云母和电气石矿物化学特征对西昆仑大红柳滩地区伟晶岩型锂矿化的指示

Mineral Characteristics of Mica and Tourmaline and Geological Implication for the Pegmatite-Type Lithium Mineralization,Dahongliutan Area,West Kunlun

大红柳滩二云母花岗岩被认为是白龙山等伟晶岩型锂矿的成矿母岩.为约束大红柳滩地区花岗岩-伟晶岩的岩浆-热液演化过程.本文选取大红柳滩岩体二云母花岗岩及不同矿化程度伟晶岩中的贯穿性矿物——云母和电气石,开展了背散射结构观察(BSE)和电子探针成分分析(EPMA).二云母花岗岩(Ms_(1))和白云母-微斜长石伟晶岩(Ms_(2))中云母结构均一、化学成分变化小;白云母-钠长石-锂辉石伟晶岩(Ms_(3))中云母类型多样,除白云母外,还发育富锂多硅白云母、铁锂云母、锂云母,后者常交代白云母.Ms_(3)中Li、F含量突增,其中Li2O最高可达4.68%、F可达6.47%.二云母花岗岩(Tur_(1))和白云母-微斜长石伟晶岩(Tur_(2))中电气石为碱性黑电气石,白云母-钠长石-锂辉石伟晶岩中(Tur_(3))发育碱性锂电气石.相较于黑电气石,锂电气石具有富SiO_(2)、Al_(2)O_(3)、Li2O,贫TiO_(2)、MgO、CaO的特征.云母和电气石的成分和结构特征揭示,二云母花岗岩与白云母-微斜长石伟晶岩形成于岩浆阶段,未发生锂矿化.白云母-钠长石-锂辉石伟晶岩形成于岩浆-热液过渡期,锂元素发生了显著富集,发育锂辉石、富锂多硅白云母、铁锂云母、锂云母、锂电气石等.据此提出,岩浆-热液转换期对于锂成矿具有重要意义.在伟晶岩型锂矿床中,锂电气石与锂云母的存在表明伟晶岩具有极高的岩浆演化程度.

The Dahongliutan two-mica granite is considered as parental rock to the adjacent pegmatite-type lithium deposits such as Bailongshan.To constrain the magmatic-hydrothermal evolution of granite and associated pegmatites,we selected the penetrative minerals,mica and tourmaline,from two-mica granite and pegmatite with different degrees of mineralization,for backscattering observation(BSE)and electron probe microanalysis(EPMA).In the two-mica granite(Ms_(1))and muscovitemicrocline pegmatite(Ms_(2)),the muscovite is homogeneous,with limited chemical variations.In muscovite-albite-spodumene pegmatite,the muscovite(Ms_(3))incorporates much higher Li(Li_(2)O up to 4.68%)and F(up to 6.47%)in comparison to Ms_(1) and Ms_(2).There are additional Li-bearing phengite,zinnwaldite and lepidolite that have replaced muscovite.Tourmalines from twomica granite(Tur_(1))and muscovite-microcline pegmatite(Tur_(2))belong to alkaline schorl,while those from spodumene pegmatite(Tur_(3))are alkaline elbaite.Compared to alkaline schorl,alkaline elbaite is enriched in SiO_(2),Al_(2)O_(3),Li2O,but depleted in TiO_(2),MgO and CaO.The textural and compositional features of mica and tourmaline reveal that the two-mica granite and muscovitemicrocline pegmatite were formed by magmatic process,without lithium mineralization.By contrast,muscovite-albite-spodumene pegmatite was formed during the magmatic-hydrothermal transition,where lithium was significantly enriched,and numerous lithium-rich minerals such as spodumene,lithium-rich polysilicon muscovite,Li-bearing phengite,zinnwaldite,lepidolite and elbaite,were precipitated.Accordingly,we propose that the magmatic-hydrothermal transition is of great significance for lithium mineralization.In pegmatite-type deposits,the presence of elbaite and lepidolite indicates high degree of magmatic evolution.