In this study. XRF and ICP-MS methods were used to analyze major and trace elements o{ dolomitite and surrounding slate combined with field investigation and indoor observation. Sulfur isotopic compositions were also conducted on the samples. The results of element analyses show that the dolomitic rock in the Bayan Obo Fe-REE-Nb formation exhibits geochemical characteristics of both igneous carbonate and sedimentary carbonate, indicating that the dolomite is neither typical igneous carbonatic rock nor sedimentary carbonatic rock. Sulfur isotopic analysis show that S isotopic compositions of whole rock is characterized by normal distribution model with two distinct peaks; one at around 0‰ indicating a deep-mantle source and the other around +8‰ higher than that of mantle-derived sulfur. The results suggest two possible origins: mantle-derived and sedimentary. Niobium in ore-bearing dolomite was enriched as REE mineralization took place, but Ta enrichment was weak. This suggests that ore-forming fluid was geochemically characterized by enrichment of REE and Nb and depletion of Ta. Our element and sulfur results demonstrate that the ore-bearing dolomitic rock was the product of metasomatism of sedimentary carbonate by mantle-derived carbonatic magma but not the product of direct eruption of igneous carbonatic magma. Deep mantle-derived carbonatic magma and its REE-enriched fluid migrated along regional deep fault and metasomatised with sedimentary carbonate, resulting in the formation of the unique, giant Fe-REE-Nb deposit in Bayan Obo.%通过野外地质观察和室内镜下研究,利用XRF和ICP-MS对白云岩及其周围的板岩的主、微量元素进行分析,同时对硫化矿化的样品开展硫同位素测试.元素测试结果显示白云鄂博Fe-REE-Nb建造白云岩既具有部分火成碳酸岩的地球化学特征,也具有部分沉积碳酸岩的地球化学特征,表明白云鄂博Fe-REE-Nb建造赋矿白云岩不是典型的火成碳酸岩或沉积碳酸盐岩.硫同位素的测试结果表明,全岩的硫同位素组成不呈塔式模型分布,出现两个比较明显的峰值,一个在0‰左右,具有深源特征；另一个在+8‰左右,明显高于幔源硫,这个结果说明其来源可能有两个:地幔和海水.赋矿白云岩中Nb随着稀土的富集也发生富集作用,但是Ta的富集作用却十分微弱,显示了成矿热液强烈富REE和Nb及贫Ta的元素地球化学特征.据元素和硫同位素结果,我们认为白云鄂博赋矿白云岩是沉积碳酸盐受地幔碳酸岩岩浆及派生的流体交代的产物,而非直接源于火山碳酸岩喷发成因.地幔深部碳酸岩岩浆及其派生的富稀土流体沿区域性深大断裂上涌与沉积碳酸盐岩进行交代作用,形成了白云鄂博独特巨大的Fe-REE-Nb矿床及区域性的稀土矿床.
展开▼
