碳酸岩型稀土矿床成矿流体演化机制研究现状及展望

Research status and prospect of the evolution mechanism of ore-forming fluids for carbonatite-hosted REE deposits

与碱性岩有关的碳酸岩型内生稀土矿床在中国乃至世界上轻稀土资源储量中占有极为重要的地位,诸如我国内蒙古的白云鄂博稀土矿床、川西冕宁—德昌稀土成矿带中的牦牛坪、大陆槽等稀土矿床、山东微山县郗山稀土矿床以及美国的Mountain Pass稀土矿床等都属于这种类型的稀土矿床。当前,对于这类稀土矿床的成矿流体演化机制,学界主要存在结晶分异作用、不混溶作用(熔体-熔体不混溶、熔体-流体不混溶以及流体-流体不混溶)以及热液交代蚀变作用之间的分歧。结晶分异作用可以使具有不相容性的稀土元素在残余熔体相中逐渐富集,直至形成稀土矿物。不混溶作用能够使稀土元素在不混溶后形成的两相或多相中的某一相中发生选择性富集,形成稀土矿化。成矿流体演化晚阶段的热液流体对早期生成的矿物或围岩进行交代蚀变,使其释放出能与稀土元素在热液中形成络合物的F-、(CO_3)~(2-)以及(SO_4)~(2-)等阴离子(团),并最终在合适的构造控矿部位和外界环境条件下,重结晶或沉淀出稀土矿物。上述3种观点各有其理论依据,但是在解释一些碳酸岩型稀土矿床地质现象或实验地球化学模拟结果的时候都或多或少存在一定程度上的不足。前人的研究结果表明,碳酸岩型稀土矿床中发育了大量的熔体包裹体、熔体-流体包裹体以及富CO_2的流体包裹体,以往在利用Linkam TS1400XY以及Linkam THMS600等这类常规高温热台,在101325 Pa条件下对其进行热力学测温时,这些包裹体大多在尚未达到完全均一状态前就已发生爆裂或泄露,极大制约了人们对这类稀土矿床在高温岩浆阶段和中高温岩浆-热液阶段成矿流体演化过程的认知。另外,对于稀土元素在成矿流体演化过程中的含量变化特征及其地球化学行为的研究,目前主要是通过包裹体成分组成的拉曼光谱分析,以及对矿体和围岩进行的全岩地球化学分析,尚缺乏单个包裹体中元素含量的原位微区分析方面的数据。未来,对碳酸岩型稀土矿床中发育的熔体包裹体、熔体-流体包裹体和富CO_2的流体包裹体,利用热液金刚石压腔开展高温高压原位均一实验模拟研究,以及对单个包裹体中微量元素的含量利用LA-ICP-MS进行原位微区分析,将是揭示该类稀土矿床成矿流体演化机制的关键。

Carbonatite-hosted endogenetic rare earth element(REE)deposits related to alkaline rocks are very important in light rare earth resources in China and even in the world.At present,controversies for the evolution mechanism of ore-forming fluids for carbonatite-hosted REE deposits are mainly among crystallization differentiation,liquid immiscibility(melt-melt,melt-fluid and fluid-fluid immiscibility)and hydrothermal metasomatic alteration.Crystallization differentiation can gradually enrich the incompatible REEs in the residual melt phase until the REE minerals are formed.Immiscibility can lead to selective enrichment of REEs in one of the two or multiple-phases formed after immiscibility,resulting in REE mineralization.The hydrothermal fluids formed in the late stage of ore-forming fluids evolutionary process have metasomatic reaction with the early-formed minerals or surrounding rocks and release anions(anion clusters)such as F-,(CO3)2-and(SO4)2-,which can form complexes with REEs in the hydrothermal aqueous solution,and finally recrystallize or precipitate REE minerals in appropriate ore-hosting structures under suitable external conditions.Each of the above three viewpoints has its own theoretical basis,but they are more or less inadequate in the explanation of some geological phenomena or experimental geochemical simulation results of carbonatite-hosted REE deposits.The previous study results shown that there are a large number of melt inclusions,melt-fluid inclusions and CO2-rich fluid inclusions in carbonatite-hosted REE deposits.In the past,most of these inclusions decrepitated or were leaked before reaching the total homogenization status when heated at 101325 Pa by using conventional high temperature heating stages,such as Linkam TS1400XY and Linkam THMS600,which greatly restrict our understanding of the evolutionary process of ore-forming fluids in high temperature magmatic stage and medium-high temperature magmatic-hydrothermal stage for this type of REE deposits.In addition,studies on the contents variation characteristics and geochemical behavior of REEs in the ore-forming fluid evolutionary process is mainly through the Raman spectroscopy analysis of the components of inclusions,as well as the whole rock geochemical analysis of ore bodies and surrounding rocks,and there is still a lack of in-situ microanalysis data about the element contents of individual inclusions.In the future,for melt inclusions,melt-fluid inclusions,and CO2-rich fluid inclusions that trapped in this type of deposits,in-situ high temperature and high pressure microthermometry experiments by employing hydrothermal diamond-anvil cell together with in-situ LA-ICP-MS microanalysis of trace elements contents in individual inclusions,are supposed crucial to reveal its evolution mechanism of ore-forming fluids.