As with nearly all significant deposits, the Thor Lake site has a long pre-discovery history, from early GSC bedrock mapping (J.F. Henderson in 1937), staking (K.D. Hannington in 1970), airborne anomaly detection1 (GSC gamma ray survey, 1971, Figure 1), intermittent exploration and discovery by a number of optionors, and finally, purchase/development (Avalon Rare Metals Inc. 2005-present). The resulting Nechalacho deposit is a major reserve of rare metals (see Table below), particularly rich in the heavy rare earth elements (REE)2.
Mineralization includes light and heavy rare earth elements, yttrium, zirconium, niobium, tantalum and gallium in a variety of minerals within a flat-lying, vertically zoned, undeformed system hosted by the alkaline Blatchford Lake Complex (BLC).
Intrusive phases within the zoned BLC range from older pyroxenite-gabbro to granite, peralkaline granite, and younger syenite. Strong geochemical zoning across the BLC is reflected by radioactive element zoning, best illustrated on the Ternary Map, and by strong magnetic total field patterns (Figure 2). The maps show how small the radiometric footprint of the discovery anomaly is, (bright yellow spot on Ternary) so it was perhaps lucky that the 1971 survey flight line passed directly over the zone.
Nechalacho Deposit Mineral Resource Estimate May 3/132
A 20-year mine is expected to begin production in 2017/18. Material will be mined, crushed and concentrated on-site, barged/trucked to Pine Point for hydrometallurgical processing, then shipped by rail to Geismar, Louisiana for separation of the REEs.
The U and Th contents of the Nechalacho resource are low to moderate (28 ppm U3O8, 139 ppm ThO2), thus posing no environmental or health concerns related to extraction, transport or processing3. However, the concentrations are sufficient to provide a very useful exploration signal (aka “anomaly) relative to the local background concentrations, serving as pathfinders to the mineralizing system.
The airborne gamma ray spectrometric surveys played an important role in the early recognition of the potential of this area, and supported subsequent regional and local bedrock mapping.
1. Charbonneau, B.W.C. and Legault, M.I. 1994. Interpretation of airborne geophysical data for the Thor Lake area Northwest Territories; in Studies of Rare Metal Deposits in the Northwest Territories, Sinclair, W.D. and Richardson, D.G., editors. Geological Survey of Canada Bulletin 475, pp. 17-31
2. http://www.avalonraremetals.com/_resources/projects/may_2013_ni43_report.pdf
3. http://www.avalonraremetals.com/_resources/projects/pine_point_pathways.pdf