Controls on REE ore formation in peralkaline igneous complexes: Insights from the deposits at Thor Lake, Northwest Territories

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Technological breakthroughs ranging from the development of environmentally friendly hybrid cars to more powerful lasers and supermagnetic materials are creating a strong demand for the REE. Currently, 97% of REE production is from deposits in China, which is now curtailing exports of this commodity. The deposits currently being explored at Thor Lake in the NWT of Canada have the potential to fill this impending shortfall. These deposits are hosted primarily by a large, silica-undersaturated peralkaline intrusion. Most importantly, unlike similar but uneconomic deposits elsewhere, the deposits at Thor Lake have undergone strong hydrothermal alteration, which has enhanced the REE mineralisation. The purpose of the proposed research is to expand on research conducted as part of a recent NSERC-CRD project, which has demonstrated that the REE mineralisation is hosted by a layered igneous complex, that primary REE concentration was achieved by fractional crystallization and the formation of REE-rich cumulates, and that hydrothermal fluids further concentrated the REE. The research proposed in this application will identify the mechanisms that produced the cumulate zircon and eudialyte which constitutes the upper and lower ore zones in the Nechalacho deposit, and the conditions that permitted the breakdown of the primary ore minerals and the mobilisation of the REE both within the Nechalacho layered sequence and into satellite bodies beyond it. This research will lead to a comprehensive genetic model for the deposits that will be achieved by reconstructing 1) the igneous petrogenesis of the layered igneous complex using a combination of petrographic, mineral chemical, bulk rock geochemical and isotopic methods, and 2) the subsequent hydrothermal mobilisation using petrographic, mineral chemical, fluid inclusion, phase equilibrium, experimental and thermodynamic analyses. The results of the study will shed new light on the origin of a very poorly understood strategic class of deposits and identify new guidelines for their exploration in Canada and abroad.