Volker Möller


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Möller, V. and Williams-Jones A.E., 2011.

Controls on the formation of rare earth and other high field strength minerals in the Nechalacho rare metal deposit - Thor Lake, Northwest Territories (NWT Geoscience Forum, Yellowknife 2011)


The Nechalacho rare metal deposit in the Northwest Territories comprises indicated reserves of 88.13 Mt grading 1.53% total rare earth oxide (TREO, including Y) and inferred reserves of 223.57 Mt grading 1.31% TREO. Its remarkable enrichment in heavy rare earth elements (REE) in a laterally continuous cumulate layer and high niobium, tantalum and zirconium concentrations render the Nechalacho deposit a world-class resource of the REE and high field strength elements (HFSE). The dominant REE- and HFSE-bearing minerals are zircon, bastnäsite, allanite, fergusonite, monazite, xenotime and columbite. Based on geological relationships, bulk-rock and mineral chemistry and textural evidence, we present a preliminary model for the magmatic and hydrothermal processes leading to the enrichment of rare metals in the Nechalacho deposit.


Located 100 km East of Yellowknife on the north shore of Great Slave Lake, the Nechalacho deposit is hosted by the Paleoproterozoic Nechalacho layered syenite suite (NLSS), which forms a chemically distinct, dome-shaped intrusive body in the centre of the Blachford Lake Intrusive Suite. While the surrounding Grace Lake Granite and Thor Lake quartz-syenite are dominated by riebeckite amphibole and hypersolvus K-feldspar, the silica-undersaturated NLSS contains end-member aegirine, iron-rich biotite, subsolvus feldspars in the upper part and secondary riebeckite and arfvedsonite. Metasomatism in the Thor Lake syenite is evidenced by common aegirinisation and fluorination near the contact to the NLSS. The at least 1300 m thick layered syenites can be divided from top to bottom into a sodalite cumulate, feldspar-rich syenites, aegirine-biotite foyaites which host the zircon and eudialyte cumulate ore zones, micro-layered aegirine syenites, a biotite-dominated sequence and lower aegirine foyaites. Modal and phase layering is present on both, a macro- and micro-scale and is likely driven by density flotation and gravitational settling within the magma chamber. The previously unknown biotite-sodalite syenites contain perthite, primary fluorite, pyrochlore, eudialyte, britholite and REE-fluorocarbonates. The presence of cryptic layering is indicated by variations in biotite composition between siderophyllite and locally more Mg-enriched varieties. Clinopyroxene compositions of all units range from pure acmite in the most evolved, central upper part of the NLSS to aegirine-augite near the margins and bottom of the currently known succession. From nepheline thermometry and perthitic-exsolved alkali feldspar compositions minimum liquidus temperatures of 650 to 900 °C can be inferred.

Low-viscosity flow textures and the presence of primary villiaumite (NaF) and fluorite provide evidence of an unusually fluorine-rich magma which was capable of effectively dissolving and concentrating the REE and other HFSE. We propose that in-situ upward fractional crystallization within the peralkaline NLSS led to the enrichment of incompatible elements in the aegirine-biotite foyaites, forming the zircon and eudialyte cumulates. While the upper rare metal zone contains primary REE-enriched zircon, in the lower REE-mineralized layer zircon only became stable during the alteration stage, where it replaced a primary eudialyte-group mineral. Further alteration of the mineralized zones led to the formation of secondary bastnäsite-(Ce), other REE-fluorocarbonates and allanite.

The evolved and fluorine-rich nature of the magma, the formation of cumulus layers and intense alteration were all key factors for the formation of the Nechalacho rare metals deposit.



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