Direct UPb dating and Nd isotopes of REE carbonate mineral unravel protracted history of ore precipitation at the Thor Lake (Nechalacho) deposit, NWT Canada

Rare Earth Elements are classified as critical resources in Canada and the Thor Lake (Nechalacho) REE deposit is one of the largest in North America. The Thor Lake area is composed of the Nechalacho Layered Suite (previously dated at ca. 2176 Ma) and multiple pegmatitic bodies with variable REE enrichment. This study focuses on the pegmatitic T-zone where bastnäsite, the main REE mineral, is abundant and displays both magmatic and hydrothermal features. The T-zone pegmatite is believed to be a late magmatic event and this study uses UPb geochronology on bastnäsite as well as Nd isotopic constraints to understand the late stages of emplacement and REE metal sources that led to mineralization. This study determines the first direct ages on the ore mineral of this deposit. Several populations of REE carbonate, corresponding to different formation ages, were distinguished according to their Ca enrichment. Unaltered bastnäsite yields a weighted mean age of 2050.9 ± 34.9 Ma (propagated; MSWD = 2.2) that we interpret as the exsolution of a hydrothermal fluid from the T-zone pegmatite melt. This shows that the T-zone was emplaced ca. 100 Ma after the Nechalacho Layered Suite, coeval with the rift initiation along the southern margin of the Slave craton. Nd isotope compositions further show that the T-Zone and the Nechalacho Layered Suite, although disconnected in time, share the same REE-enriched source, demonstrating that mineralization was reactivated but dominated by the same original REE source that was probably remelted, despite subsequent “up-grading”. The alteration of bastnäsite into parisite is traced by its enrichment in Ca. The altered bastnäsite (parasite) yields one UPb age of 1763.9 ± 47.6 Ma (propagated; MSWD = 7.5) that we interpret as correlated to crustal hydrothermal fluid events. The ca. 1764 Ma age is coeval to the Great Slave Lake shear zone reactivation and likely generated through large-scale crustal fluid flow related to that event. Another calculated age at ca. 820 Ma could not be correlated to any specific event and is likely the result of Pb loss and other fluid-related alteration process. This study highlights the complex multiphased history of the Thor Lake (Nechalacho) deposit and the importance of hydrothermal processes for bastnäsite mineralization.