【作者】 赖小东；

【导师】 常印佛； 杨晓勇；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2013， 博士

【摘要】 白云鄂博超大型稀土-铌-铁矿床的稀土储量居世界第一,铌储量居世界第二,其又是一个大型的铁矿床。该矿床的成因和成矿年龄受到中、外地质学家的关注,也是至今仍在争论的问题,其中两个主要问题是,成矿年龄和赋矿H8白云岩的成因和年龄。鉴于上述存在问题,本论文在全面总结前人的研究成果的基础上,对白云鄂博地区的一些基本问题进行了详细的研究,试图通过解决本地区的几个基本问题,进而讨论矿床的成因及年龄。通过详细的地层剖面对比,我们发现宽沟南北及白云鄂博和黑脑包两个地方的地层存在对比性。白云鄂博东南的黑脑包基底角闪片麻岩的时代为2.5Ga左右(2522±18Ma和2495±11Ma),形成于新太古代晚期。这和前人得到的白云鄂博基底中的糜棱岩化花岗片麻岩的形成年龄(2588±15Ma)和碳酸岩墙中捕获锆石的年龄(2.55Ga)相近。对基底的研究结果说明,在华北北缘白云鄂博地区除具有约2.0Ga的岩浆活动和约1.9Ga的强烈变质事件外,还存在有华北克拉通太古代古老岩石(2522±18Ma),从而证明白云鄂博矿床发育在华北克拉通基底之上。黑脑包处的腮林忽洞群不整合于基底片麻岩(毛忽洞组)之上,白云鄂博群不整合于基底杂岩(五台群?)之上,两者相似的基底年龄限制了腮林忽洞群和白云鄂博群具有相同的下限年龄,从而为两个群组的具有可比性提供了一定程度上的间接证明。不过,白云鄂博基底属于过铝质钙碱性系列岩石,其原岩为太古代沉积岩；而黑脑包基底属于准铝质高钾钙碱性系列岩石,其原岩可能是富钾的碱性火山岩。构造环境判别结果表明其构造环境可能是岛弧,说明在此时期内白云鄂博(华北北缘)属于岛弧环境,这些基底片麻岩的形成与岛弧作用引起的岩浆活动有关,不排除是造山后的结果。详细的年龄讨论以及碳酸岩墙中锆石的研究,表明白云鄂博地区的碳酸岩墙应属于同一期形成,形成年龄为1.30Ga～1.40Ga左右。其形成是中元古代(1.20～1.70Ga)华北克拉通从哥伦比亚超大陆分离而处于拉张环境形成大陆裂谷系时,碳酸岩浆的侵入形成。碳酸岩墙中的锆石年龄有两组,分别为2507±35Ma和1904±19Ma,这两组年龄和前人获得的基底的年龄1.9～2.0Ga和～2.5Ga相一致,说明碳酸岩墙中的锆石确实主要来自基底,是碳酸岩浆侵入过程中从基底捕获的。锆石中εHf(t)大部分为负值,分布于-20.4～1.7之间,单阶段模式年龄(tDM1)分布于2259Ma-3207Ma之间,平均为2572±120Ma,和锆石年龄一致。说明白云鄂博地区基底的形成与华北克拉通化过程密切,是亏损地幔的分异作用导致的本区基底的形成,这间接支持华北克拉通新太古代晚期包括BIF在内的表壳岩系可能形成于岛弧构造环境。同时,白云鄂博地区碳酸岩墙的地球化学研究表明,这些碳酸岩墙具有明显的火成碳酸岩特征,具有极高的稀土含量。而且,单矿物研究表明,白云石具有比方解石更强的容纳稀土元素的能力,碳酸岩中的稀土主要还是以副矿物的形式存在。白云鄂博矿床的成因和这些碳酸岩墙有密切的关系是毫无疑问的,碳酸岩浆及其流体具有强的成矿潜力。白云鄂博群年龄测试结果表明,宽沟北沉积灰岩的Pb-Pb年龄为1581±100Ma,其东南部黑脑包沉积白云岩的Pb-Pb年龄为1425±420Ma。黑脑包地区获得Pb-Pb年龄在误差范围内跟白云鄂博群H8沉积灰岩是一致的,同样说明了这两个群组之间存在可比性。黑脑包较小的年龄值说明腮林忽洞群顶部的微晶丘可能更相似于白云鄂博群的上部,而不是H8白云岩。H9板岩中的锆石分析结果表明其沉积年龄为1.65Ga左右,其中也出现有加里东和海西期年龄,这是本地区存在加里东期和海西期岩浆活动的证据。同位素年代学的结果支持白云鄂博群的形成时代为中元古代。白云鄂博矿区黑云母岩中的黑云母的Ar-Ar年龄为289.1±1.8Ma,说明黑云母为海西期岩浆活动作用的结果,而海西期岩浆活动已被公认跟白云鄂博的成矿没有关系,因此,黑云母岩的存在不能成为H8是碳酸岩的证据。采自条带状矿石中钠闪石脉中的钠闪石的Ar-Ar年龄为389.5+3.0Ma,这个年龄代表的是改造事件的晚期年龄。Sm-Nd年龄研究结果表明在白云鄂博矿区,稀土的主要成矿时代是1.200a左右,稍晚于碳酸岩的年龄,而且,成矿可能只有一次。虽然不少稀土矿物的Th-Pb年龄显示有加里东期的成矿作用,但同样矿物的Pb-Pb年龄却相似于Sm-Nd年龄,说明其Th-Pb体系在加里东时期受到过扰动,得到的年龄更像是一次改造事件,而不是成矿事件。年龄的讨论说明,在白云鄂博矿区,稀土的主要成矿时代是1.20Ga左右,晚于碳酸岩的年龄,而且,成矿作用可能只有一次,不存在加里东期成矿作用。H8白云岩全岩地球化学研究表明,粗粒白云岩,尤其是那些稀土含量较低的,具有和沉积碳酸盐岩相似的特征,而细粒白云岩则跟火成碳酸岩的特征相似,利用某些微量元素能够很好的将沉积碳酸盐岩和碳酸岩区分开。单矿物地球化学研究表明,虽然全岩的成分上看,粗粒白云岩和细粒白云岩具有较不相同的特征,表现为细粒白云岩的特征更相似于碳酸岩,粗粒白云岩则更相似于沉积碳酸盐岩,但他们的白云石单矿物的特征是相似的,完全不同于碳酸岩。H8白云岩(包括细粒白云岩)中的白云石中的稀土元素含量较低,总稀土含量最高的只有146ppm,远远低于碳酸岩中方解石或者白云石的稀土含量：并且,在粗粒白云岩中白云石的核部保留有沉积碳酸盐岩的信息,这些都表明白云岩是沉积交代成因的。全岩和黄铁矿的硫同位素组成不呈塔式模型分布,出现两个比较明显的峰值,一个在0‰左右,平均值为+0.02‰,具有深源特征：另一个在+8‰左右,平均值为+6.88‰,明显高于幔源硫。重晶石具有明显的比全岩高的834S值,平均值为+12.50‰。通过大本模式计算得到的δ34S∑S值分别为6.1‰和10.1‰。计算得到的值都具有幔源硫交代或混合地壳或海水的特点。我们认为,全岩及硫化物的硫有两个来源：地幔和海水,因混合得不均匀,形成双峰。重晶石硫主要是海水硫,但有幔源硫的加入。H8白云岩中碳氧同位素范围落在岩浆碳酸岩到元古宙海相碳酸盐岩之间,可解释为沉积碳酸盐岩受到地幔流体的交代。富钾板岩若为火山岩或变质火山岩,则其中应该存在1.6Ga左右的岩浆锆石,但出现典型岩浆环带的锆石被证实形成于海西期,且不存在16亿年的锆石,结合白云鄂博群形成环境属浅海相(郭光裕等,1982；中国科学院地球化学研究所,1988),因此,富钾板岩是火山岩或火山变质岩的可能性不大。同时,地球化学研究结果也表明H9富钾板岩的形成以正常沉积为主,后来受到碳酸岩浆或(和)流体的交代作用。综合以上研究,我们认为白云鄂博赋矿白云岩是沉积碳酸盐受地幔碳酸岩浆及派生的流体交代的产物,其中粗粒白云岩是弱矿化的结果而细粒白云岩则是强烈矿化的结果。中元古代约1300～1400Ma时,白云鄂博地区处于拉张和沉降环境,白云鄂博裂谷系活化,地幔物质上涌,伴随火成碳酸岩浆活动,上涌的稀土含量极高的碳酸岩浆及其流体交代白云鄂博群H8白云岩形成稀土矿床。加里东期时,华北地块向西伯利亚板块俯冲并随后发生碰撞拼合,使白云鄂博地区褶皱隆起,发生花岗岩岩浆活动,此变形和岩浆活动造成白云鄂博矿床强烈扰动,形成晚期的黄铁矿、辉钼矿和钠闪石,并使部分同位素体系重置,显示加里东期的表面年龄。更多还原

