白云鄂博超大型稀土—铌—铁矿床的成矿时代及成因探析——兼论P—T之交生物群灭绝事件和“阿蒙兴造山运动”

Analysis on the Oreforming Time and Genesis of the Bayan Obo REE—Nb—Fe Deposit:With a Discussion on the Mass Extinction at the P—T Boundary and “AMH Orogeny”

白云鄂博矿床是世界上最大的稀土矿床,同时也是一个特大型铌矿和大型铁矿,其成矿时代及成因至今仍有多种不同认识。本研究从东部接触带(菠萝头山)3号铌矿体内的金云母岩中获得残留锆石和热液独居石,测得其U-Pb年龄分别为269.5±3.1 Ma和249±13 Ma;从巴音敖包伟晶岩中获得热液锆石,测得其U-Pb年龄为248.9±2.5 Ma。结合阿尔泰—天山—北山—内蒙古—大兴安岭—小兴安岭造山带中的成矿年龄资料,笔者推测,在248%251 Ma白云鄂博地区发生了一次强烈的热液活动,该热液活动的时间可以代表白云鄂博矿床的成矿年龄。基于绝大多数稀有金属热液矿床都是与花岗质岩浆活动关联的,本研究对白云鄂博地区出露的两类花岗岩进行了岩相学、矿物学、地球化学和锆石学分析,从而确定白云鄂博花岗岩基中的黑云母二长花岗岩(主体相)为同碰撞花岗岩,其定位年龄为269.8±2.0 Ma;而白云鄂博花岗岩基中的二云母碱长花岗岩(补体相)为碰撞后花岗岩,其定位年龄为250.5±6.0 Ma。根据二云母碱长花岗岩的成岩年龄等于白云鄂博矿床的成矿年龄,以及大量的野外地质现象和区域地质资料,笔者认为:(1)该二云母碱长花岗岩为白云鄂博矿床的成矿母岩,它的岩浆直接来自地壳深部岩浆房;(2)该岩浆房就是同碰撞花岗岩浆的岩浆房,这意味着留存在该岩浆房中的巨量花岗岩浆经历了近20 Ma的分离结晶作用,从而在岩浆房上部聚集了富含成矿物质的残余花岗岩浆;(3)当构造环境由挤压转为拉张时,该残余花岗岩浆沿着张性断裂被动侵位。由于快速上升引起压力和温度的骤降,富含稀有金属(稀土和铌)、卤素(氟)和碱金属的硅质热液从残余花岗岩浆中分离出来;(4)这种硅质热液沿断裂构造率先进入白云鄂博群H_8白云岩岩层,与碳酸盐发生交代反应,其稀土和铌金属元素沉淀成矿;同时,H_8白云岩岩层中的菱铁矿和铁白云石分解,释放出Fe^(2+)和[CO_3](2-),前者(Fe^(2+))经近距离迁移后沉淀成铁矿,后者([CO_3]^(2-))与少量稀土—铌元素结合成金属—碳酸络合物,呈脉状穿插在H_8白云岩中,或迁移至H_8白云岩的外围。该认识首次将白云鄂博地区的构造、成岩和成矿有机地统一起来,从而阐释了一个致白云鄂博矿床形成的能量和物质的运移过程;同时,该认识可以整合多种流行的白云鄂博矿床成因认识("正常或热水沉积说"、"火成碳酸岩说"、"热液交代说"等等)中的合理因素;笔者认为,它是一个全面、系统而又新颖的白云鄂博矿床的成矿模式。

The Bayan Obo deposit is the largest REE deposit in the world, and is also a giant Nb and Fe deposit,its metallogenic mechanism is very sophisticated.This research has firstly collected phlogopitite from the #3 Nb-ore body in eastern contact zone（Boluotoushan） , and obtained 269.5±3.1 Ma and 249±13 Ma U-Pb ages for its relict zircon and hydrothermal monazite respectively;then, we have collected the Bayin Obo pegmatite, and obtained 248.9±2.5 Ma U-Pb age for its hydrothermal zircon. Combining with the reported mineralization ages in the AMH orogenic belt （i. e. Altay Mts.--Beishan Mts.--Inner Mongolia--Greater Hinggan Mts.--Lesser Hinggan Mts. orogenic belt） , this research deduces that a strong hydrothermal activity happened at 248±251 Ma in the Bayan Obo area, the time of this hydrothermal activity may represent the mineralization age of the Bayan Obo deposit. On the basis of close relationship between granitic magmatism and hydrothermal ore deposit, we has carried out petrographic, mineralogical, geochemical and geochronological studies on two types of granites （biotite monzogranite and two-mica alkali-feldspar granite）occurred in the Bayan Obo area, and determined that the biotite monzogranite （main phase of the Bayan Obo granitic batholith）has syn-collisional nature with its emplacement age of 269.8±2.0 Ma, and the two-mica alkali-feldspar granite （ subsequent phase of the Bayan Obo granitic batholith ） has post- collisional nature with its emplacement age of 250.5± 6.0 Ma. According to the similarity in age between the two- mica alkali-feldspar granite and the Bayan Obo deposit with the observation in the field, this research concludes：（1） The two-mica alkali-feldspar granite is parental rock for the Bayan Obo deposit, its magma came from the deep- seated magma chamber in the lower--middle crust;（2） The magma chamber is the same for the syn-collisional granitic magma, meaning that the huge amount of magma in the magma chamber went through ca.20 Ma of fractional crystallization,thus the ore-forming material-enriched residual magma occurred at the top of the magma chamber;（3） When the tectonic regime converted from the compressional to the extensional states, the residual magma could ascend along extensional fault in the permissive mechanism. A large amount of rare metal （ REE and Nb） -- halogen （F） -- alkali metal-enriched siliceous fluid was separated from the residual magma with intensely decreasing pressure and temperature ;（4） Such siliceous fluid entered the dolomite Hsalong the fracture, and REE and Nb ores formed after its metasomatism with carbonates.At the same time ,the siderite and ferrous dolomite in the dolomite H8 decomposed, and released Fez＋ and [ CO3 ] 2-, the former（ Fe2＋） precipitated as Fe ore after a short migration, and the latter（ [ CO3 ] 2-）combined a small quantity of REE and Nb into metal--carbonate complex,which precipitated as REE and Nb ores in the form of veins in the dolomite H8 or in the rocks outside the dolomite H8. This metallogenic model firstly combines tectonic movement, granitic magmatism and mineralization in the Bayan Obo area,then determines a migration process of energy and material up to the formation of the Bayan Obo deposit; Moreover,this metallogenic model takes account of the profitable factors from the main genetic hypotheses （e. g., ＂general or hot-water sediment ＂, ＂ carbonatite intrusion ＂ and ＂ hydrothermal metasomatism ＂, etc. ） on the metallogenic mechanism of the Bayan Obo deposit;Therefore, it is a comprehensive and distinctive metallogenic model for the Bayan Obo deposit.