摘要:东矿西缘的 A-A′剖面延伸约500 m，从下盘白云岩一直到上盘板岩为连续剖面。在白云岩与板岩接触处出现一个过渡带，其矿物组成和结构不同于白云岩和其上覆的板岩，为富含稀土，铁和萤石的条带状矿石，与主东矿的条带状矿石相似，其间还有富铁的薄的夹层。A-A′剖面的白云岩与主东矿下盘的白云岩是相似的，除其含有稍高的铁之外。推测，该剖面的白云岩来自于含大量碱金属，REE和铁的碳酸岩浆，并有高含量的挥发分。在碳酸岩浆侵位和结晶分异的过程中，这些组分会富集在残余岩浆之中，最终聚集在白云岩的顶部与板岩接触处，从而形成了该剖面过渡带中富稀土、铁、铌和萤石的条带状矿石以及富铁的薄的夹层。这些矿石类型分割开了白云岩和上覆的板岩。但是，该剖面未出现主东矿强烈而广泛的钠交代现象，即未形成钠质霓长岩。但是，该剖面的板岩中则钾钡交代广泛发育，形成了富钾和钡的板岩，可定义为钾质霓长岩。通过对A-A′剖面的系统研究及与主东矿的对比表明，白云鄂博地区的稀土、铁和微量元素的高度富集，主要出现在白云鄂博碳酸岩与围岩的接触处。但是形成一定规模的矿体还是需要具备一定的条件。首先要有来自深部规模宏大可聚集大量挥发分、稀土、铁和其它微量元素的碳酸岩（白云鄂博矿即为白云岩），白云质碳酸岩的母岩浆分异结晶的晚期阶段在其顶部与H9板岩的接触处即在H9的底部，由于H9板岩的屏蔽作用，其下聚集了更加富集萤石，稀土和铁等的残余碳酸岩熔体形成条带状矿石，并对所接触板岩继续进行广泛而强烈的钠质交代作用即钠质霓长岩化，在这一过程中仍然发育了强烈的稀土和铁的矿化，这样即形成了超大规模的白云鄂博矿。但处于白云鄂博赋矿白云岩边缘且位置较浅的碳酸岩，钠交代作用甚微，在白云岩与板岩的接触带中虽有稀土和铁的明显富集，但不能形成具一定规模的矿体。表明，稀土的强烈矿化与钠质交代作用密切相关，而一旦钾质交代作用替代了钠质交代作用，稀土矿化明显减弱。
关键词: 白云鄂博矿/
赋矿白云岩/
稀土元素地球化学
Abstract:The A-A′ profile located at the western margin of the East ore body. The profile is about 500 meters long passing through the dolomite of the footwall into the slate of the hanging wall, the slate is capping on the dolomite. There is a transitional zone between the dolomite and slate, in which the rocks are different from the dolomite and slate and mainly consist of banded ores containing REE minerals, magnetite and fluorite similar with the banded ores of the Main and East ore bodies. The dolomites of the profile are similar to the dolomites from the footwall of the ore bodies except the former has higher Fe content. The carbonatite magma from which the dolomite of A-A′ profile was derived may have a high alkaline, REE and Fe component and high volatile content, during the process of fractional crystallization of the carbonatite magma, these components were further enriched in the evolved residual magma and accumulated to the upper portion of the dolomites of the profile and producing the banded ore and massive thin iron layers rich in Fe, Nb, REE and F, which contact with the covering slates. However, in comparison with the Main and East ore bodies, the Na-alteration had been much weeker at the transitional zone of the profile, in which the aegirine ore and Na-amphibole ore did not occur. In terms of a clear distinction in mineralization scale between the ore bodies and the A-A′ profile, it has been suggested that the REE and Fe mineralization mainly occur at the contact zone between the dolomite and its wall rocks, however, when the dolomite shows a large scale and very rich in REE, Fe, Nb and volatile, the REE and Fe mineralization may reach a very large scale with strong Na-fenitization forming the Main and East ore bodies, however, if the dolomite occurs as small scale or at the margin of a large dolomite body, there would not occur a large scale of mineralization just like the case of A-A′ profile, in which the K and Ba metasomatism developed well in the slate and can be named potassium fenitization.
Key words:Baiyan Obo mine/
Ore-bearing dolomite/
Geochemistry of rare earth elements
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