The role of fenitising fluids in the formation of Nb, Zr and REE ore deposits

在本研究中，我们打算研究利用流体对稀土元素（REE）、Nb和Zr的再动员、运输和进一步富集的能力，以及它们在碳酸盐中经济稀有金属矿床形成中的作用。碳酸盐岩是世界上稀土、铌等重要原料的重要来源。粉煤化流体是一种富含碱的含水流体，具有很强的流动性和反应性，通常是从碳酸盐岩中排出的，它是造成乡村岩石广泛高温交代蚀变的原因，即粉煤化流体的形成。该提案旨在验证以下工作假设：碳酸盐中的bastnäsite (（Ce,La,Nd，Y)(F，OH)CO3）、焦绿石(（Na,Ca)2Nb2O6(OH，F，O)）和差辉石（ZrO2）沉积物是由成分介于盐水和水合盐熔体（4-20质量% H2O）之间的高浓度碱碳酸盐流体自动再活化REE、Nb和Zr所致。这个想法是，稀有金属可以通过流体与岩浆、早期结晶的磷灰石、含REE和nb的钙钛矿固溶体和二析出物的反应而重新活化。原生岩浆方解石的重结晶也可能释放出大量的稀土元素。本研究的具体目标包括：(1)利用热液金刚石砧池，通过目视观察和原位拉曼光谱，对低至中等含水量下na20 - nacl - co2 - h2o±REE2O3体系的相平衡进行实验研究。将特别注意流体的不混相性。(2)在0.1 ~ 0.2 GPa、500 ~ 800℃条件下，实验研究了关键成矿矿物（钙钛矿-绿长石固溶体、含ree磷灰石、独居石、焦绿石、锆石、双析液和坏辉石）在碳酸-氯化物-硫酸盐盐水中的溶解度。(3)将合成体系的实验结果与碳酸盐（焦绿石、磷灰石、磁铁矿等）中形成较早或相对较早的矿物（尤其是石英，也包括碱长石等）中形成的天然流体包裹体的分析结果进行比较。研究中最重要的样本将在选定地点的实地工作中收集。

In the proposed study, we intend to investigate the capacity of fenitising fluids for remobilization, transport, and further concentration of rare earth elements (REE), Nb and Zr, and thus their role in the formation of economic rare metal ore deposits in carbonatites. Carbonatites are the world’s most important source for the critical raw materials REE and niobium. Fenitising fluids are alkali-rich hydrous very mobile and reactive fluids typically expelled from carbonatites, and that are responsible for extensive high-temperature metasomatic alteration of country rocks, i.e. for the formation of fenites. This proposal is aimed at testing the working hypothesis that bastnäsite ((Ce,La,Nd,Y)(F,OH)CO3)), pyrochlore ((Na,Ca)2Nb2O6(OH,F,O)) and baddeleyite (ZrO2) deposits in carbonatites result from autometasomatic remobilisation of REE, Nb and Zr by highly concentrated alkali carbonate fluids with compositions intermediate between brines and hydrated salt melts (4–20 mass% H2O). The idea is that the rare metals can be remobilised by reactions of fluid with magmatic, earlier crystallised apatite, REE- and Nb-bearing perovskite solid solutions, and eudialyte. Significant amounts of REE may be also released by recrystallisation of primary magmatic calcite. Specific objectives of the proposed research include: (1) Experimental study of phase equilibria in the system Na2O–NaCl–CO2–H2O±REE2O3 at low to intermediate water contents by visual observations and in situ Raman spectroscopy using a hydrothermal diamond-anvil cell. Particular attention will be paid to fluid immiscibility. (2) Experimental study of the solubility of key ore-forming minerals (perovskite-loparite solid solutions, REE-bearing apatite, monazite, pyrochlore, zircon, eudialyte and baddeleyite) in carbonate-chloride-sulphate brines at 0.1–0.2 GPa and 500–800 °C. (3) Comparison of experimental results in synthetic systems with analyses of natural fluid inclusions in minerals formed early or relatively early in carbonatites (pyrochlore, apatite, magnetite, and others) and in minerals from fenites (particularly quartz, also alkali feldspar, and others). The most important samples for the study will be collected during field work at selected locations.