白云鄂博碳酸岩中矿物的次生反应边结构

在白云鄂博粗粒方解石碳酸岩中,发现了三种围绕硅酸盐矿物的次生反应边结构:1)在方解石中的石英被磷硅钙铈矿反应边环绕与方解石分隔开;2)在方解石中的碱性闪石被磷硅钙铈矿反应边环绕与方解石分隔开;3)方解石中的榍石被硅钛铈矿反应边环绕与方解石分隔开。这种情况表明,在后期阶段该区有富稀土的流体活动,该流体沿石英或硅酸盐矿物与方解石矿物颗粒接触边缘进行交代,生成了次生稀土矿物的反应边环带。这进一步表明,白云鄂博矿床稀土的富集至少与一期富稀土流体的交代作用有关。流体除含REE外,还含有铁和钠,该种流体的成分与以往有关白云鄂博矿床流体研究所得的成分结论是完全一致的。

Secondary reaction textures in the Bayan Obo carbonatite

In Bayan Obo coarse-grained calcite carbonatite there are three kinds of reaction texture: 1) the quartz within the calcite surrounded by the rim of britholite (±aegirine); 2) the alkaline amphibole within the calcite surrounded by the rim of britholite; 3) the sphene within the calcite surrounded by the rim of chevkinite. The total REE content of britholites is about 67% (average content of three samples), the rest constituents of which consist of silicon and calcium, if the silicon and calcium came from surrounded quartz (or sodium amphibole) and host calcite, respectively, the REE can be only derived from metasomatic fluid. Similar case is that the chevkinites have total REE content reaching to about 52% (average content of three samples), however, the sphene only contain about 3% total REE, therefore, if the silicon and titanium came from the surrounded sphene, the REE as well iron of the chevkinites can be only derived from the metasomatic fluid. The case suggests that in late stage there was some mobilisation of REE-rich fluid which metasomatized the original silicate minerals along their margin contacting with the calcites developing the reaction rim of secondary REE minerals. It has been further suggested that REE enrichment at Bayan Obo deposit is related to, at least, one period of REE-rich fluid metasomatism, in addition to the fluid may also has iron and sodium. The results based on the data are quite consistent with the conclusion of previous study on the fluid composition at the Bayan Obo deposit.