Interpretation of airborne geophysical data for the Thor Lake area, Northwest Territories

Abstract

Radiometric, magnetic, and electromagnetic patterns detected over the Blatchford Lake Intrusive Suite were investigated on the ground in order to gain a better understanding of this complex and the related Thor Lake rare-metal deposits (beryllium, yttrium, rare-earth elements, niobium, tantalum, zirconium, gallium). Bedrock radioelement concentrations in the Grace Lake Granite and Thor Lake Syenite, two major units of the Blatchford Lake complex, average 4.6% K2O (3.8% K), 3 ppm equivalent uranium, and 18 ppm equivalent thorium. Principal radioactive minerals in these rocks are zircon and bastnaesite with minor REE-rich apatite and monazite. Although the radioactivity of the Thor Lake deposits is typically one to two orders of magnitude higher than surrounding rocks, there is no apparent halo of uranium or thorium around the deposits. Felsic dykes within the Grace Lake Granite and Thor Lake Syenite are one order of magnitude more uraniferous or thoriferous than host rocks and are enriched in elements found in the Thor Lake deposits. A prominent magnetic anomaly in the western part of the complex is related to the Caribou Lake Gabbro. Two significant magnetic anomalies are present in the eastern part of the complex. One occupies the western half of the Thor Lake Syenite, and the other forms a crescent-shaped feature within the Grace Lake Granite. These two anomalies are related to magnetite produced by alteration of ferromagnesian minerals. The alteration may be either autometasomatic or related to fenitization by a nepheline syenite body which underlies the rare-metal deposits. The electromagnetic patterns relate primarily to conductive lake bottom material and faults.