白云鄂博火成碳酸岩墙的研究

CARBONATITE DYKES AT BAY AN OBO, INNER MONGOLIA,CHINA

对白云鄂博稀土矿床附近的富方解石岩墙进行了阴极射线发光分析、x 射线萤光分析和电子探针分析。结果表明,该岩墙为方解石质的火成碳酸岩。其矿物组成有含锶锰的方解石、镁钠铁闪石、磷灰石、烧绿石、钾长石和黑云母。岩墙两侧有强烈稀士矿化,磷灰石和环带独居石较为发育,并经受多次次生蚀变作用。岩墙一侧的混合岩经受霓长岩化作用,变为镁钠铁闪石-钠长石岩石。白云鄂博的沉积白云岩(H8)主要由含锰、锶、稀土的铁质白云石所组成,并含有磁铁矿、磷灰石、独居石、萤石和带云母。通过微量元素和同位素分析结果看出,火成碳酸岩岩墙和沉积白云岩相似。这些证据支持了 H8是白云岩化火成碳酸岩中的凝灰岩观点。白云鄂博稀土矿化也显示出与火成碳酸岩墙的地球化学属性的相似性,提供了它们在成因方面的相互关系。

Calcite-rich dykes occur in the thrust fold belt near the Bayan Obo rare earth element (REE) deposit. They are cream-coloured, 1-2 metres wide and cut an inlier of granite migmatite within folded Bayan Obo Group sediments. Compositional data from cathodoluminescence, X-ray fluorescence and microprobe studies show that the rock is a normal calcite carbonatite with Sr-Mn-bearing calcite, magnesio-riebeckite, apatite, pyrochlore, K-feldspar and biotite. One dyke was chosed for detailed analysis. Its margin is strongly REE-mineralized with much monazite developed adjacent to zoned apatite. Besides mineralization, there is much secondary alteration, marked by the introduction of Fe and Mn. The adjacent migmatite is fenitized to a magnesio-riebeckite-albite rock. The sedimentary dolomite H8 of the Bayan Obo Group is composed mainly of Mn-Sr-RE-bearing ferroan dolomite and contains bands of opaque grains, apatite, monazite, fluorite and taeniolite. Many trace element and isotope similarities between the carbonatite dyke and the sedimentary dolomite are revealed, and the evidence supports the possibility that H8 is a dolomitized carbonatite tuff. The Bayan Obo REE mineralization also shows geochemical similarities with the mineralization seen in the carbonatites, and a possible genetic connection is presented.