中国不同类型铍矿床中绿柱石流体包裹体特征研究

Characteristics of Fluid Inclusions in Beryl from Different Beryllium Deposits in China

采用偏光显微镜薄片观察、激光拉曼成分分析等方法,对伟晶岩型和岩浆热液型铍矿床中绿柱石的流体包裹体进行了岩相学观察和成分分析。结果表明,绿柱石中原生流体包裹体形态多样,常孤立或成群沿晶体生长带定向分布,大小从5~80μm不等。流体包裹体类型以富液相型气液两相包裹体最常见,其次为含液相CO_(2)的三相包裹体和含子矿物多相包裹体,偶见固体包裹体和熔融包裹体。其中,伟晶岩型绿柱石中包裹体数量和种类更为丰富,常见含子晶多相包裹体和气液包裹体共存,岩浆热液型绿柱石中包裹体则相对较少,可见熔融包裹体与富液相CO_(2)流体包裹体共存。流体包裹体气相成分主要以CO_(2)和N_(2)为主,液相主要为H2O和CO_(2)以及CO_(3)^(2-)、HCO_(3)^(-)等离子。伟晶岩型绿柱石中常见含石英、云母、钠长石等子矿物的多相包裹体,由伟晶岩中晶体快速结晶形成;岩浆热液型绿柱石中的有机质气体更丰富,与氧化剂Al_(2)O_(3)活度较低而形成相对还原环境有关。富含CO_(2)、H_(2)O成分的流体更有利于绿柱石的形成。结合流体包裹体的生成机制,认为岩浆热液型绿柱石形成于岩浆演化晚期的热液阶段,伟晶岩型绿柱石形成于岩浆-热液过渡→热液阶段,绿柱石的形成机制为岩浆的结晶分异和液态不混溶作用。

The fluid inclusions in beryl in pegmatitic and magmatic hydrothermal beryllium deposits were studied using polarizing microscope observations of thin sections and laser Raman composition analysis.The results showed that the primary fluid inclusions in beryl had various forms.Beryl was usually distributed in isolation or in groups along the crystal growth zone,ranging in size from 5μm to 80μm.Liquid-rich gas–liquid two-phase inclusions were the most common types of fluid inclusions,followed by liquid-phase CO2-containing three-phase inclusions and multiphase inclusions containing daughter minerals.Occasionally,solid inclusions and melt inclusions were also observed.Pegmatitic beryl had substantial quantity and variety of inclusions,and it was common to have multiphase inclusions with carrier crystals and gas-liquid inclusions.However,there were relatively few inclusions in magmatic hydrothermal beryl,and melt inclusions coexisted with liquid-rich CO_(2) fluid inclusions.The gaseous components of the fluid inclusions were primarily CO_(2) and N_(2),and the liquid phase was primarily H_(2)O,CO_(2),CO_(3)^(2-),HCO_(3)^(-),and other ions.Pegmatitic beryl often contained multiphase inclusions containing quartz,mica,albita,and other daughter minerals,which are formed by the rapid crystallization of pegmatitic crystals.Organic gas,which is related to the relative reduction environment formed by the low activity of the oxidant Al2O3,was more abundant in magmatic hydrothermal beryl.Fluids rich in CO_(2) and H_(2)O were more conducive to the formation of beryl.Combined with the formation mechanism of fluid inclusions,it is suggested that magmatic hydrothermal beryl was formed in the hydrothermal stage of late magmatic evolution.Pegmatitic beryl was formed during the transition from the magmatic-hydrothermal stage to the hydrothermal stage.The formation mechanism of beryl is crystal differentiation and liquid immiscibility of magma.