白云鄂博碳酸岩墙的稀土和微量元素地球化学及对其成因的启示

根据矿物组成白云鄂博矿区的碳酸岩岩可墙可分为白云石型、白云石－方解石共存型和方解石型三种类型。REE和微量元素地球化学表明，这三类碳酸岩岩墙为碳酸岩浆演化不同阶段的产物，白云石型和白云石－方解石共存型对应于早期岩浆阶段，其（La/Nd)n、(La/Yb)n比值随稀土总量的增加而增大，方解石型则对应于碳酸岩浆演化的晚期热液阶段，其稀土总量明显富集，但其（La/Nd)n、(La/Y)n和（La/Yb)n比值随稀土总量的增加却有减小的趋势，热液阶段也是白云鄂博稀土矿化的主要阶段。     

Geochemical constraints on the genesis of the Bayan Obo Fe-Nb-REE deposit in Inner Mongolia, China

Trace element and isotopic compositions of carbonate from ore bodies, country rock which hosts the ore bodies (H8 dolomite), a carbonatite dyke exposed in Dulahala near Bayan Obo, and rare earth element (REE)-rich dolomite in Bayan Obo have been determined to understand the genesis of the Bayan Obo Fe-Nb-REE ore deposit, the world’s largest resource of REE. The REE and trace element distribution patterns of samples from the REE-rich carbonatite dykes are identical to those of mineralized carbonate rocks, indicating a genetic linkage between the REE-rich carbonatite and mineralization in this region. By contrast, carbon and oxygen isotopes in the mineralized carbonate varied significantly, δ¹³C = −7.98‰ to −1.12‰, δ¹⁸O = 8.60-25.69‰, which are distinctively different from those in mantle-derived carbonatite. Abnormal isotopic fractionations between dolomite and calcite suggest that these two minerals are in disequilibrium in the carbonatite dyke, ore bodies, and H8 marble from Bayan Obo. This isotopic characteristic is also found in mineralized sedimentary marine micrite from Heinaobao, ∼25 km southeast of the Bayan Obo Fe-Nb-REE ore deposit. These facts imply that the carbonate minerals in the Bayan Obo deposit have resulted from sedimentary carbonate rocks being metasomatised by mantle-derived fluids, likely derived from a REE-enriched carbonatitic magma. The initial Nd isotope values of ore bodies and carbonatite dykes are identical, indicating that ore bodies, carbonatite dykes and veins may have a similar REE source.     

白云鄂博地区碳酸岩脉侵位序列与稀土元素富集机制

白云鄂博地区发育大量的火成碳酸岩脉。按照矿物组成,碳酸岩脉可分为白云石型、白云石-方解石共存型和方解石型。野外穿插关系表明,白云石型碳酸岩脉形成得早,而方解石型碳酸岩脉形成得晚。白云鄂博地区的碳酸岩浆存在由白云石型到共存型再到方解石型的先后结晶顺序和演化趋势。碳酸岩脉的主量、稀土和微量元素组成特征表明,随着碳酸岩脉中方解石矿物组分的增加,轻稀土元素的含量呈明显富集趋势,而长期的结晶分异作用正是稀土元素,尤其是轻稀土元素在晚期岩浆中强烈富集的内在机制。     

New evidence from a calcite-dolomite carbonatite dyke for the magmatic origin of the massive Bayan Obo ore-bearing dolomite marble, Inner Mongolia, China

