Trace element geochemistry of Amba Dongar carbonatite complex,India: Evidence for fractional crystallization and silicate-carbonate melt immiscibility

Carbonatites are believed to have crystallized either from mantle-derived primary carbonate magmas or from secondary melts derived from carbonated silicate magmas through liquid immiscibility or from residual melts of fractional crystallization of silicate magmas. Although the observed coexistence of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement in many, and overlapping initial⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios are supportive of their cogenesis; there have been few efforts to devise a quantitative method to identify the magmatic processes. In the present study we have made an attempt to accomplish this by modeling the trace element contents of carbonatites and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace element data suggest that the carbonatites and alkaline silicate rocks of this complex are products of fractional crystallization of two separate parental melts. Using the available silicate melt-carbonate melt partition coefficients for various trace elements, and the observed data from carbonatites, we have tried to simulate trace element distribution pattern for the parental silicate melt. The results of the modeling not only support the hypothesis of silicate-carbonate melt immiscibility for the evolution of Amba Dongar but also establish a procedure to test the above hypothesis in such complexes.     

Unusual rocks and mineralisation in a new carbonatite complex at Kandaguba, Kola Peninsula, Russia

The paper presents the results from a reconnaissance investigation of carbonatites in a newly discovered alkaline complex in the Kola peninsula, Russia. The Kandaguba complex differs from other carbonatite plutons of the Kola alkaline province by (a) the absence of ultrabasic rocks, (b) widespread occurrence of nepheline-, cancrinite- and nepheline–cancrinite syenites and carbonatites and (c) presence of apatite–calcite ijolites and feldspar ijolites as separate intrusive phase. The Kandaguba carbonatites are notable for the predominance of late ferromagnesian varieties together with quartz and numerous accessory mineral species. The association of phosphates (monazite, gorseixite, goyazite, apatite), sulphides and tellurides (pyrite, sphalerite, galena, hessite), ilmenorutile, barite with quartz and ankerite is a remarkable feature of these carbonatites. The Kandaguba carbonatites are inferred to have been generated as the products of liquid immiscibility followed by differentiation of the carbonatite melt.     

Postorogenic carbonatites at Eden Lake, Trans-Hudson Orogen (northern Manitoba, Canada): Geological setting, mineralogy and geochemistry

The Eden Lake pluton in the Trans-Hudson Orogen is the first known occurrence of carbonatites in Manitoba. The pluton is largely made up of modally and geochemically diverse syenitic rocks derived from postorogenic magma(s) of shoshonitic affinity. Their diversity can be accounted for by a combination of crystal fractionation and fluid release in the final evolutionary stage (crystallization of quartz alkali-feldspar syenite). At Eden Lake, carbonatites, represented predominantly by coarse-grained massive to foliated sövite, occur as branching veins and lenticular bodies up to 4 m in thickness showing crosscutting relations with respect to all of the syenitic units. The host rocks are intensely fenitized at the contact, and there is also abundant mineralogical and textural evidence for assimilation of silicate material by carbonatitic magma through wallrock reaction and xenolith fragmentation and digestion. The bulk of the carbonatites are composed of (in order of crystallization): Sr–REE-rich fluorapatite, aegirine–augite, and coarse calcite crystals surrounded by fine-grained calcite (on average,  90 vol.% of the rock). Noteworthy accessory constituents are celestine, bastnäsite-(Ce) (both as primary inclusions in calcite), Nb–Zr–rich titanite, low-Hf zircon, allanite-(Ce) and andradite. The calcite is chemically uniform (Sr-rich, Mg–Mn–Fe-poor and low in ¹³C), but shows clear evidence of ductile deformation and syndeformational cataclasis. Geochemically, the carbonatites are enriched in Sr, Ba, light rare-earth elements, Th and U, but depleted in high-field-strength elements (particularly, Ti, Nb and Ta). The stable-isotope composition of coarse- and fine-grained calcite from the carbonatites and interstitial calcite from syenites is remarkably uniform: ca. − 8.16 ± 0.27‰ δ¹³C (PDB) and + 8.04 ± 0.19‰ δ¹⁸O (SMOW). The available textural and geochemical evidence indicates that the Eden Lake carbonatites are not consanguineous with the associated syenites and may have been derived from a Nb–Ti-retentive and ¹³C-depleted source such as the subducted crustal material underlying the Eden Lake deformation corridor.     

