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.     

A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China

Summary ?A carbonatite dyke, extremely enriched in rare earth elements (REE), is reported from Bayan Obo, Inner Mongolia, North China. The REE content in the dyke varies from 1 wt% to up to 20 wt%. The light REEs are enriched and highly fractionated relative to the heavy REEs, and there is no Eu anomaly. Although carbon isotope δ¹³C (PDB) values of the carbonatites (−7.3 to −4.7‰) are within the range of normal mantle (−5±2‰), oxygen isotope δ¹⁸O (SMOW) (11.9 to 17.7‰) ratios apparently are higher than those of the mantle (5.7±1.0‰), indicating varying degrees of exchange with hydrothermal fluids during or after magmatic crystallization. The carbonatite is the result of partial melting followed by fractional crystallization. Primary carbonatite melt was formed by less than 1% partial melting of enriched mantle, leaving a garnet-bearing residue. The melt then rose to a crustal magma chamber and underwent fractional crystallization, producing further REE enrichment. The REE and trace element distribution patterns of the carbonatites are similar to those of fine-grained dolomite marble, the ore-host rock of the Bayan Obo REE–Nb–Fe giant mineral deposit. This fact may indicate a petrogenetic link between the dykes described here and the Bayan Obo mineral deposit. Received November 1, 2001; revised version accepted June 16, 2002     

白云鄂博铁、铌、稀土矿床:研究进展、存在问题和新认识

白云鄂博是世界第一大稀土元素矿床,其稀土元素资源量占世界目前已知稀土元素资源总量的三分之二以上,具有重要的经济和战略地位。白云鄂博自1927年发现至今已有92年,对其的开发和研究历史悠久,在赋矿碳酸岩的成因、成矿年代、稀土矿物学等方面取得了一系列重要进展,为碳酸岩型稀土元素成矿理论发展做出了突出贡献。由于矿床经历了成矿后复杂的构造变形和热液蚀变,致使其地质、地球化学特征复杂,学界对其成因和成矿过程一直存在不同认识,对成矿碳酸岩的岩浆演化、稀土元素迁移与富集机理等方面的研究也相对薄弱。笔者最新研究表明,矿区过去被认为是下白云鄂博群的H1、H2、H9岩石单元并非沉积变质岩或变质火山岩,而应为岩浆侵入成因,矿区其他岩石单元(H3～H7)的成因也值得商榷。H9岩石单元中的黑云母岩(前人称为黑云母板岩)和富含黑云母的碳酸岩(前人称为暗色板岩)为成矿碳酸岩的一部分,是碳酸岩不同岩相带的表现。黑云母岩和富黑云母碳酸岩相对于含矿碳酸岩具有相对低的稀土元素含量和轻、重稀土元素比值,表明岩浆演化可能对稀土元素的富集和分异具有重要贡献。矿区主要岩石单元成因的新认识不仅为矿床成因、成矿背景研究提供了新的依据,同时也为研究区域构造演化、正确厘定矿区构造式样和指导矿区深部和外围找矿提供了新的思路。     