【Abstract】 The Bayan Obo REE-Nb-Fe deposit is the largest LREE deposit and the second largest Nb deposit in the world, and it is also a large Fe deposit. The genesis and forming age of the deposit have been debated for decades by geologists both of China and worldwide. The two most important problems are the mineralization age and how and when the H8dolomite was formed.In order to clarify the above questions, on the basis of a comprehensive summarizing and reviewing of existing results, a lot of works have been done. We try to solve some basic problems in the Bayan Obo area, and then analyze the REE mineralization age and its genesis.Through detailed strata research, it is found that strata in north and south Kuangou fault are the same, and the Bayan Obo Group is correlated with Sailinhudong Group in the Heinaobao, southeast Bayan Obo deposit. Zircons from basement rocks in the Heinaobao yield an age of～2.5Ga (two LA-ICP-MS results are2522±18Ma and2495±11Ma, respectively), they are formed at late New Archean, this age is similar with the age of basement rocks and zircons from carbonatite in the Bayan Obo. The chronology studies of basement rocks show that there is not only magmatic activity and intense metamorphic thermal event during early Proterozoic(1.9～2.0Ga), Archean rocks (2522±18Ma) of North China craton are normal in this area, the Bayan Obo deposit is indeed developed upon the North China Craton basement. The same basement age is indirect evidence for that stratum develop upon them (thus the Bayan Obo Group and Sailinhudong Group) are comparable.Basement in the Bayan Obo are peraluminous cal-alkaline series rocks, and their protolith is Archaean sedimentary rocks; while basement in the Heinaobao are aluminous high-K calc-alkaline series, whose protolith is probably K-rich alkaline volcanic rock. Tectonic study indicated an island arc environment, the formation of these basement rocks is related to the island arc related magmatism.Carbonaities in the Bayan Obo are formed in the age of1.3～1.4Ga, and there is no other carbonatite magmatism in this area in the long geologic period. In the Mesoproterozoic(1.20～1.70Ga), North China Craton broke away from Columbia supercontinent, carbonaitie magma intruded into basement and Bayan Obo Group in rift environment. Zircons from carbonatite gain two ages of2507±35Ma and1904±19Ma, respectively. Both ages are consistent with the age of basement rocks. Furthermore, inclusions in these zircons are silicate, indicating that zircons in the carbonatite are entrapped from the basement during carbonatitie magma intrusive process. Most zircons in the carbonatite have variableεHf(t) values of-20.4to1.7, with Paleoproterozoic to Neoarchean Hf model ages of2259Ma to3207Ma (with an average of2572±120Ma). The Hf model age is the same with the zircon age, indicating the formation of basement is closely related to cratonization of North China craton, indirectly supported that supracrustal sequences