白云鄂博矿床霓长岩类矿石成因

The fenite mainly consisting of aegirine, riebeckite and alkaline feldspar is caused by alkali metasomatism associated with the carbonatite.The question is that there distribute widespread the aegirine ore and riebeckite ore caused by alkali metasomatism in the Bayan Obo ore bodies , are they the fenites ? Can they be comparable with the fenites developed in the wall rocks of the carbonatite dykes at the area. In fact, the occurrence of the fenite ores of the ore bodies is related to the ore-bearing dolomite which emanated the fluid rich in alkalis, REE and volatile metasomatized the wall rocks(the Banyan Obo Group) forming the fenite ores rich in alkalis and REE. This study will carry out a detailed and combined investigation on fenites developed at the ore bodies and the wall rocks of carbonatite dykes including petrography, mineral chemistry and geochemistry .Special emphasis is given to discuss the factors constraining the generation of the fenite ores in the ore bodies, the elementary gains and losses, REE、 Nb and Fe transfer and enrichment during fenitization process. Thus, the generation of the fenite ores of the ore bodies was implicated in the origin of the Bayan Obo deposit.

霓长岩是碳酸岩对围岩碱交代作用的产物，其主要组成矿物为钠辉石，钠闪石和碱性长石等。白云鄂博矿体内广泛发育碱交代作用形成的矿石如钠辉石型和钠闪石型矿石，是否为碳酸岩对围岩碱交代作用的产物，是不是霓长岩？是否可与该区碳酸岩墙围岩中发育的霓长岩对比？本项申请拟通过矿体霓长岩类矿石的空间分布特征，说明其产出受控于赋矿白云岩。该白云岩释放出富碱，富挥发分和稀土等元素的流体对围岩（白云鄂博群）进行了交代，形成富碱和稀土等元素的霓长岩类矿石。通过对这些霓长岩类矿石的岩石学，矿物化学和地球化学的深入研究，并与碳酸岩墙相关霓长岩进行对比，阐明矿体霓长岩类矿石形成的控制因素，霓长岩化过程中元素的带出和带入，稀土、铌和铁等成矿元素的迁移和富集规律，从一个新的角度去诠释白云鄂博矿床成因。

通过四年的研究，对白云鄂博这一存在巨大争议的矿床，提出了主要为霓长岩化交代蚀变型矿床（注：还存在岩浆型和热液型矿床）。首次阐明了主东矿体内广泛分布的钠辉石和钠闪石型矿石，都是霓长岩化的产物。在这一过程中，下盘白云岩的母岩浆聚集了大量的挥发分，碱金属，铁，稀土和铌等，并往地壳的浅部迁移，最后聚集在H9板岩之下。在这一过程中，碳酸岩浆释放出的大量流体交代了板岩之下的白云鄂博群的富硅的围岩，产生了钠闪石和钠辉石等矿物，并沉淀了大量的稀土和铁。本研究解释了为什么钠辉石型矿石较之钠闪石型矿石通常含更高的稀土。并对矿体内广泛发育的氟交代，钠交代和铁交代都作了描述和解释。并将白云质碳酸岩中的磁铁矿的氧同位素，与钠辉石和钠闪石型矿石及其它矿石类型中的磁铁矿进行了对比，发现是一致的，说明，其它矿石中的铁的来源即来自白云质碳酸岩。总之，主东矿白云岩有最大的厚度，其所蕴含的流体及所携带的稀土铁等也是最多的，因此，在主东矿人们获得了最丰厚的稀土和铁等资源。与碳酸岩浆释放的流体的交代作用有关的矿床，通常会聚集在地壳的上部，超过深度为1700多米的钻孔中的白云岩其稀土含量明显低于现在采集到的样品的稀土含量。