白云鄂博碳酸岩脉的显微共聚焦激光拉曼光谱分析

LASER RAMAN SPECTROSCOPIC ANALYSIS OF BAYAN OBO CARBONATITE DYKES AND ITS PETROGENETIC SIGNIFICANCE

白云鄂博矿区发育的脉状稀土碳酸岩,由于其结晶迅速,矿物颗粒细微,其中的微小矿物的鉴定一直是一个难题。应用显微共聚焦激光拉曼光谱仪则能较好地解决这一问题。研究表明,白云鄂博地区存在富稀土白云质岩浆碳酸岩脉,早期阶段形成碱性长石和铁白云石,无稀土矿化;铁白云石常常出溶铁质而自身则形成方解石。霓石和方解石形成略晚,常常与氟碳铈矿等稀土矿物共生,出现强烈的稀土矿化;而无解理的方解石则形成于更晚的岩浆期后热液阶段,发育大量的流体包裹体,并出现强烈的铌、稀土矿化。铌铁矿分布在氟碳铈矿中和赤铁矿边缘,为热液交代作用的产物。早期结晶的矿物如碱性长石、铁白云石稀土矿化弱,岩浆晚期分异出大量的流体相,稀土元素和Sr等进入岩浆热液中,并在热液结晶矿物中富集,甚至在非平衡结晶的石英中产生强烈的稀土矿化。结合岩相学显微观察,显微拉曼探针很好的揭示了这一地质过程。同时为白云鄂博矿床铌、稀土矿化的热液交代成因提供了依据。

Laser Raman spectroscopy is an approach for mineral identification and composition determination based on the chemical bond and composition of determined objects. In the Bayan Obo district, several decades of carbonatite dykes were developed. The identification of mineral species for micro-sized grains is usually difficult because of their very small grain size formed in rapid crystallization unless the electron microprobe analyzer is used. Laser Raman spectroscopy is also a poweffial tool for this purpose. The investigation showed that there are REE-rich magmatic carbonatite dykes in the ore district. During the early magmatic crystallization stage, alkali feldspar and ankerite dolomite were formed with no REE mineralization. Ankerite dolomite is often converted to calcite due to the exsolution of ferric matter. Aegirine and calcite were formed later and are frequently associated with REE minerals such as bastnaesite. Calcite with no cleavage was formed in the post-magmatic hydrothermal stage and a large amount of fluid inclusions are found within it, associated with strong REE mineralization. Niobite is distributed within bastnaesite and also at the margin of hematite as a hydrothermal metasomatic product. REE mineralization was weak during the early magmatic stage when alkali feldspar and ankerite dolomite crystallized, tending to become strong in the late magmatic hydrothennal stage while a large amount of fluid phase was differentiated. REE and Sr entered into magmatic flmds to form strong REE mineralization. In combination with petrographic observations, Laser Raman spectroscopy can well reveal this geological process, and provides some evidence for the magmatic hydrothermal metasomatic origin of the Bayan Obo Nb-REE-Fe deposit.