内蒙古敖包吐萤石矿床的Sr、Nd、Pb同位素地球化学特征

敖包吐萤石矿床是内蒙古北部苏莫查干地区单一萤石矿集区中的一个代表性矿床,产于早二叠世大石寨组火山-沉积岩与早白垩世敖包吐花岗岩的接触带上。文章通过分析该矿床岩、矿石的微量元素和稀土元素,揭示出萤石的成矿作用可分为2个阶段,即交代作用和充填作用。交代作用过程中大石寨组的结晶灰岩可能为萤石的形成提供了部分Ca来源,萤石矿石的稀土元素配分模式与海水基本类似,具有Ce负异常;成矿作用后期主要表现为充填作用,形成颗粒粗大的萤石,表现为重稀土元素富集的特征,并随着萤石的沉淀析出,稀土元素总量逐渐下降,反映出成矿流体经历了较长期的演化过程。各地层单元、花岗岩体和萤石矿石的Sr、Nd、Pb同位素研究表明,萤石的放射性同位素组成具有壳、幔源混合的特点,成矿物质来源具有多源性。早白垩世敖包吐花岗岩可能是萤石中F的主要来源,而大石寨组的结晶灰岩则可能提供了Ca。另外,Pb、Nd同位素的极大不均一性,有可能是成矿流体在运移过程中对艾力格庙群放射性组分的选择性吸收的结果。萤石成矿作用与钾玄岩的时空关系暗示了萤石的成矿过程可能是中国东部岩石圈减薄和下地壳的置换地质事件的结果。在构造转型的过程中,燕山中期富碱的酸性花岗岩浆的活动分异出富含F的成矿流体,与幔源流体混合,沿区域重新活化的深大断裂和大石寨组的层间破碎带上升,交代其间的灰岩透镜体,从而形成敖包吐中型萤石矿床。

Sr, Nd and Pb isotopic geochemistry of fluorite from Obotu fluorite deposit, Inner Mongolia

The Obotu fluorite deposit is a representative deposit in the nomomineral fluorite ore concentration area of Sumochagan Obo within Inner Mongolia, lying in the contact zone between volcanic-sedimentary rocks of Lower Permian Dashizhai Formation and Early Cretaceous granites. Geochemistry of rare and trace earth elements of wall rocks and ores from the Obotu deposit shows that mineralization of fluorite can be divided into two stages, i.e., metasomatism and space-filling. Crystallized limestone of Dashizhai Formation might have offered Ca source for fluorites during metasomatism. The chondrite-normalized REE patterns of light purple layer-like fluorite ores at this ore-forming stage are similar to the REE patterns of the ocean, both with obvious Ce negative anomaly. The coarse-grained fluorite ores show heavy rare earth enrichment, and the total rare earth elements decrease gradually with the precipitation of fluorite from ore-forming fluids, suggesting that the ore-forming fluids have undergone a long period of evolution. Studies of Sr, Nd and Pb isotopes of various stratigraphic units, granite plutons and fluorite ores indicate that the metallogenic materials of fluorite were derived from multiple sources, and the isotopic composition of fluorite is characterized by crust-mantle mixed sources. Early Cretaceous Obotu granites probably provided F sources for fluorite, and crystallized limestone might have supplied Ca. He-terogeneous Nd and Pb isotopic compositions resulted probably from selective absorption of radioactive composition of Ailigemiao Group. The close time-space relationship between shoshonite in the northern part of Siziwang Banner and fluorite mineralization implies that the fluorite deposit might have resulted from the lithospheric thinning of eastern China and the replacement of the lower crust. In the course of structural transition, Early Cretaceous Obotu alkali-rich acid magmas gave birth to fluorine-rich fluids through differentiation, which, combined with mantle fluids, migrated upward along the deep-seated fault of the activated area and the interlayer fracture zone of Dashizhai Formation and replaced carbonate lenses, forming the supelarge Sumochagan Obo fluorite deposit and the medium-sized Obotu fluorite deposit as well as star-studded fluorite occurrences.