山东微山稀土矿矿床成因:来自云母Rb-Sr年龄、激光Nd同位素及流体包裹体的证据  

Genesis of the Weishan REE deposit, Shandong Province: Evidences from Rb-Sr isochron age,LA-MC-ICPMS Nd isotopic compositions and fluid inclusions

作　　者：蓝廷广[1] 范宏瑞[1] 胡芳芳[1] 杨奎锋[1] 王永[1] 

机构地区：[1]中国科学院地质与地球物理研究所矿产资源研究重点实验室,北京100029

出　　处：《地球化学》2011年第5期428-442,共15页Geochimica

基　　金：国家自然科学基金(40625010)

摘　　要：微山稀土矿位于华北克拉通东南缘的鲁西地块,为与碱性岩相关的伟晶岩型稀土矿床。根据单颗粒云母Rb-Sr定年,其形成于119.5 Ma,属于华北克拉通早白垩世大规模成矿事件的一部分。稀土矿物氟碳铈矿和独居石的激光Nd同位素研究表明,稀土矿的成矿物质与其围岩碱性岩一样,来源于富集地幔。对稀土矿脉中的石英、萤石和重晶石的流体包裹体研究显示,流体包裹体主要分为以下4类:(1)H2O包裹体(H类),(2)纯CO2包裹体(C类),(3)H2O+CO2包裹体(HC类)和(4)H2O+CO2+子矿物包裹体(HCD类),其中H类包裹体为次生包裹体,其余为原生包裹体,HCD包裹体中的子矿物为无水芒硝、重晶石、天青石、方解石、钾芒硝和钙芒硝等。HC和HCD类包裹体的完全均一温度变化于205～433℃之间,捕获压力在120～200 MPa之间,结合子矿物大量发育,成矿流体应为中压环境下的中高温、高浓度流体。这种流体富含CO 32?、SO 42?、F-阴离子以及Na+、K+、Ca2+、Sr2+和Ba2+等阳离子,并且其中的S同位素组成具有岩浆硫的特征,因此,其可能来自碱性岩浆的分异,通过岩浆-流体不混溶作用形成。C类、HC类和HCD类包裹体的密切共生以及不同充填度包裹体的均一温度的相似表明流体可能发生不混溶作用。稀土元素主要以[REEF]2+形式运移,运移过程中的流体不混溶作用可能是稀土矿物沉淀的决定因素。成矿晚期可能发生流体混合作用,但对稀土矿的沉淀贡献不大。Weishan REE deposit, a pegmatite-type REE deposit related to alkaline rocks, is located in Luxi Block,southeastern North China Craton. Muscovite Rb-Sr isochron yields an age of 119.5 Ma, which belonged to the Early Cretaceous large-scale mineralization in the North China Craton. LA-ICPMS Nd isotopic compositions of bastnaesite and monazite indicate that the source of the REE deposit is the enriched lithospheric mantle. Four types of fluid inclusions have been identified in quartz, fluorite and barite, including （1） H2O inclusions （H）, （2） pure CO2 inclusions （C）, （3） H2O＋CO2 inclusions （HC） and （4） H2O＋CO2＋daughter mineral inclusions （HCD）.The H inclusions are secondary while others are primary. The daughter minerals in HCD inclusions include thenardite, barite, celestine, calcite, aphthitalite and glauberite. The homogenization and capture pressures of HC and HCD inclusions range from 205-433 ℃ and 120-200 MPa, respectively. Coupled with the existence of abundant daughter minerals and S stable isotopic compositions, it can be deduced that the initial ore-forming fluids were high-temperature, moderate-pressure and high-concentration orthomagmatic fluids, which were characterized by enrichment nt of HCO^-3 /CO^2-3,SO^2-4 and F– and multi-component （e.g., Na＋, K＋, Ca＋, Ba2＋, Sr2＋ and REE3＋）. The coexistence of C, HC and HCD inclusions and the wide range of liquid/vapor ratios between these inclusions suggest that fluid unmixing may have occurred during ore-forming process. REE were most probably transported as [REEF]2＋ and precipitated through fluid unmixing. Fluids mixing, which contributed little to the REE precipitation, also happened in the late stage of the ore-forming process.

关 键 词：RB-SR等时线年龄 激光Nd同位素 流体包裹体 不混溶作用 微山稀土矿 华北克拉通 

分 类 号：P597[天文地球—地球化学] P611[天文地球—地质学]