CH4 – Exploration for Uranium – Bancroft Area, Ontario, Canada

GamX CH4 Bancroft U Exploration Maps



In the Bancroft area, Ontario, Canada, Precambrian uranium deposits (black dots, Figures 1,2) occur within the margins of large gneissic domes (pink, Figure 4), either in enclosing metasediments (crystalline limestone, paragneiss, amphibolite) or plutonic rocks (older gabbro, younger granite, syenite, and pegmatitic phases). The U deposits have been divided into pegmatite, metasomatic or vein types1. The most common radioactive minerals include uraninite, thorite, uranothorite, REE-bearing minerals, secondary alteration products (uranophane) and accessory mineral phases.


Episodic exploration and development in the area dates back to 1929, with a mid-1950s peak in activity following airborne and ground scintillometer surveys. Uranium was produced mainly from four mines, at Faraday, Bicroft, Dyno and Greyhawk (labeled in Figure 3): Several hundred additional showings and prospects have been discovered in the area.



The map images are derived from regional surveys flown by the Geological Survey of Canada2. Note the strong eU and eTh anomalies associated with known radioactive deposits.




GamX CH4 Bancroft High eU-eTh ratio profile

GamX CH4 Bancroft Low eU-eTh ratio profile





The absolute concentrations of all three radioactive elements (K, eU and eTh) and the relative concentrations indicated by their ratios (eU/eTh, eU/K and eTh/K) provide important clues to deposit geology and mineralogy.


Deposits enriched in uranium relative to thorium appear as strong positive eU/eTh ratio anomalies, with low K. An example is illustrated in profile and map forms in Figure 5.














Other deposits, where both uranium and thorium bearing minerals may occur, produce no eU/eTh ratio anomaly and may have abundant K as shown in Figure 6.




Therefore, in addition to the obvious, primary role in U exploration targeting, the airborne gamma ray spectrometric patterns also support strategic discrimination of end-member types of deposits in the area.


Elsewhere in Canada, significant airborne eU anomalies overlie known and new U discoveries. Examples include Elliot Lake, Uranium City, Eldorado, portions of the Athabasca and Thelon Basins, South March, South Maitland and many other areas, in nearly all Provinces. Indeed, the early successes of gamma ray spectrometry applied to uranium exploration in Canada generated the long-standing misconception that the method was a “uranium-only” exploration tool.  Fortunately, this has been clearly demonstrated to not be the case!






  1. Satterly, J., and Hewitt, D.F., (1955): Some radioactive mineral occurrences in the Bancroft Area; Geological Circular No.2, Ontario Department of Mines.


  1. Killeen, P.J., Mwenifumbo, J., Ford, K.L., (in press). Tools and Techniques: Radiometric Techniques, in Treatise on Geophysics, 2nd Edition, Volume 11.