in the late New Archean of North China craton (including BIF) are in an island arc environment.Geochemical studies of carbonatite dykes in the Bayan Obo show obviously igneous carbonatite features, with extremely high REE contents. Moreover, mineral analysis indicate that dolomite has a stronger ability to hold rare earth elements than calcite, and rare earth elements in the carbonatite exist in the form of accessory minerals. Carbonatite magma and fluids have high ore-forming potential, there is no doubt that the Bayan Obo REE-Nb-Fe deposit is related to carbonatite.The207Pb-206Pb isochron age (158±100Ma) of the sedimentary limestone of the Bayan Obo Group in the north Kuangou fault may represent its deposition time, indicating the Bayan Obo Group was deposited in Mesoproterozoic. The207Pb-206Pb isochron age of sedimentary dolomite of the Sailinhudong Group in the Heinaobao is1425±420Ma, though it is a little younger than the age of sedimentary limestone in the Bayan Obo, similar age indicate that Bayan Obo Group is comparable with Sailinhudong Group, and comparative studies between H8dolomite and sedimentary carbonate rocks in the Heinaobao is reasonable. Zircon SIMS U-Pb age dating of K-rich slate show that they was form at1.65Ga, one Group of age concentrates in the Late Archean-Early Paleoproterozoic (2519.9Ma), consisting with the zircon ages of basement rocks in the Bayan Obo region, and the age of518.8Ma represent the Caledonian magmatic event. Chronology studies support that the Bayan Obo Group was formed in Mesoproterozoic.The Ar-Ar age of biotite from biotitite from Bayan Obo is289.1±1.8Ma, indicating it is the result of Hercynian magmatism, since Hercynian magmatism is generally accepted to have nothing to do with REE mineralization, the exist of biotitite should not be the evidence of H8dolomite is carbonatite. Riebeckite from riebeckite vein of banded type ore has Ar-Ar age of389.5±3.0Ma, represented the age of reworking process rather than second mineralization age. Sm-Nd isochron ages of H8dolomite in the Bayan Obo are concentrated in-1.20Ga, younger than the age of carbonatite, as Bayan Obo is an REE deposit, it is clear that the Sm-Nd age stand for mineralization age. Though many REE minerals have Th-Pb isochron ages of Caledonian, their Pb-Pb ages are similar to the Sm-Nd isochron age. This shows that Th-Pb system was reset in Caledonian, thus these ages are more likely the age of reworking process rather than another mineralization. Besides, the mineralization process has only occur once, about1.2Ga, after carbonatite intrusion.The whole rock geochemical studies of H8dolomite show that coarse-grained dolomite especially those with low rare earth content, have similar characteristics to sedimentary carbonate rocks, fine-grained dolomite have nearly the same features as igneous carbonatite, and using some trace elements can easily distinguish sedimentary carbonate rock and carbonatite. Though the whole rock geochemical features between coarse and fine grained dolomite are