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

在这项拟议的研究中，我们打算调查成岩流体对稀土元素(REE)、Nb和Zr的再动员、迁移和进一步浓缩的能力，从而探讨它们在经济的碳酸盐岩稀有金属矿床形成中的作用。碳酸盐岩是世界上最重要的稀土和Nb原材料来源。辉长岩流体是富碱、含水、流动性极强的反应性流体，通常从碳酸盐岩中排出，并对围岩的广泛高温交代蚀变负责，即辉长岩的形成。这一建议旨在验证碳酸盐中的碳酸盐((Ce，La，Nd，Y)(F，OH)CO3)、焦绿石((Na，Ca)2Nb2O6(OH，F，O))和斜方石(ZrO2)矿床的工作假说，这是由于成分介于卤水和水合盐熔体(4-20质量%H2O)之间的高浓度碱性碳酸盐流体对REE、Nb和Zr的自生再活化。这个想法是，稀有金属可以通过流体与岩浆、较早结晶的磷灰石、含稀土和含Nb的钙钛矿固溶体以及真透析岩的反应而重新活化。原生岩浆方解石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)通过目测和原位拉曼光谱实验研究Na2O-NaCl-CO2-H2O+REE2O体系在中低含水率下的相平衡。将特别注意流体的不混溶。(2)主要成矿矿物(钙钛矿-磷灰石固溶体、含稀土磷灰石、独居石、焦绿石、锆石、透辉石、斜长石)在碳酸盐-氯酸盐-硫酸盐卤水中的溶解度实验研究(0.1-0.2 Gpa和500-800℃)。(3)合成体系中实验结果与碳酸盐(焦绿石、磷灰石、磁铁矿等)形成较早或较早形成的矿物中的天然流体包裹体分析结果的对比。这项研究最重要的样本将在选定地点的实地工作期间收集。

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.