四川牦牛坪稀土矿床成因研究——来自包裹体的证据

四川牦牛坪稀土矿床是中国的第二大轻稀土矿床。碳酸岩-正长岩杂岩体是矿区主要的围岩,萤石是主要的脉石矿物。在四川冕宁牦牛坪稀土矿床霓辉重晶石型矿体早期萤石中笔者发现了大量的流体-熔体包裹体,对萤石矿物包裹体研究结果表明,牦牛坪矿床的晚期萤石矿物中至少存在6种类型包裹体：（1）气液包裹体（V-L）;（2）含CO2三相包裹体（L-L-V）;（3）含有少量子晶的多相包裹体（L-V-D或L-L-V-D）;（4）流体-熔体包裹体或多子晶包裹体（L-L-V-MS）;（5）固体包裹体（S）和（6）已经爆裂的高压包裹体等。各种类型的包裹体代表了不同的成岩阶段,并存在成因上的内在联系。气液包裹体（V-L）初融温度为-24．5--22．0℃,冰点温度为-2．5～-6．0℃,盐度为4．2％～9．2％。均一温度集中在142-146℃。含CO2三相包裹体（L—L—V）Tm-CO2为-56．6℃,Tm-clathe为＋2．1-＋10．0℃,盐度为0．1％～13．0％;Th-CO2为＋25～＋30．3℃,均一温度为230～265℃。含子晶的多相包裹体（L—V—D或L-L—V—D）子晶溶化温度在260～285℃之间,与前两类包裹体具有明显的成因上联系。流体-熔体包裹体或多子晶包裹体（L—L—V—MS）CO2含量约为10vol％;固相成分含量30vol％到90vol％,为长板状或立方体子晶,有些子晶呈圆形,正高突起,一轴负晶,可能为稀土碳酸盐矿物;Tm-CO2为-65．0～-58．0℃,初熔温度为-55．0--52．0℃,Th-CO2为＋26．5～＋28．3℃,气体水合物溶解温度为＋26．0～＋31．0℃;CO2相在220～276℃均一到水溶液相。固体组分一般会出现4次明显的溶解过程,即200～240℃、240—300℃、270～370℃和370～720℃四个阶段。包裹体的显微测温和拉曼成分分析结果显示早期流体中富含H2O、CO2、H2S和心等挥发分,并富含大量的重晶石、氟碳铈矿的子矿物,晚期成矿流体中则以H2O和CO2为主。根据矿床地质特征和矿物包裹体的研究结果,我们认为源区地幔岩石的低程度部分熔融形成富集稀土的碱性碳酸岩岩浆,侵位过程中产生液态不混熔现象,分异出携带大量高度富集REE的富F和CO2的高温流体,并主要以气相形式迁移稀土元素,在气成热液阶段REE富集成矿。矿床的成矿作用是由碳酸岩岩浆气液流体的沸腾、充填和交代过程而实现的。萤石的形成经历了气相迁移、深部成岩、上升交代、侵位成岩的过程。萤石的形成与稀土矿化有密切的关系。     