Summary New data on Sr and Nd isotope composition and major and trace element distribution in dolomite-calcite carbonatite dykes at Bayan Obo are provided, and a Mid-Proterozoic age is deduced. The dykes and the neighbouring massive dolomite (H8) body have similar geochemical characteristics, interpreted to indicate a carbonatitic magmatic origin. The occurrence of riebeckite-bearing fenitized quartzites marginal to both dykes and H8 dolomite body, and the presence of xenoliths in the latter, supports this conclusion. Taken together with previously published stable isotope data, these data confirm a mantle-derived origin for the H8 body. The oxygen isotope composition of the dolomite and magnetite in the dykes is lower than that in the fine-grained dolomite. Oxygen data from samples of the coarse-grained dolomite host are either similar to the dykes or to the fine-grained type in agreement with their other geochemical characteristics. The carbonate-magnetite thermometric pairs of the fine-grained dolomite indicate a range of 350–540 °C, which is probably lower than that of the original main magmatic emplacement. This supports the distinction made between the original coarse-grained dolomite marble and dyke composition from the later fine-grained dolomite. Thus the large H8 dolomite is interpreted as a carbonatite intrusion that contains wall-rock xenoliths and caused fenitization of the hanging wall, foot wall and the xenoliths, and that the coarse-grained portions of the H8 marble are those portions that, in the Late Proterozoic to Palaeozoic, escaped recrystallization to fine-grained dolomite and subsequent REE-Fe mineralization. Currently at National Oceanography Centre, University of Southampton, Southampton, UK An erratum to this article is available at .     

内蒙古白云鄂博矿床的稀土矿化时代与期次

内蒙古白云鄂博REE-Fe-Nb矿床是世界罕见的超大型多金属矿床, 但矿床成因一直没有解决, 而年代学研究是解决矿床成因的基础。本文结合矿床地质特征, 系统地分析了前人的年代学研究成果, 并对白云鄂博矿床的Sm-Nd同位素体系进行了深入的分析和讨论。结果表明, 白云鄂博稀土的成矿时代约为1.3 Ga, 与碳酸岩墙的形成时间一致, 成矿物质来源于地幔。加里东期的热事件(约0.44 Ga)导致了白云鄂博矿床晚期稀土矿脉的形成和原有矿体中部分稀土矿物的重结晶, 但成矿物质主要来源于矿体内部的稀土再循环, 外源物质的贡献不明显。约1.3 Ga到约0.44 Ga间的一系列中间年龄为后期热扰动的结果, 并不代表成矿事件。简言之, 白云鄂博矿床只在中元古代发生过一次实质性的稀土矿化作用, 地幔是稀土物质的单一源区。     

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits,Inner Mongolia,China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~ 1 Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3 Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~ 450 to 420 Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450 Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3 Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~ 450 Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.     

Integrated U–Pb and Sm–Nd geochronology for a REE-rich carbonatite dyke at the giant Bayan Obo REE deposit,Northern China

The Bayan Obo deposit in North China contains the largest rare-earth element (REE) resources in the world, but its forming time remains controversial. Nearly one hundred carbonatite dykes occur around the Bayan Obo deposit, including dolomite, calcite and calcite–dolomite carbonatite varieties. Zircons from a REE-rich carbonatite dyke and wallrock quartz conglomerate at Bayan Obo have been analyzed for U–Pb to determine the age of the dyke. Zircon from the carbonatite dyke, analyzed by conventional isotope dilution thermal ionization mass spectrometry (ID-TIMS), yielded an upper intercept age of 1417 ± 19 Ma. This age is confirmed by SHRIMP U–Pb analysis of zircon from the same carbonatite dyke, which gave a ²⁰⁷Pb/²⁰⁶Pb weighted mean age of 1418 ± 29 Ma. In situ Nd isotope measurements of monazite collected from the carbonatite dyke gave an isochron age of 1275 ± 87 Ma. These results demonstrate that the dyke intruded ~ 1400 Ma. In view of predecessor's results, it is clarified that the REE mineralization at Bayan Obo occurred at ca. 1400 Ma, consistent with the timing of carbonatite dyke intrusion in the region. The youngest detrital zircons from the quartz conglomerate yielded a ²⁰⁷Pb/²⁰⁶Pb weighted mean age of 1941 ± 7 Ma using LA ICP-MS U–Pb method. Detrital zircons in the carbonatite dyke also gave a mean apparent age of 1932 ± 3 Ma using ID-TIMS U–Pb method and 1914 ± 14 Ma using SHRIMP U–Pb method. These ages constrain the beginning active time of the Zha'ertai–Bayan Obo rift in the northern margin of the North China Craton after ~ 1900 Ma.     

