Geophysical Exploration for Unconformity-Related Uranium Deposits in the Northeastern Part of the Thelon Basin, Northwest Territories, Canada

Recent progress in the use of geophysical exploration techniques for unconformity-related uranium deposits within permafrost terrain in the northeastern part of the Thelon basin, Northwest Territories, Canada, is summarized using exploration case histories. These exploration techniques have attempted to use the unique physical and petrological propertiesof altered rocks that accompany uranium mineralization. Altered rocks are detectable as low resistivity anomalies by airborne resistivity and ground VLF-resistivity surveying, and the influence of thin surficial thawed zones on the responses are minimal except for the ground underneath large lakes. Gravity surveys are very effective for detecting low density zones associated with alteration and mineralization provided that the overburden is uniform in thickness (or the actual thickness is known), and a low density alteration zone is relatively shallow and/or large enough to produce a recognizable anomaly. A geophysical model can be established to link the physical properties of various alteration zones. Resistivity can be significantly decreased in the early stages of alteration (or relatively weak alteration) by hydrolysis of feldspar (sericitization). However, it is not the hydrolysis of feldspar that is responsible for the significant change of density that is typical of inner part of alteration zones, but rather the dissolution of quartz followed by replacement by water. This suggests that an influx of a considerable volume of hydrothermal solutions has contributed to promote desilication, and that the mineralization must be related to this influx of solution, which can be inferred by the close spatial relationship between desilication and uranium mineralization. Introduction Unconformity-related deposits have been a major uranium exploration target within the last two decades due to their high grade nature. The northeastern part of the Thelon basin, located in District of Keewatin, Northwest Territories, Canada, is one of the prospective areas for unconformity-related deposits. The first discovery of an economically significant uranium deposit in the subject area was made by Urangesellschaft Canada Ltd. in 1977 (BUNDROCK, 1981). An airborne radiometric survey and ground follow-up led to the discovery of the deposit which was formerly called Lone Gull and was later renamed the Kiggavik deposit (FUCHS and HILGER, 1989). During the successive exploration history of the area, the effectiveness of radiometric surveys has been marginal as the exploration target has shifted to blind deposits, and non-radiometric surveys consisting of electromagnetic and gravity surveying have begun to play a major role in detecting alteration zones. This paper summarizes the development of geophysical exploration techniques in the area which have attempted to use the unique Received on March 1,1990, accepted on March 10, 1990. *PNC Exploration (Canada) Co ., Ltd., 2401-650 W. Georgia St., Vancouver, B.C., V6B 4N8, Canada. Present address: Power Reactor and Nuclear Fuel Development Corporation, Chubu Works, 959-31, Jorinji-Sonodo, Izumi-cho, Toki, Gifu 509-51, Japan. **PNC Exploration (Canada) Co ., Ltd. ***Urangesellschaft Canada Limited , Suite 2812, Toronto-Dominion Bank Tower, Toronto, Ontario,