白云鄂博矿床稀土富集过程:来自成因矿物学的证据

REE enrichment process in the Bayan Obo Fe-REE-Nb deposit:Genetic and mineralogical evidence

稀土富集机制一直是白云鄂博矿床的重要研究内容之一,深刻理解稀土矿物的形成过程对于高质化开发利用稀土资源具有重要意义。白云鄂博主矿区矿石样品扫描电镜观察显示:最主要的两种稀土矿物氟碳铈矿和独居石常常呈细脉状产出,局部稀土细脉明显是由微细裂隙充填所致。需要特别关注的是,稀土矿物细脉外多见氟碳铈矿或独居石呈细小的浑圆状晶体被白云石包裹或赋存于白云石晶界处。同时,稀土细脉中也零星可见一些氟碳铈矿或独居石保留浑圆状的形态特点。如上这些现象暗示稀土物质最初是伴随白云质碳酸盐岩浆侵位而迁移到地壳浅部,并伴随岩浆固结成岩过程呈细小浑圆状晶体被白云石包裹或赋存于白云石晶界处,浑圆状似“珠滴”的晶体形态抑或是白云质碳酸盐岩浆固结成岩时发生了强烈的不混溶作用所致。此外,细脉状稀土矿物中局部可见富含Nb和Cr的独居石晶体,也可见一些独居石中包裹了早期自形富Si独居石,也说明氟碳铈矿或独居石的富集成矿具有岩浆属性。还需注意的是,在氟碳铈矿或独居石组成的稀土细脉中,除了少量晶体保留浑圆状形态外,大多呈半自形或它形晶体形态。同时,多处可见白云石晶体遭受溶蚀后被氟碳铈矿、独居石或萤石等包围的现象。特别是伴随氟碳铈矿或独居石细脉,有大量的萤石富集,局部还有较多的磷灰石结晶,其中一些磷灰石晶体中也包裹有细小浑圆状独居石。这些现象暗示早期伴随岩浆作用形成的氟碳铈矿、独居石和白云石,与后期富含F和P的流体发生了强烈相互作用。即氟碳铈矿或独居石细脉的形成,同时具有显著的热液作用属性。上述矿物学现象综合表明,白云鄂博稀土富集成矿至少经历了两次明显的成矿作用。首先是伴随碳酸盐岩浆侵位作用,稀土被运移到地壳浅部,并由于不混溶作用而得以初步富集;之后由于富含F和P的热液流体作用,使稀土物质发生了再活化和循环迁移,并得以再次富集形成稀土矿体。

The enrichment mechanism of rare earth element(REE) in the Bayan Obo deposit is of global interest, and it is of great importance to define the REE mineralization process for high-quality development and utilization of REE resource. According to scanning electron microscope(SEM) observation of samples from the main orebody, two main REE minerals, bastnaesite and monazite, occur commonly as veinlets formed by filling localized micro fissures of host minerals. Of note, small and globular-shaped bastnaesite and monazite are wrapped by dolomite or occur on the grain boundaries of dolomite outside the veinlets of REE-bearing minerals;occasionally they also appear within the veinlets. The above observations indicate that REE is initially hosted by dolomitic carbonate magma, then migrates to the shallow crust, and eventually assumes its occurrence state as solidification of carbonate magma occurs. The globular droplet-like crystals also might be attributed to strong liquid immiscibility in carbonate magma during solidification. Furthermore, Nb-Cr-rich monazite can be observed in the REE-bearing mineral veinlets, also seen is Si-rich monazite from the early stage wrapped around by Si-poor monazite, suggesting that the enrichment of bastnaesite and monazite is closely associated with magma at depth. In addition, most of bastnaesite and monazite are characterized by subhedral-anhedral crystals apart from a few globular ones in the veinlets. Dolomite commonly undergoes dissolution and is wrapped by bastnaesite, monazite and fluorite. Especially, abundant fluorite and apatite are enriched together with bastnaesite and monazite veinlets, and globular-shaped monazite wrapped by apatite can be observed. These features indicate that bastnaesite, monazite and dolomite crystalized from early-stage magmatism are strongly modified by late-stage F-P-rich hydrothermal fluids—that is, the formation of bastnaesite and monazite veinlets is also closely linked to hydrothermal fluids. The above mineralogical phenomenon shows that REE enrichment in the Bayan Obo deposit underwent metallogeny at least twice: REEs are firstly carried by carbonate magma and migrate to the shallow crust where magmatic immiscibility leads to the initial enrichment;then F-P-rich hydrothermal fluids cause re-mobilization and cyclic migration of REEs, resulting in further enrichment.