http://www.sciencedirect.com/science/article/pii/S1674987115001310

Bayan Obo ore deposit is the largest rare-earth element(REE) resource,and the second largest niobium(Nb) resource in the world.Due to the complicated element/mineral compositions and involving several geological events,the REE enrichment mechanism and genesis of this giant deposit still remains intense debated.The deposit is hosted in the massive dolomite,and nearly one hundred carbonatite dykes occur in the vicinity of the deposit.The carbonatite dykes can be divided into three types from early to late:dolomite,co-existing dolomite-calcite and calcite type,corresponding to different evolutionary stages of carbonatite magmatism based on the REE and trace element data.The latter always has higher REE content.The origin of the ore-hosting dolomite at Bayan Obo has been addressed in various models,ranging from a normal sedimentary carbonate rocks to volcano-sedimentary sequence,and a large carbonatitic intrusion.More geochemical evidences show that the coarse-grained dolomite represents a Mesoproterozoic carbonatite pluton and the fine-grained dolomite resulted from the extensive REE mineralization and modification of the coarse-grained variety.The ore bodies,distributed along an E-W striking belt,occur as large lenses and underwent more intense fluoritization and fenitization.The first episode mineralization is characterized by disseminated mineralization in the dolomite.The second or main-episode is banded and/or massive mineralization,cut by the third episode consisting of aegirinerich veins.Various dating methods gave different mineralization ages at Bayan Obo,resulting in long and hot debates.Compilation of available data suggests that the mineralization is rather variable with two peaks at~1400 and 440 Ma.The early mineralization peak closes in time to the intrusion of the carbonatite dykes.A significant thermal event at ca.440 Ma resulted in the formation of late-stage veins with coarse crystals of REE minerals.Fluids involving in the REE-Nb-Fe mineralization at Bayan Obo might be REE-F-C02-NaCI-H20 system.The presence of REE-carbonates as an abundant solid in the ores shows that the original ore-forming fluids are very rich in REE,and therefore,have the potential to produce economic REE ores at Bayan Obo.the Bayan Obo deposit is a product of mantle-derived carbonatitic magmatism at ca.1400 Ma,which was likely related to the breakup of Columbia.Some remobilization of REE occurred due to subduction of the Palaeo-Asian oceanic plate during the Silurian,forming weak vein-like mineralization.     

白云鄂博矿床研究若干问题的探讨

白云鄂博巨型稀土-铌-铁矿床是与火成碳酸岩(即H8)有关的矿床。在碳酸岩岩浆阶段,在其蚀变围岩(霓长岩)中以及晚期的热液阶段都有稀土、铌和铁等的富集。因此,白云鄂博矿床不是单一矿床类型,其涵盖了稀土-铌-铁碳酸岩岩浆型矿床、稀土-铌-铁交代蚀变岩型和热液型矿床,这种复合类型的矿床是十分罕见的,可称之为白云鄂博式矿床。根据已发表的年代数据,白云鄂博大规模碳酸岩的形成时代和伴随的稀土矿化的高峰期在1.3～1.4Ga。加里东构造热事件对本区的影响广泛和强烈,不仅有广泛发育的大型褶皱、冲断层和韧性剪切作用,并伴有广泛的流体交代作用和局部的热液活动,某些稀土矿物的同位素体系受到重置,表现为其Sm-Nd年龄和Th-Pb年龄的不一致。本区的碱性岩在基底杂岩中即有出露,随后在白云鄂博群中和1.3～1.4Ga白云鄂博碳酸岩形成时皆有产出。目前所获资料表明,至少1.3～1.4Ga的碱性杂岩在成因上与碳酸岩有密切的联系。本区基底杂岩显示了白云鄂博地区在2.0Ga左右曾经有一个弧地质体,该区在2.0～1.9Ga间经历了从被动大陆边缘到活动大陆边缘增生碰撞的一个完整的造山过程。     

白云鄂博REE-Nb-Fe矿床的富钠岩石类型析及成因分

白云鄂博稀土-铌-铁矿是世界最大的稀土多金属矿床之一,主要产于中元古界富钠岩石、白云岩和富钾板岩中。富钠岩石与含矿碳酸岩、富钾板岩一起在白云鄂博广泛分布,特别是在主矿和东矿南侧分布广泛,也产在含矿围岩和底盘围岩中。富钠岩石包括富钠长石岩、含钠闪石钠长岩脉、钠闪石岩脉及钠闪石碳酸岩脉和钠闪石化蚀变白云岩。富钠岩石在一个地质单元内出现,但成分略有不同,其岩石化学分析表现出富钠特征,大部分样品的w(Na2O)在6%以上,w(Na2O+K2O)>8%,w(Na2O)>w(K2O)。富钠长石岩的里特曼指数δ为2.96~8.56,n(A l)/n(K+Na)>1,n(A l)/n(K+Na+Ca)<1,表明为弱碱性偏铝质岩石。富钠岩石以富集地壳不相容元素、亏损地幔元素为特征,其稀土总量低于矿化白云岩,但轻稀土元素高度富集,具弱负铕异常,与含矿白云质火山岩一致。根据岩石产状、组构及化学成分分析,富钠长石岩属于富钠火山岩,而其他含钠闪石岩脉则属于岩浆分异侵入的岩脉与岩浆热液充填岩脉。钠化交代以霓石化、钠铁闪石化和钠长石化出现为特征。另外,富钠流体与白云岩反应形成了钠铁闪石化白云岩,强烈的钠化蚀变作用主要出现在稀土强烈富集的主矿和东矿。比较蚀变与未蚀变岩石的化学成分,表明蚀变作用引起稀土的重新分配和弱负铕异常。钠铁闪石化白云岩的w(S iO2)、w(TiO2)、w(K2O)和w(Na2O)升高,w(P2O5)降低。钠化蚀变前的碳酸岩中稀土元素已经富集,热液蚀变引起稀土元素的活化和重新分配,但是没有提供新的稀土来源。     

