Abstract

The Thor Lake rare metal (Zr, Nb, REE, Ta, Be, Ga) deposits in Canada’s Northwest Territories represent one of the largest resources of zirconium, niobium, and the heavy rare earth elements (HREE) in the world. Much of the potentially economic mineralization was concentrated by magmatic processes. However, there is also evidence of autometasomatic processes and remobilization of Zr and REE by hydrothermal fluids.

The deposits are situated at the southern edge of the Slave province of the Canadian Shield, within the 2094 to 2185 Ma alkaline to peralkaline Blachford Lake 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 the 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 which Zr is hosted primarily by zircon, Nb primarily by ferrocolumbite and fergusonite-(Y), and HREE by fergusonite-(Y) and zircon. The LREE are present mainly in monazite-(Ce), allanite-(Ce), bastnäsite-(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 μm) anhedral grains in pseudomorphs after probable eudialyte. In the upper zone, zircon is a magmatic cumulate mineral, which was replaced locally by secondary REE-bearing minerals. Element distribution maps of zircon crystals in the upper zone indicate that the HREE 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 remobilization of primary zirconium.

A model is proposed in which injection of separate pulses of miaskitic and agpaitic magma resulted in the crystallization of an upper zone rich in zircon and a lower zone rich in eudialyte. Primary eudialyte was later altered in situ to zircon-fergusonite-(Y)-bastnäsite-(Ce)-parisite-(Ce)-synchysite-(Ce)-allanite-(Ce)-albite-quartz-biotite-fluorite-kutnahorite-hematite–bearing pseudomorphs by an inferred fluorine-enriched magmatic hydrothermal fluid. Zirconium, niobium, and REE 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 REE (albitization generally dispersed REE and high field strength elements (HFSE)) and created new secondary REE-bearing phases.

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