The Genesis Of The Bayan Obo Ree Deposit

As the most famous rare earth elements?REE?deposit in the world,the Bayan Obo ore deposit has drawn worldwide attention since it was discorved.Due to the origin of ore-hosting dolomites remains controversial,there is no conclusive theory on the origin of the Bayan Obo ore deposit,even to this day.So,this paper is going to focus on this problem again,as well as discuss the ore-forming process and the subsequent modifications.The covered dolomites not far away from the deposit,the dolomites from a carbonatite dike and the ore-hosting dolomites were chosen as objects of this study and relevant researches have been carried out on them.The coverd dolomites are the fisrt igneous carbonatite unit with considerable scale discovered near the deposit,as evidenced by bulk rock geochemistry,mineral chemistry and isotope geochemistry,which indicates that carbonatite occurrence in Bayan Obo,including but not limited to dikes.The coverd dolomites show intial(¹⁴³Nd/¹⁴⁴Nd)_i ratios similar to those of ore-hosting dolomites and carbonatite dikes tested by previous study,implying they probably have similar origin.SEM observation shows there are many LREE-beraing mineral assemblages distributing along the boundary of dolomite grains.This means that intense hydrothermal fluids overprinting happened after the formation of the dike.The metasomatism also caused the formation of LREE-riched apatite in the dike.It has been found that REE contents of carbonatite dike depend on the degree of post-magmatic hydrothermal metasomatism.This theory is also suitable for ore-hosting dolomites.Petrographic observations indicate that fine-grained dolomites were shaped after the fragmentation,graining and recrystallization of coarse-grained dolomites and this kind of course occurs before REE mineralization.In general,the fine-grained dolomites experienced more intense fluid modifications than coarse-grained dolomites.As a result,coarse-grained dolomites retained more original information.Hence,taking the apatite in coarse-grained dolomites as the object,the author explores the nature of the dolomites.BSE images show there is a great deal of mircro-monazite inclusions contained in them.These inclusions were formed via secondary fluid-induced“dissolution-reprecipitation”process.The apatite offered REE to these monazite inclusions.It can be deduced that the original apatite was rich in LREE and their REE patterns shall be similar as those of primary apatite in carbonatite dikes.In consideration of their high Sr and Ba contents,it can be concluded that the apatite has an igneous origin,and so are the host dolomites.Combining the analysis above with previous studies,the author thinks that carbonatite magmatism happened at around 1.3Ga in the Bayan Obo area.Some carbonatite magma quickly emplaced to shallow crust and formed dolomites,while part of them stored in deep magma chamber,keeping their evolution.The low degree of magma evolution led to a relatively weak REE mineralization in crystallized dolomites.Because of continuous tectonization,the dolomites crystallized partly become small graining and miliolite.At the same time,the alkaline hydrotherm fluids with LREE enrichment stored in deep magma chamber were able to rise along fault or weak structure,and therefore find a“vantage points”in dolomites to form REE minerals or reaction with dolomites,which leads to a huge enrichment of REE and fenitization in dolomites.Through“dissolution-reprecipitation”,Caledonian therm-fluids activity resulted in activation and re-enrichment of REE in the deposit,but the fluids provide negligible REE materials.