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.     

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。     

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。     

白云鄂博碳酸岩中矿物的次生反应边结构

在白云鄂博粗粒方解石碳酸岩中,发现了三种围绕硅酸盐矿物的次生反应边结构:1)在方解石中的石英被磷硅钙铈矿反应边环绕与方解石分隔开;2)在方解石中的碱性闪石被磷硅钙铈矿反应边环绕与方解石分隔开;3)方解石中的榍石被硅钛铈矿反应边环绕与方解石分隔开。这种情况表明,在后期阶段该区有富稀土的流体活动,该流体沿石英或硅酸盐矿物与方解石矿物颗粒接触边缘进行交代,生成了次生稀土矿物的反应边环带。这进一步表明,白云鄂博矿床稀土的富集至少与一期富稀土流体的交代作用有关。流体除含REE外,还含有铁和钠,该种流体的成分与以往有关白云鄂博矿床流体研究所得的成分结论是完全一致的。     

In-situ LA-ICP-MS trace elemental analyses of magnetite: The Bayan Obo Fe-REE-Nb deposit,North China

The Bayan Obo Fe-REE-Nb deposit in northern China is the world's largest light REE deposit, and also contains considerable amounts of iron and niobium metals. Although there are numerous studies on the REE mineralization, the origin of the Fe mineralization is not well known. Laser ablation (LA) ICP-MS is used to obtain trace elements of Fe oxides in order to better understand the process involved in the formation of magnetite and hematite associated with the formation of the giant REE deposit. There are banded, disseminated and massive Fe ores with variable amounts of magnetite and hematite at Bayan Obo. Magnetite and hematite from the same ores show similar REE patterns and have similar Mg, Ti, V, Mn, Co, Ni, Zn, Ga, Sn, and Ba contents, indicating a similar origin. Magnetite grains from the banded ores have Al + Mn and Ti + V contents similar to those of banded iron formations (BIF), whereas those from the disseminated and massive ores have Al + Mn and Ti + V contents similar to those of skarn deposits and other types of magmatic-hydrothermal deposits. Magnetite grains from the banded ores with a major gangue mineral of barite have the highest REE contents and show slight moderate REE enrichment, whereas those from other types of ores show light REE enrichment, indicating two stages of REE mineralization associated with Fe mineralization. The Bayan Obo deposit had multiple sources for Fe and REEs. It is likely that sedimentary carbonates provided original REEs and were metasomatized by REE-rich hydrothermal fluids to form the giant REE deposit.     

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.     