A Geochemical Study of an REE-rich Carbonatite Dyke at Bayan Obo, Inner Mongolia, Northern China

An REE-rich carbonatite dyke was found in Dulahala, close to the Bayan Obo superlarge REE-Nb-Fe mineral deposit in Inner Mongolia, northern China. The REE content in the dyke varies greatly, from 1% up to 20% (wt), which might constitute rich REE ores. Light REEs in the carbonatite are enriched and highly fractionated relative to heavy REEs and there is no Eu anomaly. The REE and trace element distribution patterns of the carbonatite are identical to those of fine-grained dolomite marble which is the host rock of the Bayan Obo REE-Nb-Fe superlarge mineral deposit. This indicates a petrogenetic linkage between the REE-rich carbonatite and the mineralizations in this region.     

白云鄂博矿区周围火成碳酸岩岩墙地质特征

首次填出白云鄂博矿区周围火成碳酸岩岩墙的分布图,深入系统地研究了岩墙的地质产状、主矿物类型、岩石结构、人工重砂矿物组成、稀土元素含量等特征。反映了白云鄂博矿区周围火成碳酸岩岩墙的岩浆演化分异过程存在差异。对于研究白云鄂博矿区铁与稀土的矿化提供了物质来源的证据。     

http://www.sciencedirect.com/science/article/pii/S1674987110000125

<正>Carbonatites are commonly related to the accumulation of economically valuable substances such as REE.Cu,and P.The debate over the origin of carbonatites and their relationship to associated silicate rocks has been ongoing for about 45 years.Worldwide,the rocks characteristically display more geochemical enrichments in Ba,Sr and REE than sedimentary carbonate rocks.However,carbonatite's geochemical features are disputed because of secondary mineral effects.Rock-forming carbonates from carbonatites at Qinling.Panxi region,and Bayan Obo in China show REE distribution patterns ranging from LREE enrichment to flat patterns.They are characterized by a Sr content more than 10 times higher than that of secondary carbonates.The coarse- and fine-grained dolomites from Bayan Obo H8 dolomite marbles also show similar high Sr abundance,indicating that they are of igneous origin.Some carbonates in Chinese carbonatites show REE(especially HREE) contents and distribution patterns similar to those of the whole rocks.These intrusive carbonatites display lower platinum group elements and stronger fractionation between Pt and Ir relative to high-Si extrusive carbonatite.This indicates that most intrusive carbonatites may be carbonate cumulates.Maoniuping and Daluxiang in Panxi region are large REE deposits.Hydrothermal fluorite ore veins occur outside of the carbonatite bodies and are emplaced in wallrock syenite.The fiuorite in Maoniuping has Sr and Nd isotopes similar to carbonatite.The Daluxiang fiuorite shows Sr and REE compositions different from those in Maoniuping.The difference is reflected by both the carbonatites and rock-forming carbonates,indicating that REE mineralization is related to carbonatites.The cumulate processes of carbonate minerals make fractionated fluids rich in volatiles and LREE as a result of low partition coefficients for REE between carbonate and carbonatite melt and an increase from LREE to HREE.The carbonatite-derived fluid has interacted with wallrock to form REE ore veins.The amount of carbonatite dykes occurring near the Bayan Obo orebodies may support the same mineralization model,i.e.that fluids evolved from the carbonatite dykes reacted with H8 dolomite marble,and thus the different REE and isotope compositions of coarse- and fine-grained dolomite may be related to reaction processes.     

