Behaviour of zirconium, niobium, yttrium and the rare earth elements in the Thor Lake rare-metal deposit, Northwest Territories, Canada

The Thor Lake rare-metal (Zr, Nb, Y, REE, Ta, Be, Ga) deposit in the Northwest Territories of Canada represents one of the largest resources of zirconium, niobium, yttrium and the heavy rare earth elements in the world. Much of the potentially economic mineralization was concentrated by magmatic processes. However, there is also evidence of remobilization of Zr, Y and REE by hydrothermal fluids. Geologically, the deposit is situated at the southern edge of the Slave Province of the Canadian Shield, within the 2150 Ma alkaline to peralkaline Blachford Lake Complex (Davidson, 1978).;A model is proposed in which REE and HFSE enriched layers were formed by injection of separate miaskitic and agpaitic magmas, to form an upper zone rich in zircon and a lower zone rich in eudialyte. Primary eudialyte was later altered to zircon-fergusonite-(Y)-bastnasite-(Ce)-parisite-(Ce)/synchysite-(Ce)-allanite-(Ce)-albite-quartz-biotite-fluorite-kutnahorite-hematite-bearing pseudomorphs by an inferred fluorine-enriched magmatic hydrothermal fluid. Yttrium and REE were also mobilized from the cores of primary zircon crystals in the upper zone and locally precipitated as fergusonite-(Y) along micro-fractures. Hydrothermal fluids altered the distribution of REE, within the upper and lower mineralized zones, locally enriching and depleting them, and created a set of secondary minerals which will be the main target of future exploitation.;The rare metal mineralization occurs in two sub-horizontal tabular layers in the Nechalacho deposit, an upper and a lower zone, with Zr hosted primarily by zircon, Nb primarily by ferrocolumbite and fergusonite-(Y), and Y + HREE by fergusonite-(Y) and zircon. The LREE are hosted by monazite-(Ce), allanite-(Ce), bastnasite-(Ce) and parisite-(Ce)/synchysite-(Ce).