白云鄂博REE-Nb-Fe稀土矿赋矿岩系建造研究评述

白云鄂博特大型铌、稀土、铁矿床,由于其矿物组成的多样性、地质构造的复杂性、成矿的多期性使之成为一类典型稀土矿床,成为研究稀土矿的天然实验室。尽管已有60多年的开采及研究历史,但其含铁及稀土的矿床建造机制仍具争议。本文通过搜集、整理国内外近几年来发表的有关赋矿岩性(白云岩、富钾板岩、富钠岩石及碳酸岩脉)的研究成果,结合笔者的认识,对各种不同观点进行了对比分析和总结,最终提出了白云鄂博碱性碳酸岩-热水沉积岩系的观点,为白云鄂博稀土矿的研究提供一定的参考价值。     

Spatial distribution pattern of ore forming elements in the Bayan Obo deposit and exploration implicationsSCIEI北大核心CSCD

白云鄂博矿床是全球最大的稀土-铌-铁矿床。本文对白云鄂博矿床主矿、东矿赋矿白云岩、不同类型稀土-铌-铁矿石和富钾板岩进行详细的野外地质调查和地球化学成分分析,揭示了成矿元素稀土、铌和主要蚀变元素钠、钾、氟、磷的空间分布规律。结果表明,白云鄂博主采坑比东采坑具有更高的稀土含量,矿体由外围向中心、由浅部向深部有明显的稀土元素(尤其是中-重稀土元素)富集的现象,矿体深部具有更大的中-重稀土元素找矿潜力;成矿元素铌的异常范围与重稀土的异常范围相吻合,表明二者具有密切的共生关系;蚀变元素氟的异常范围与稀土-铌-铁矿体的分布范围相一致,是重要的沉淀成矿元素;蚀变元素磷的异常要明显超过稀土-铌-铁矿体的分布范围,是重要的迁移载体元素;钠化蚀变主要出现在矿体下盘含稀土白云岩中,代表早期高温蚀变过程;钾化蚀变主要发育在矿体上盘的富钾板岩中,代表晚期低温蚀变过程;钾化、萤石化及磷的异常是碳酸岩型稀土矿床最主要的浅部和外围示矿指标。     

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。     

白云鄂博碳酸岩型REE-Nb-Fe矿床——一个罕见的中元古代破火山机构成岩成矿实例

白云鄂博碳酸岩型REE-Nb-Fe矿床是世界上最大的稀土矿床。稀土矿石产于整个白云石碳酸岩体和部分脉状碳酸岩中。对比世界上20余个火成碳酸岩地区的特征后发现,白云鄂博地区完全具备国外火成碳酸岩区的地质特征。在岩石、矿石组合上,本区也发育一套碳酸岩+超基性岩+碱性基性岩(含基性熔岩)+碱性岩+稀土矿石+铁矿石组合;在矿物组合上,以白云石为主,方解石次之,伴生一套碱性闪石、长石、霓石、磷灰石、萤石、磁铁矿、稀土矿物组合;在全岩化学成分、微量元素、稀土元素和Sr、Nd、Pb、C、O同位素上,这些岩石具有一定的亲缘关系,有着共同的来源;在岩体的形态与岩石组构上,它们以岩席、岩筒和脉岩的形式出现,并发育有强烈的熔离作用与流动构造;在区域构造上,发育隐伏穹窿构造、岩筒构造和巨型断裂汇聚构造。综合分析上述特征表明:白云鄂博地区具有中元古代破火山机构的痕迹,赋矿白云石碳酸岩体则是顺层侵入的火成碳酸岩体,东矿、主矿可能是一个火山颈构造控矿,而赋矿白云石碳酸岩体西南侧的苏木图矿床则是隐伏岩筒构造控矿。     

Bayan Obo Controversy: Carbonatites versus Iron Oxide‐Cu‐Au‐(REE‐U)

The Bayan Obo Fe‐REE‐Nb deposit is the world’s largest rare earth element (REE) resource and its genesis has been the subject of much debate for many years. The most popular are the carbonatite‐related and hydrothermal Fe oxide‐Cu‐Au‐(REE‐U) genetic models. Comparisons of geologic setting, lithology, mineral assemblages, metal associations, geochemistry (particularly REE and light REE/heavy REE ratios), fluid chemistry and isotopics indicate that the Bayan Obo deposit shares features of both types, which are classified differently; that is, the carbonatites model is host‐rock based, while the Fe oxide‐Cu‐Au‐(REE‐U) model is essentially mineral assemblage and metal association based. A speculative classification scheme is tentatively put forward to link the two models, but many questions remain for further studies.     

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。