白云鄂博磷灰石的矿物化学特征及其成因意义

本文对白云鄂博地区广泛分布的磷灰石进行了系统的电子探针分析。结果表明,赋矿粗粒白云石大理岩中磷灰石的矿物化学特征与碳酸岩岩墙中的非常相似,它们均属于富含锶和稀土元素的氟磷灰石,具有典型的火成碳酸岩特征。然而,它们在稀土元素分布模式以及轻稀土元素之间的分馏作用方面却存在着差异。造成这些差异的原因是,磷灰石的寄主岩石受富含氟和稀土元素的流体影响的程度不同：粗粒白云石大理岩受流体的影响比碳酸岩岩墙要大。     

白云鄂博碳酸岩脉的显微共聚焦激光拉曼光谱分析

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。     

白云鄂博碳酸盐矿物的矿物化学成分标型特征

对白云鄂博地区不同产状的碳酸盐矿物进行了系统的电子探针分析,其结果表明白云鄂博ＲＥＥ－Ｎｂ－Ｆｅ矿床的赋矿白云岩中,无论是粗粒结构的,还是细粒结构的白云岩,其主要造岩矿物白云石或者铁白云石均具高锰（ＭｎＯ〉０．５ｗｔ％）和高锶（ＳｒＯ〉１．５ｗｔ％）的特征,与矿床北东方向０￣３．５ｋｍ范围内切割五台群花岗片麻岩－混合岩及白云鄂博群碎屑岩的火成碳酸岩岩墙中的碳酸盐矿物非常相似。它们完全不同于宽沟段裂以北典型沉积石灰岩厦白云质石灰岩中的方解石和白云(MnO<0.1wt％ ,SrO<0.1wt％ )。赋矿白云岩中自云石的锶和锰含量系统变他反映碳酸岩浆发生了分离结晶作用,其结果可以导致其余岩浆中REE高度富集．本文认为碳酸盐矿物的锶和锰含量可以作为识别其成囡的重要标型特征,其氧化物MnO>0.15wt％ 和Sr>0.15wt％为火成碳酸岩的标志。     

白云鄂博超大型稀土矿床成因评述

稀土元素(Y+Sc+La-Lu)作为现代工业的“维生素”,是当今社会重要的战略资源。白云鄂博超大型REE-Nb-Fe矿床位于中国内蒙古,是世界上最大的稀土矿床。因此,该矿床的成岩成矿模式受到全球地质工作者的强烈关注,并对此进行了大量的分析研究工作。以往的工作主要集中在对H8赋矿白云岩全岩年代学和地球化学的研究上,但由于白云鄂博稀土矿床经历了后期复杂的变形变质作用,矿石的结构构造非常复杂,全岩的研究结果造成了对该矿床成岩成矿模式不同的认识和争论。近些年,随着现代地球化学分析技术的快速发展,人们可以直接对白云石和稀土矿物(如独居石、磷灰石、氟碳铈矿等)进行高精度的微区原位同位素组成和原位U-Th-Pb定年分析,从而获得能够明确指示矿物时代、成矿流体和成矿物质来源的重要证据。结合近年来的研究成果,本文总结了目前关于白云鄂博超大型稀土矿床成岩成矿模式的不同认识,提出白云鄂博矿床初始稀土成矿源于中元古代(1.3 Ga)的碳酸岩岩浆活动,后期经历了早古生代的大规模流体作用,矿床中稀土元素发生再活化、富集和沉淀,稀土矿物巨量堆积成矿的新认识。     

白云鄂博主、东矿三个剖面地球化学特征及其研究意义

近年来关于白云鄂博Nb-REE-Fe矿床H8岩体火成水成归属的争论已渐息,而矿床成矿过程以及REE富集机制仍是学术界关注的热点。文章对白云鄂博矿床白云石碳酸岩体、霓长岩化蚀变带、尖山组板岩3个典型剖面开展系统的岩石地球化学工作,发现赋矿碳酸岩、霓长岩、铁矿石微量与稀土元素配分模式具有相似性,靠近H8岩体的尖山组板岩往往有着更高的稀土元素含量,以及与成矿碳酸岩相近的微量元素配分模式。区内各类岩石单元稀土元素分馏明显,全岩La_N/Nd_N比值的变化规律显示,H8岩体内部比边缘更富La,边缘比岩体内部更富Nd。通过岩（矿）石薄片BSE图像结合矿物电子探针分析显示,H8岩体内的稀土元素矿物（主要是独居石、氟碳铈矿等）可分为2组,一组相对富La,呈半自形-他形,星点状分布;另一组相对富Nd,呈他形细粒,脉状分布。两组矿物中不同元素的富集特征可能代表了结晶过程中流体环境的改变。上述实验结合地质勘查结果表明,白云鄂博矿床初始成矿物质的起源与H8碳酸岩一致,均来源于中元古代碳酸岩岩浆活动,而不同类型的稀土元素矿物对应了白云岩成岩阶段与萤石矿化阶段两个不同的稀土矿化阶段。     

