In light of systematic microtexture and chemical composition studies of the dolomite minerals of the ore-bearing dolomites from the Bayan Obo deposit,it has been proposed that the ore-bearing dolomites were originally a carbonatitic sub-volcanics.Most orebearing fine-grained dolomites are magnesio-carbonatite and ferro-carbonatite;they generally have high FeO,MnO and SrO contents and contrast with the typical sedimentary limestone and dolostone with very low FeO,MnO and SrO contents.The carbonatite magma may rise slowly in crust to fractionate and produce a evolved dolomite carbonatite magma with high alkali,REE and Fe components and high volatile (F,Cl,P,S) contents,which were lost to the stratigraphically upper portion of the Bayan Obo Group and concentrated at the contact zone between the dolomite carbonatite of the footwall and the overlying Bayan Obo Group,thus producing extensive fenitizing aureole consisting broad zoned various ore types rich in Fe,Nb,REE,F and alkali metals in ore bodies.In terms of a clear distinction between the aegirine and Na-amphibole ores in their REE contents and distribution patterns,this indicates that the fluids for forming aegirine ore and Na-amphibole ore may have clear difference in XCO2 and halogen content.The formation of REE minerals is correlative to the oxidized fluids rich in CO2;the case may be responsible for the formation of ore with higher REE abundance like aegirine ore.If the fluids reducing in XCO2 and increasing in XH2O in falling temperature then the Na-amphibole ore may be formed and the case was unavailable for great enrichment of REE.Following the fenitization process the CO2/H2O ratios and oxygen fugacity and halogen content of the fluids may be changed,the case plays an important role in forming various fenitized ore types.Magnetites selected from the ore-bearing dolomite rock having oxygen isotope composition are similar with those of various ore types;this implies that the magnetites of various ore types have an origin consistent with that of the ore-bearing dolomite.The normalized REE patterns for ore-bearing dolomite and fenitizing ore types show strong LREE enrichment and strong HREE depletion indicating that LREE and HREE experienced extreme fractionation;however,there may be some large difference in their LREE contents for some fenitized ore types providing evidence for the greater mobility of LREE under fenitizing conditions.It is worth mentioning that the Main and East ore bodies are located the place where the ore-bearing dolomite is the thickest,the fenitization is the strongest and the Fe,Nb and REE resources are the richest.The case indicates that the fenitizing ore formation occurred at the place where a large amount of fluid escaped from the dolomite carbonatite magma was accumulated and the fluid metasomatism may be strong and widespread.%通过对组成赋矿白云岩的白云石矿物系统的显微结构和化学成分的研究表明,赋矿白云岩是碳酸岩质次火山岩,因此白云鄂博矿床是与火成碳酸岩有关的矿床.赋矿白云岩属于镁质碳酸岩(MgO＞ FeO+ MnO)和铁质碳酸岩(MgO＜ FeO+ MnO)系列,FeO、MnO和SrO含量高,这与FeO、MnO和SrO含量很低的沉积碳酸岩完全不同.赋矿白云岩的这一特点还表明它来自经历过分异结晶后的白云质碳酸岩浆而不是初始白云质碳酸岩浆.赋矿白云岩复杂的矿物组合表明,其母岩浆是富含F、Cl、P和S这些挥发分以及REE,Na、K和Fe这些元素的.在碳酸岩浆上升和侵位的过程中由于温度压力的降低,碳酸岩浆释放的含上述组分的流体会向上部地壳集中并对其接触的围岩进行交代,这就形成了矿体内呈不对称带状分布的霓长岩化矿石带.在大量的萤石和稀土氟碳酸盐矿物形成以后(其对应于萤石-稀土条带状矿石),流体的成分仍相对富钠和CO2,钠可以和围岩中的硅结合形成钠辉石,流体较高的CO2/H2O比值,有利于稀土的富集,此时形成的钠辉石型矿石的稀土含量是相当高的,仅次于条带状矿石.随着流体继续迁移和交代,流体中CO2浓度下降而H2O含量增加,温度也有所下降,但是流体中的钠依然活跃,所以出现了含有结构水的钠闪石,形成了钠闪石型矿石.流体中H2O的增加,CO2/H2O比值的下降,不利于稀土的大量富集,因此,钠闪石型矿石的稀土含量明显低于钠辉石型矿石.这说明,从碳酸岩浆中外逸的流体,在迁移交代围岩的过程中其成分、温度和氧逸度都是有变化的.从初期富CO2,温度和氧逸度较高,到后期富H2O,温度和氧逸度都有所降低.不同矿石类型的形成与这种变化有直接的关系.不同矿石类型中的磁铁矿的生成方式虽然不同于白云岩中的磁铁矿,但前者的氧同位素继承了后者的特点,表明了它们的亲缘关系和成因上的联系.稀土分布型式表明,不同矿石类型的稀土分布型式与赋矿白云岩的大体一致,有明显的LREE富集和明显的HREE亏损,且总稀土含量越高,轻重稀土分离程度越高.但是,不同矿石类型在轻稀土含量上有一定程度的差别,表明在霓长岩化过程中轻稀土活动性更强.总之,氟、钠和铁的交代作用在主东矿最强,稀土、铌和铁资源也最为富集,这里的白云岩厚度也最大,表明白云鄂博矿的霓长岩化成矿作用与赋矿白云岩衍生的流体的大量聚集以及流体对围岩广泛强烈的交代作用直接相关.
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