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Use of Gamma Spectrometry in Ontario for Rare Earth Element Exploration

The Thorium-bearing Allan Lake carbonatite, located in Algonquin Park, Ontario, is a small (0.4 km), unexposed ankeriticsideritic instrusion hosted by gneisses. Although the intrusion is surrounded by a poorly exposed fenitic aureole, characterized by radial, hematitic veins (fig.B) and breccia dykes (fig. C) it does not outcrop. However, glaciation has produced a large, distinctive dispersal train which was discovered and effectively mapped using airborne (fig. A) and ground gamma ray spectrometry.

During ground investigations, in-situ spectrometry, as provided by the GR-320, rapidly delineated the extent of the dispersal train (fig. D) and lead to the discovery of many carbonatite boulders, particularly whose with high concentrations of rare earth elements (REE's)

Geochemical analysis of drill core, boulder and till samples shows that carbonatite is enriched in FE, Mn, Zn, Th, Ce, La, and Ba, with elevated concentrations of Y, P, Cu, Pb, Mo, Co, and U. The intimate association of Th with Nb and Zn, and the diminishing, "down-ice" dispersion of these elements, is illustrated in fig. D.

Unlike many other alkalic/ carbonatite complexes, the Allen Lake instrusion lacks a prominent magnetic signature. This underscores the importance of incorporating modern, calibrated, digital airborne and ground gamma ray spectrometric techniques into regional and detailed mapping or exploration programs relating to the search for these deposits.

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