究及意义

可可托海3号脉伟晶岩型稀有金属矿床是阿尔泰造山带产出的规模最大的伟晶岩脉,其完美的同心环状结构分带举世闻名。云母和长石作为3号脉9个结构带的贯通性矿物,由外向内表现不同的结构和成分特征。其中,云母由白云母系列向锂云母系列演化,白云母呈黄-绿色中细粒→白色或绿色中粗粒-巨晶→白色或绿色书状集合体→白色或绿色中粗粒-巨晶,锂云母呈玫瑰紫中细粒鳞片状或楔状集合体,BSE图像下云母表现出成分分带及不平衡和交代结构;长石主要为钾长石和钠长石,及少量斜长石,钾长石主要呈块体产出,钠长石呈细粒→叶片状→薄片状产出。本次研究通过电子探针(EMPA)和激光剥蚀等离子质谱(LA-ICP-MS)获得3号脉各结构带云母和长石的主微量成分。3号脉云母具有高Li(249×10^-6~35932×10^-6)、Rb(1240×10^-6~22825×10^-6)、Cs(35.9×10^-6~13980×10^-6)、Ta(13.3×10^-6~447×10^-6)含量、低K/Rb值(4.23~59.4)和K/Cs值(6.53~2368),钾长石具有低K/Rb值(35.4~1865),且由外向内,随K/Rb值降低,云母的Li、Rb、Cs、F、Ta含量升高,表明3号脉是一个由外向内结晶的分异演化程度较高的伟晶岩脉。另外,连续相邻结构带中云母和长石的主微量成分呈振荡变化,该现象主要受熔体不混溶过程的控制,也受矿物结晶不平衡影响,而熔体不混溶过程也是控制3号脉结构分带的机制之一。外部带(I-IV带)和内部带(V-VIII带)的云母和碱性长石在成分(FeO、Li、Rb、Cs、F、Ta含量和K/Rb值及K/Cs值)和结构(不平衡和交代结构)上具有明显差异,内部带演化程度明显加大,流体组分比例升高,表明体系由以熔体为主的阶段(外部带)进入以熔流体为主相对不稳定的阶段(外部带)。结合野外观察的证据,促使体系在IV带和V带间发生突然转变而进入熔流体阶段的是一个泄压事件。

Mineralogy and significance of micas and feldspars from the Koktokay No.3 pegmatitic rare-element deposit, Altai

The Koktokay No.3 pegmatitic rare-element deposit with perfect and world-famous concentric internal zones is the largest pegmatite of the Altai orogen belt, hosting about 100,000 pegmatites. The micas and feldspars show different features of structure and composition in 9 internal zones. The micas belong to muscovite through I-VIII zones and lepidolite in inner zones. From outer to inner, muscovites occur in yellow-green middle to fine grains, white or green middle-coarse-huge grains and white or green book assemblages. Lepidolites occur in middle-fine scaly or wedge-shaped assemblages. There are chemical zoning, disequilibrium and metasomatic structures of micas in backscattered images. The feldspars from the Koktokay No.3 pegmatite include K-feldspar, albite and some plagioclase. The K-feldspars occur in massive and albites occur in saccharoidal, blade and slice from outer to inner. The micas of the Koktokay No.3 pegmatite contain high Li(249×10^-6~35932×10^-6), Rb(1240×10^-6~22825×10^-6), Cs(35.9×10^-6~13980×10^-6), Ta(13.3×10^-6~447×10^-6), low K/Rb values(4.23~59.4)and K/Cs values(6.53~2368)and the K-feldspars contain low K/Rb values(35.4~1865). Also, from outer to inner, Li, Rb, Cs, F and Ta contents increase and K/Rb values decrease. Thus, the Koktokay No.3 pegmatite is a highly evolutionary pegmatite with increasing fractionation from outer to inner. The oscillation changes of major and trace element contents of micas and feldspars from continuous adjacent zones are possibly controlled by melt-immiscible process with minor effect of mineral crystallization and fractionation. Also, the melt-immiscible process possibly results in the internal zonation of the Koktoka No.3 pegmatite. There are significant differences of compositions(FeO, Li, Rb, Cs, F, Ta contents and K/Rb and K/Cs values)and structures(disequilibrium and metasomatic)of micas and feldspars between outer zones(I-IV)and inner zones(V-VIII). Compared with outher zones, inner zones show more fractionation features and contain more fluid, suggesting that the system transformed from melt stage(outer zones)to instable melt-fluid stage(inner zones). On account of fieldwork, a pressure-relief event results in the the system abrupt change between zone IV and zone V and contributes to more fluid exoslution and the formation of melt-fluid stage.