different, their carbonate minerals (dolomite) have similar geochemical characteristics, which are completely different from the carbonatite. Rare earth element content of dolomite mineral in the H8dolomite (both coarse and fine-grained dolomite) is low, with the highest total rare earth content is146ppm, much lower than the calcite or dolomite in carbonatite. Moreover, dolomite from coarse-grained dolomite retain signatures of sedimentary carbonates in the core, indicating that dolomite is the result of mineralization of sedimentary carbonate rocks by REE-rich fluids derived from carbonatite magma. Moreover, SrO content in carbonate minerals is a more sensitive indicator than MnO to distinguish carbonatite from sedimentary carbonate rocks.The sulfur isotopic composition of the intensively REE-mineralized dolomites and pyrite separated from those dolomites show two obvious peaks; one peak in δ34S values is around0‰, which is characteristics of a mantle-source; another is around+8‰, being significantly higher than the mantle sulfur source value and falling into the sedimentary carbonate field. The sulfur isotopic composition of barite samples indicates that they are richer in34S than the dolomites, with an average δ34S value of+12.50‰, this value is lower than+17‰that was suggested by Holser et al.(1996) to be the composition of sulfates precipitated from Precambrian seawater. The calculated834S∑s results using Ohmoto model are6.1‰and10.1‰, respectively, both values have characteristics of mantle source that exchanged and/or mixed with the sulfur source in the crust or seawater. Sulfur in the whole rock and sulfides came from two sources:the mantle and seawater. Metasomatism unevenly formed two sulfur isotope peaks in the mineralized dolomites.The sedimentary age of K-rich slate are about1.65Ga, if slate is volcanic rock or metavolcanic rock, there should be zircon with age of～1.65Ga, whereas typical magmatic zircon is formed at Hercynian, and there is no1.6Ga zircon. Geochemical studies of H9K-rich slate show that they normal metamorphic sedimentary rocks, and they also have suffered from carbonatite magma. In summary, mineralized dolomites in the Bayan Obo REE-Nb-Fe deposit are the product of sedimentary carbonate hydrothermally metasomatised by carbonatite magma and/or associated fluids, and coarse-grained dolomite marbles are the result of weak mineralization of sedimentary carbonate rocks, and fine-grained dolomite marbles are the product of intensive mineralization. The Bayan Obo rift mobilized at about1.3～1.4Ga, accompanied by extremely high REE carbonatite magmatism, H8sedimentary carbonate of the Bayan Obo Group was mineralization and metasomatised to form the whole deposit. In Caledonian era, the North China Block subducted toward the Siberian plate, and then collided each other and combined together. Meanwhile, the Bayan Obo deposit was strongly disturbed, later phase minerals, such as pyrite and riebeckite, were formed, and some isotopic systems were reset and show Caledonian apparent ages.更多还原