REPLACEMENT PROCESSES INVOLVING HIGH FIELD STRENGTH ELEMENTS IN THE T ZONE, THOR LAKE RARE-METAL DEPOSIT

Abstract

The Thor Lake rare-element (Y-REE-Nb-Ta-Zr-Be) deposit, Northwest Territories, Canada, is a world-class deposit hosted by alkaline syenite and granite and has two main mineralized zones, namely the Nechalacho and T Zones deposit. The T Zone is shown to have originally been a pegmatite, but has experienced a strong hydrothermal overprint. Coarse-grained to megacrystic, elongate crystals defining the pegmatitic texture are completely pseudomorphed and are interpreted to have originally been nepheline. The T Zone underwent three main alteration stages, which are (from earliest to latest): (I) silicification of the principal phases of the pegmatite and the associated replacement of igneous aegirine and arfvedsonite by quartz + magnetite + high field strength element (HFSE)-bearing minerals; (II) polylithionite [KLi 2 AlSi 4 O 10 (F,OH) 2 ] alteration (Li metasomatism) that caused pervasive replacement of albite by polylithionite; and (III) phenakite (Be 2 SiO 4 ) alteration (Be metasomatism). Replacement of the primary mineralogy resulted in the formation of abundant pseudomorphs that are characterized by various HFSE minerals [typically, bastnasite-(Ce), zircon, rutile, xenotime-(Y), and columbite] and phenakite (the predominant Be mineral). The habit and mineralogy of the pseudomorphs, as well as textural relationships in partially altered crystals, make aegirine and mica-group minerals the most likely precursors of the HFSE-rich and Be-rich pseudomorphs, respectively. Although aegirine and arfvedsonite from alkaline rocks can contain HFSE, LA-ICP-MS analysis of aegirine and arfvedsonite from the T Zone indicates that the HFSE concentrations in these two minerals are insufficient to account for the bulk HFSE concentrations of the HFSE-rich pseudomorphs. Therefore, HFSE must have been added to the pseudomorphs during fluid-mineral interaction, requiring hydrothermal mobility of HFSE. An open-space filling process may have been involved in the precipitation of HFSE minerals, including zircon, rutile, anatase, and REE fluorcarbonates. Initial replacement of aegirine by quartz + magnetite increased porosity in some of the T Zone rocks, which may have facilitated HFSE precipitation. Our study also demonstrates that most hydrothermal zircon shows no association with fluorite and that the fluorite in REE-rich pseudomorphs commonly postdates the REE fluorcarbonate minerals [ e.g. , bastnasite-(Ce)]. These relationships indicate that HFSE precipitation in the T Zone requires a model that does not rely on the mixing of HFSE-, F-bearing fluids with Ca-rich fluids, as has been proposed for other HFSE deposits hosted by alkaline igneous rocks. As the precipitation of HFSE appears to be restricted to pseudomorphs where aegirine and micas have been replaced, the degree of hydrothermal enrichment of HFSE in the T Zone, and possibly in other deposits containing such minerals, may be dictated by the primary abundance of these minerals.