秦岭造山带碳酸岩稀土成矿机理研究

Rare earth elements (REE) are an important strategic resource. China contains abundant REE from diversity and unique types of deposits, which attracts many geologists worldwide. The carbonatite associated REE deposits in Qinling Orogen are especial, and are composed of magmatic and hydrothermal types. The rocks contain abundant phosphate and fluorcarbonate REE minerals, especial rare xenotime. The carbonatite-derived fluid metasomatized the wall-rocks and formed REE ore veinlets. The carbonatites in south and north Qinling show different geochemical features and REE mineral compositions, which is an natural laboratory for comparatively studying the evolution of the REE ore-forming fluids and the fractionation of light and heavy REE. associated with carbonatites. In this application, the REE deposits (spots) associated with carbonatites from Miaoya in South Qinling and Huayangchuan, Yuantou and Huanglongpu in Lesser Qinling are compared based on geological and geochemical studies. Moreover, high T and P experiments are designed to study the REE partition behavior between magma-hydrothermal interaction system. The aim is to study the relationship between carbonatite source compositions and REE mineralization, and REE fractionation, enrichment processes, and to probe similarity and difference of ore-forming fluids from magmatic and hydrothermal deposits, and LREE and HREE fractionation. Comparison with studied results of REE mineralization in Panxi region, this will be perfect to establish the metallogenic model of the important carbonatite REE deposits in China.

稀土是重要的战略资源，我国稀土资源丰富，矿床类型多样且独特，为世界矿床学界所瞩目。秦岭造山带内碳酸岩型稀土矿床（点）最为典型，包括了岩浆和热液两种类型。岩体内富含磷酸盐和氟碳酸盐稀土矿物，特别是罕见的磷钇矿；碳酸岩岩浆分离的热液流体交代围岩也形成了大量的细网矿脉。南、北秦岭两地碳酸岩的地球化学和稀土矿物组成各有特色，它们是对比研究碳酸岩稀土成矿流体演化和轻、重稀土分异的天然实验室。本申请拟对南秦岭庙垭和秦岭北端小秦岭华阳川、垣头和黄龙铺四处矿床（点）进行系统的地质、地球化学对比分析，高温高压实验模拟稀土在富挥发份碳酸岩岩浆-热液体系中的配分行为，研究碳酸岩源区物质组成、演化与稀土成矿的联系以及稀土在碳酸岩岩浆和热液体系的分异、富集过程，揭示两种类型成矿流体的共性和差异，探讨轻、重稀土元素分异的内在因素。对比攀西稀土成矿的研究成果，完善我国重要且具特色的碳酸岩型稀土矿床的成矿模式。

稀土元素是现代工业的“维生素”，也是各国目前竞相争夺的战略资源。相对于轻稀土，重稀土是未来新能源技术更为重要的原材料，并且面临着严重的资源短缺风险。目前，全球几乎所有的重稀土来自我国南岭地区的花岗岩风化壳型稀土矿床，研究其它类型的重稀土矿床迫在眉睫。本项目针对我国秦岭造山带南北两地碳酸岩稀土成矿作用进行对比研究。两地碳酸岩的稀土富集程度明显不同，南秦岭碳酸岩富集轻稀土，是大型轻稀土矿床，北方小秦岭碳酸岩富集重稀土。同时，开展了内蒙古白云鄂博和新疆巴楚碳酸岩型稀土矿床的对比研究，丰富了碳酸岩稀土成矿理论。首次在小秦岭碳酸岩内发现原生的重稀土矿物，包括磷钇矿、富重稀土硅酸盐矿物、富重稀土氟碳酸盐矿物和富Y烧绿石和褐钇铌矿，说明小秦岭碳酸质流体富含重稀土；在相邻草滩发现白云石与方解石-橄榄石共生的碳酸岩，两者显示C-Sr同位素解耦，前者显示高的δ13CVPDB（–3.09至–3.58 ‰）和低的Sr同位素（0.70368-0.70560），方解石碳酸岩则具有更低的δ13CVPDB（–6.11至–7.19 ‰）和更高的Sr同位素（0.70560-0.70619），后者与小秦岭其它方解石碳酸岩相似。根据草滩岩石学和地球化学证据，我们提出小秦岭方解石碳酸岩是初始白云质岩浆交代地壳富钙物质的产物，导致方解石碳酸岩富集重稀土。对南秦岭庙垭正长岩和碳酸岩内锆石和独居石原位定年分析，确定碳酸岩内锆石是捕获的产物，与正长岩内锆石U-Pb年龄相似约为440 Ma，碳酸岩内原生独居石和正长岩内次生交代独居石均有相似Th-Pb年龄约为220 Ma，限定庙垭碳酸岩与正长岩并无成因联系，成岩成矿年龄为三叠纪，与勉略洋俯冲有关。在白云鄂博富稀土白云岩内识别三种类型的磷灰石和四种独居石。三类磷灰石具有不同的稀土元素和配分模式，都具有高的Sr含量（5461-6892 ppm），与秦岭碳酸岩内磷灰石相似，指示火成碳酸岩起源；识别裂谷型巴楚富稀土碳酸岩具有低的初始Sr同位素比值（0.7037-0.7041）和高的εNd(i) (1.2-4) 以及沉积的Mg同位素组成（-0.99至-0.65‰），指示裂谷型碳酸岩地幔源区混合俯冲循环地壳物质是稀土富集的关键因素。