西藏羌塘增生杂岩带内中—晚三叠世碳酸盐岩地层的发现及大地构造意义

羌塘中央增生(变质)杂岩带地层的解体有助于厘定龙木错—双湖缝合带和古特提斯洋俯冲-碰撞的时空演化过程。长期以来,该缝合带三叠纪地层研究薄弱,已有认识缺乏与洋陆俯冲古地理格局的联系。蓝岭高压变质带北侧3个露头中新发现一套中—晚三叠世台地相碳酸盐岩夹细碎屑岩、硅质岩沉积组合,它们以断块状叠置于中二叠世蛇绿混杂岩之上,内部层理近水平状,层序并未倒转,但顶底不全。依据岩性组合、相对层厚和化石分布,将层序较完整的西南侧岩块大致划分出三个沉积亚相:局限台地相、开阔台地相和台地边缘礁相。边缘礁相砾屑、生屑灰岩中含丰富的六射珊瑚化石,已鉴定出的珊瑚总计6属1种,分别为Conophyllia sp.,Omphalopylliasp.,Coryphylliasp.,Pamirastraeasp.,Margarophylliacf.capitata,和Craspedophylliasp.,指示地层时代为中三叠世拉丁期—晚三叠世诺利期。通过对羌塘盆地中—上三叠统的对比,发现北羌塘地体、中央增生杂岩带和南羌塘地体分别发育不同的沉积体系。构造接触关系及地层特征综合显示蓝岭北侧的中上三叠统代表北羌塘活动大陆边缘的弧前沉积,具体类型为增生楔上叠盆地。结合区域内中上三叠统的产出背景和生物化石时代,推测古特提斯洋闭合于晚三叠世诺利期中期。     

滇中易门里士北部水下碳酸质火山-喷流岩铁、铜矿化及其大型隐伏矿床找矿研究

易门里士北部磁异常区中元古代是一个比周边相对低洼的碱性-碳酸质火山水下活动环境。因民组含矿层除有(次)火山岩和火山辞屑沉积岩外,还有粘土质-绿脱石岩、铁质岩、铁质硅质岩、铁质泥岩、碳酸盐岩、泥质白云岩、硅质白云岩等热液喷流岩含钠质硅酸盐团砾的碳酸质岩及碳酸盐晶屑组成的岩石是碳酸质火山岩。岩石、矿石的高场强元素对比值、稀土元素球粒陨石标准化比值、硫化物 Co/Ni 和 S 同位素比值显示喷流岩源于碱性-碳酸质火山岩,热液交代对成矿作用也有重要贡献,喷流岩除了铁矿化,还有铜、钴、锌、银等。如果将本区矿化岩石与典型喷流矿床——日本黑矿对比,果树村附近的网脉状矿化次火山岩、角砾熔岩、矿化角砾岩类似于喷流中心的矿化英安岩穹和角砾岩;磁异常北缘相同层位的含石膏富BaO 铁质泥岩和铁质硅质岩则相当于低洼地的石膏和重晶石沉积岩。本区磁异常与黑矿主矿一样在喷流岩层内处于喷流口与膏盐沉积洼地之间,其周边的铁矿点绝不是矿化的主体。大型含铜铁矿应该隐伏在深部因民组含矿层个于喷流中心和石膏、重晶石层之间的磁异常体中。     

岩溶水系统对大气CO2的潜在影响——基于热力学的研究

认识不同条件下岩溶水释放或吸收CO2的反应过程是研究碳酸盐岩对碳循环响应的前提和基础。本文从吉布斯自由能的热力学原理出发,对全球不同岩溶地区162组岩溶水(河水、溪水、湖水等)进行了热力学研究,结果显示:1)河水、溪水、湖水和洞穴滴水等岩溶水所处的环境因方解石矿物沉积而释放CO2成为大气CO2一个潜在的源;2)地下水在所处的环境下由于方解石的溶解而吸收CO2,成为大气CO2一个潜在的汇;3)少数出露点的泉水所处的环境既可发生方解石的溶解而吸收CO2,成为大气CO2的潜在汇,也可发生方解石的沉积而释放CO2,成为大气CO2的潜在源;4)在洪水期,泉水的水化学特征变化并未导致对大气CO2潜在贡献在源汇之间的跨跃性转变。162组岩溶水数据中,所有河水与溪水皆无一例外地在释放CO2。结果表明,从吉布斯自由能的热力学原理出发,研究岩溶水系统对大气CO2潜在源汇的贡献,没有条件约束,是一种较好的途径。     

