Controls on the Concentration of Zirconium, Niobium, and the Rare Earth Elements in the Thor Lake Rare Metal Deposit, Northwest Territories, Canada

The Thor Like rare metal (Zr, Nb, REE, Ta, Be, Ca) deposits in Canada's Northwest Territories represent one of the largest resources of zirconium, niobium. and the heavy rare earth elements (FIB E IF,) in the world. Much of the potentially ecolunnic mineralization was concentrated by magmatic processes. However, there is also evidence of autometasomatic processes and remobilization of Zr and REE by liydrot henna! fluids. The deposits are situated at the southern edge of the Slave province of the Canadian Shield, within the 2091 to 2185 Ma alkaline to peralkaline Blachford Like Intrusive Complex. A layered alkaline suite dominated by aegirine nepheline syenite occurs in the center of this suite of rocks and is considered to represent die youngest phase of the complex. Much of the rare metal mineralization occurs in two subhorizontal tabular layers, which form upper and lower zones of the Nechalacho deposit (formerly the Lake zone), and in Zr is hosted primarily be zircon. Nb primarily by ferrocolumbite and fergusonite-(Y), and HREE by fergusonite-(Y) and zircon. The LREE are present mainly in nionazite-(Ce), allanite-(Ce), bastnasite-(Ce), parisite-(Ce), and synchysite-(Ce). Much of the HREE mineralization in the lower mineralized zone occurs in secondary zircon, which forms small (10-30 mu m) anhedral grains in psendomorphs after probable endialyte. In the upper zone, zircon is a magmatic cumulate mineral, which was replaced locally by secdulary BEE-bearing minerals. Element distribution maps of zircon crystals in the tipper zone indicate that the FIBEB were mobilized from the cores and locally precipitated as fergusonite-(Y) along microfractures. The light rare earth elements (LREE) were also mobilized locally from both primary zircon and inferred primary eudialyte. The occurrence of zircon in fractures, wrapped around brecciated K-feldspar fragments, and as a secondary phase in pseudomorphs are evidence of its hydrothermal origin and/or of remobilizatimi of prilflarY zirconnun. A model is proposed in which injection of separate pulses of iniaskitic and agpaitic magma resulted in the crystallization of an upper zone rich in zircon and a lower zone rich in dutiable. Primary euclialyte was later altered in situ to zircon-fergusouite-(Y)-bastniisite-(Ce)-parisite4Ce)-synchysite-(Ce)-allanite-(Ce)-albitequartz-biotite-fluorite-kutualiorite-hematite-bearing pseudomorphs by an inferred illumine-enriched magmatic hydrothernial fluid. Zirconiiim, niobinni, and BEE in both the upper and lower Zones were subsequently mobilized during multiple metasomatic events, which, for the most part, served to further enrich the primary layers in HEE (albitization generally dispersed BEE and high field strength elements (HESE)) and created new secondary H E phases.