Geochemical characteristics of the Bayan Obo giant REE–Nb–Fe deposit: Constraints on its genesis

Geochemical characteristics of different dolomites in the Bayan Obo giant REE–Nb–Fe deposit in Inner Mongolia have been studied. Intensively REE-mineralized dolomites (total REE over 800 ppm) show similar geochemical characteristics to associated carbonatite dykes, with Ba, Th, REE enrichments and Sr, Nb, Ti, Cu depletions, which is different from those of dolomites in the deposit with low REE contents (total REE less than 800 ppm). The low REE dolomites display some transitional characteristics between carbonatite dyke and sedimentary carbonate, with La depletion and Nb enrichment. This indicates that the genesis of the REE-mineralized dolomites might be related to both carbonatite magma and sedimentary carbonates. Sulfur isotope data indicates two sulfur sources, a mantle source (δ³⁴S c.a. 0‰) and seawater (δ³⁴S c.a. +25‰). It is proposed that mineralized dolomites in the Bayan Obo giant REE–Nb–Fe deposit are the product of sedimentary carbonate hydrothermally metasomatised by carbonatite magma and/or associated fluids. These dolomites formed the large-scale rare earth mineralization in the unique Bayan Obo REE–Nb–Fe deposit.     

白云鄂博超大型REE-Nb-Fe矿床的稀土成矿模式综述

白云鄂博矿床在矿物学、矿床学以及成矿理论等研究中有着极其重要的地位。本文旨在总结前人的研究成果,归纳现有的几种矿床成矿模式及其主要依据,并进行简要分析,以期对白云鄂博矿床成因的进一步研究起到些许作用。     

Samarium–Neodymium and Rubidium–Strontium Isotopic Dating of Veined REE Mineralization for the Bayan Obo REE‐Nb‐Fe Deposit,Northern China

The Bayan Obo REE‐Nb‐Fe deposit in Inner Mongolia, China, consists of later REE‐mineralizing fluorocarbonate veins cutting the earlier banded and massive ores in the deposit. Samarium–neodymium dating using the minerals including huanghoite and rubidium–strontium dating using single‐grain biotites both from the later veins show concordant isochrons corresponding to 442 ± 42 Ma (2σ uncertainty) and 459 ± 41 Ma, respectively. The isochron ages suggest that the later REE vein mineralization took place during the middle Paleozoic at Bayan Obo, consistent with geological observations and age data previously reported.     

白云鄂博矿区碱性基性岩—碳酸岩岩墙岩石学研究

白云鄂博地区广泛发育碳酸岩墙 -碱性基性岩墙,通过对其岩石学、矿物学、岩石化学、稀土元素、微量元素的系统研究表明,碳酸岩墙为火成碳酸岩,部分碳酸岩墙的REE含量高达14 6 75 %,与赋矿白云石碳酸岩体的REE含量(最高达 10 % )相近,两者均为富稀土矿石。矿区基性岩墙的K2 O +Na2 O含量( >5 % )高于普通基性岩石(4 % ),表明它们属于碱性岩类,是碱性基性岩墙。碳酸岩墙 -碱性基性岩墙的REE与微量元素的分配型式近似,Sr、Nd、Pb同位素特征亦相近,表明它们形成于一个比较特殊的、近似的源区,两者有一定的成因关系。