内蒙古白云鄂博铁-铌-稀土矿床矿化蚀变矿物组合及流体组成

Mineral Assemblages of the Mineralized Alteration and Ore-forming Fluid Composition of the Bayan Obo Fe-Nb-REE Deposit,Inner Mongolia

针对内蒙古白云鄂博铁-铌-稀土矿床是否多期成矿及底板白云岩的成因争论,本文通过光薄片岩矿鉴定和扫描电子显微镜研究,获得如下信息:(1)碳酸岩脉、磁铁矿石、蚀变岩及矿化白云岩中矿化蚀变的矿物组成类似,均以磁铁矿(碳酸岩脉中除外)、独居石、氟碳酸稀土矿物(氟碳铈矿等)、方解石、萤石、磷灰石、富钡矿物(重晶石或毒重石)、黑云母、富钠矿物(钠闪石、霓石)等为主,常见硫化物,显示为同一种富含Fe、REE、Ca、Ba、K、Na、Si及挥发分CO_(2)、P_(2)O_(5)、F、S的流体,岩(矿)石的结构构造也显示出明显的一期成矿特点,支持白云鄂博矿床主体是一期成矿事件的产物。(2)稀土矿化与底板白云岩不是同一流体的产物,稀土矿化与方解石为主的碳酸岩脉有关,有可能为火成碳酸岩或浆液过渡态流体;底板白云岩是稀土矿化的围岩,有更多的沉积成因信息,可能为热水沉积岩,或者为热液改造的沉积碳酸盐岩。(3)磁铁矿化与稀土矿化是同一成矿事件的产物,但碳酸岩中缺乏磁铁矿表明,铁质来源与稀土来源是否相同仍需要进一步研究。

In order to place new constraints to the multiple metallogenic stages and genesis of the footwall dolomite,we carried out detailed petrographic studies on the ore related samples using optical microscope and scanning electron microscope in this study.The following genetic results have been achieved.Firstly,the mineral assemblages associated with the mineralized alteration show similar patterns among carbonatite(dyke),magnetite ore,altered rock,and mineralized dolomite.This pattern is characterized by the presence of magnetite,monazite,fluorine-carbonate rare earth minerals(e.g.,bastnasite),calcite,fluorite,apatite,Ba-rich minerals(barite or witherite),biotite,Na-rich minerals(riebeckite and aegirine),and sulfide minerals.This suggests that these rocks were formed in association with a same fluid event that enriches in Fe,Ca,Ba,K,Na,Si,REE,and volatile components,such as CO_(2),P_(2)O_(5),F,and S.The textures of the rocks and ores also show distinct one-stage mineralization characterization,supporting that the Bayan Obo deposit is a product of one mineralization event.Secondly,the REE mineralization is not the product of the same fluid event as the footwall dolomite is.Instead,the REE mineralization is related to the fluid activity dominated by carbonate species,which belongs to the type of carbonatitic or magmatic-hydrothermal fluid.The footwall dolomite is REE-mineralized wall rock showing messages of sedimentary origin.The dolomite may represent hydrothermal sedimentary rocks or sedimentary carbonate rocks which have experienced hydrothermal modifications.Thirdly,we conclude that both the magnetite mineralization and the REE mineralization are the products of a same metallogenic event.However,the lack of magnetite in carbonatite raises the question of the relationship between the two sources of iron and REE.