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

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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.