CH6 – Exploration for Rare Earth Elements – Allan Lake Carbonatite, Ontario, Canada

GamX CH6 Allan Lake eTh anomaly map


The Allan Lake Carbonatite is a small (0.4 km2), unexposed, REE-enriched intrusion into felsic igneous rocks (gneisses), detected in 1977 by a regional (5000 m line spacing) airborne gamma ray spectrometric survey flown by the GSC (see stacked airborne profile, bottom right). Follow-up investigations in 1983 led to discovery of a thorium-rich glacial dispersal train (Figure 1) containing carbonatite boulders. A detailed airborne survey was completed across the train to support additional ground studies, leading to two diamond drill holes (indicated) which confirmed the presence of the carbonatite1.








GamX CH6 Allan Lake Fenite Zone radial dikes



Minor phases include biotite silico-carbonatite, orthoclase-dolomite carbonatite, apatite-rich carbonatite and narrow zones of intense chloritization. Mineralogy includes ankerite and siderite with interstitial synchisite-basnaesite. Surrounding the stock, a poorly exposed fenite contains hematite and acmite veins, radial, lamprophyre and breccia dikes, grading outwards into brecciated host rock (Figure 2).

GamX CH6 Allan Lake Till Chemistry Down-Ice Profiles


The “textbook” example of a glacial dispersal train of lithologically distinctive till covers an area of approximately 10 km2 characterized by anomalous (10–20 times background) concentrations of Ba, Nb, Th, Ce, La, Zn, Mn and Fe; and elevated (5–10 times background) concentrations of Y, P, Cu, Pb, Mo, Co and U.

Within the till, carbonatite clasts, minerals derived from their mechanical disaggregation and related geochemical elements do not occur up-ice from the stock; all appear down-ice from the small pond which overlies the stock and their concentrations diminish with distance in that direction (Figure 3).




The vast majority of applications of gamma ray spectrometry use the “more easily detectable” radioactive elements solely as pathfinders, through recognition of anomalous enrichment (or depletion) associated with the commodities of economic interest. In this excellent example, there is clear association of eTh with REEs and other metals.

GamX CH6 Allan Lake Airborne Survey Stacked Profile








The small, unexposed intrusion is overlain by water and would not have been detected if not for the development of the dispersal train, effectively increasing the detectable footprint by many magnitudes. The stacked profile shown is from a flight line that passed about 750 m west of the carbonatite, cutting obliquely across the boulder train. Note the lack of uranium response, and no magnetic signature2.








1. Ford, K.L., Delabio, R.N.W. and Rencz, A.N. 1988. Geological, geophysical and geochemical studies around the Allan Lake carbonatite, Algonquin Park, Ontario. Journal of Geochemical Exploration volume 30, pp.99-121.


2. Thomas, M.D., Ford, K.L. and Keating, P. 2011.  Exploration geophysics for intrusion-hosted rare earth metals; Geological Survey of Canada, Open File 6828 (Poster).