THE NECHALACHO HEAVY RARE EARTH AND RARE METAL DEPOSIT, THOR LAKE, NORTHWEST TERRITORIES, CANADA
The Nechalacho deposit is hosted by the Aphebian Blachford Lake alkaline layered intrusive complex emplaced within the Slave Structural Province. The Lake Zone REE mineralization is hosted in a tabular hydrothermal alteration zone, in a previously unrecognised nepheline-sodalite syenite unit of the complex. It is exposed over an area in excess of one km2 and averages 100-150m thick in thickness. It is characterized by near-complete replacement of the primary mineral assemblage by chlorite, magnetite, biotite, zircon, monazite, allanite, bastnaesite and fergusonite. The deposit exhibits a distinct horizontal layering with HREE content generally increasing towards the base of the deposit. The lowermost layer, called the “Basal Zone” contains between 1.5 and 2.5% total rare earth oxides (TREO) over thicknesses of order of 20m, with HREE ranging between 15% and 30% of the total REE present.
A geological model is presented that includes the REE and rare metals being originally precipitated as cumulate minerals as part of the magmatic process, possibly as eudyalite, zircon and other complex REE minerals. Subsequent hydrothermal activity altered these to the presently seen assemblage of REE minerals resulting in bands of REE-Nb-Ta-Zr mineralization dominated by zircon. The light and heavy rare earths may have behaved differently during this alteration process resulting in both REE enrichment and differential zonation of these elements.
The Nechalacho deposit is thus a result of a complex interplay of magmatic and hydrothermal processes, complicating geological interpretation.