白云鄂博“白云岩”地质地球化学特征及成因讨论

Geological and Geochemical Characteristics and Genesis of “Dolomite” at Bayan Obo,China

白云鄂博"白云岩"位于华北板块的北缘,宽沟背斜的南翼。蕴藏着世界最大的稀土矿床,还是大型—超大型铌、铁和钍等矿床赋存母岩。"白云岩"不是层状岩石,无明显的层理和固定的层位,而是一套大小不等,串珠状的"白云岩"带。"白云岩"与围岩呈明显的侵入关系。表现在"白云岩"切割H4石英砂岩、H5板岩和花岗岩脉,并有许多分枝脉插入到H4石英砂岩和H5板岩中。"白云岩"中存在H4石英砂岩的残留顶盖相,捕获了H4石英砂岩和H5板岩的捕掳体,并引起围岩的强烈蚀变。板岩的黑云化,石英砂岩的钠闪石化,花岗岩的碱交代。"白云岩"含有大量铌、稀土、钍等岩浆岩中常见的矿物晶体。硫、碳、氧、锶和铁等同位素组成都具有深源特征。说明白云鄂博"白云岩"不是沉积岩,而是岩浆碳酸岩。

Bayan Obo ＂dolomite＂ is distributed mainly at the south wing of the Kuangou anticline in the northern margin of the North China Craton.It is not only host to world＇s largest rare earth deposit but also parental rock for the large or very large niobium,iron,scandium and other minerals.The ＂dolomite＂ is not layered rock due to lacking no obvious bedding and fixed sequence,but a beded dolomite belt with varying sizes.The ＂dolomite＂ has the obvious relationship of invasion with surrounding rock,which is reflected by the ＂dolomite＂ cutting through quartz sandstone（H4）,slate（H5）,and granite,and by many dolomite veins intruding the quartz sandstone（H4） and slate（H5）.Residual cover phase of quartz sandstone（H4） occurs in the ＂dolomite＂.Meanwhile xenoliths of quartz sandstone and slate were also found in the dolomite,along with strong alteration of country rocks.Biotitization of slate,riebeckitization of quartz sandstone,and alkaline metasomatism are very common in the mining area.The ＂dolomite＂ contains a large amount of mineral crystals such as niobium,rare earths,thorium,etc.,which are common in igneous rocks.The sulfur,carbon,oxygen,strontium and iron isotope composition have deep-source characteristics.All the features indicate that the dolomite is magmatic carbonate but not sedimentary carbonate.