白云鄂博超大型REE-Nb-Fe矿区碳酸岩墙的侵位年龄--兼答Le Bas博士的质疑

对白云鄂博稀土-铁-铌矿矿区一号碳酸岩墙内锆石U-Pb年龄进行了测定,三颗锆石测定数据点拟合直线与谐合线的上交点年龄为1416±77 Ma,它可能表明了白云鄂博矿区火成碳酸岩墙的侵入时代,而另外1颗锆石的表面年龄(1925±8 Ma)则可能是来自围岩的捕获锆石年龄.     

Characteristics of carbonatites from the northern part of the Korean Peninsula: A perspective from distribution,geology and geochemistry

The present study introduces the carbonatite in the northern part of the Korean Peninsula for the first time.Recent exploration and development of the phosphorus-bearing carbonate rocks in the area have accumulated new geological data which gave us an opportunity to study origin of the carbonate rocks.We conducted geological survey,geochemical analyses of trace elements and rare earth elements,and carbon and oxygen isotope analyses for the carbonatites from Ssangryong,Pungnyon,Yongyu and Puhung districts of the northern part of the Korean Peninsula.This research confirms that the phosphorus-bearing carbonate rocks are carbonatite originating from the mantle.The studied carbonatites are distributed at the junctions of ring and linear structures or around their margins and contain a greater amount of REEs,Y,and Sr than carbonate rocks.The carbonatites in Yongyu and Puhung area show evidence that they were formed from mantle plume generated at the lower mantle and display similar fractionation characteristics to carbonatites in Barrado Itapirapua in Brazil and Kalkfeld and Ondurakorume in Namibia.REE patterns of the carbonatites are typical of carbonatites and the carbon and oxygen isotope analyses demonstrate that the carbonatites were originated from mantle.The carbonatites from the northern part of the Korean Peninsula have a great potential for sources of REE,Y,PGE(platinum group elements),copper,and gold.     

巴西与碱性岩- 碳酸岩杂岩体相关的关键矿产成矿作用与规律

在巴西,与碱性岩- 碳酸岩杂岩体相关成矿作用复杂,产出了世界上唯一的霞石正长岩型斜锆石矿床、全球最大的铌矿床以及巴西最大的铀矿和镍矿集区,资源储量丰富,具有重要的经济价值和战略意义,同时也是关键矿产成矿作用研究的殿堂。本文在前人研究基础之上,整理分析了35处与碱性岩- 碳酸岩相关的关键矿产地质特征。根据成矿作用特征,划分了4个成矿系列:①霞石正长岩碱性杂岩相关的U- Zr- Mo- REE 和萤石成矿系列;②碱性岩- 碳酸岩杂岩(正长岩+辉石岩+霓霞岩+碳酸岩等)相关的Nb- P- Ti- REE和蛭石成矿系列;③阿拉斯加型超基性岩- 碱性杂岩体相关的Ni成矿系列;④交代正长岩相关的U- P成矿系列。根据关键矿产资源时空分布规律,划分了5个成矿带:(Ⅰ)Lancinha- Cubato碱性岩- 碳酸岩相关REE- P- U- Zr- Ni- Mo- 蛭石成矿带;(Ⅱ)Rio de Janeiro- Rondnia碱性岩- 碳酸岩- 金伯利岩相关的Nb- REE- Ti- Zr- P- 金刚石成矿带;(Ⅲ)Ceará- Mato Grossodo Sul 超基性岩- 碱性岩- 碳酸岩相关的Ni- U- P- 蛭石成矿带;(Ⅳ)亚马逊地区碳酸岩相关Nb- REE- Th- Ti- P成矿带;(Ⅴ)巴伊亚地区与变正长岩相关的U成矿带。本文还详细介绍了典型矿床成矿地质特征和成矿条件,为走出去寻找关键矿产资源基地提供了理论支撑。