A comparison of airborne electromagnetic data with ground resistivity data over the Midwest deposit in the Athabasca basin

摘要

The Midwest deposit in the Athabasca basin of Saskatchewan, Canada, lies 200 m below an arm of South McMahon Lake. The rocks between the lake and the deposit are sediments of the Athabasca group while the deposit is hosted by graphitic gneisses or metapelites in the basement. Geological logs of holes drilled by AREVA indicate that the Athabasca sediments are strongly altered. In the Athabasca basin, alteration in combination with conductive graphitic zones in the basement is a strong indicator of the presence of economic amounts of uranium. The purpose of the airborne surveys is to determine whether airborne methods can detect the alteration and the graphitic conductor. The airborne methods tested are broad-band fixed-wing time-domain electromagnetic (FTEM) systems (TEMPEST and GEOTEM) and helicopter frequency-domain electromagnetic (HFEM) systems (RESOLVE). All the lakes in the area, particularly the one above the deposit, are mapped as conductive near-surface features with a high resolution HFEM system. On a resistivity section, the HFEM can delineate the bottoms of the conductive lakes quite well. The fixed-wing FTEM sections are unable to resolve sharp changes in near-surface resistivity; however, they clearly map the steeply dipping basement conductor and they also see another flat-lying conductor close to the basement/basin unconformity (perhaps a conductive paleoregolith or other conductive basement lithologies). Comparison of ground and airborne resistivity maps indicates there are some similar features but also some important differences. At intermediate depths, where an alteration response is expected, the ground resistivity data indicate two relatively conductive zones, one at Midwest and smaller zones to the north, including one at the Mae zone (a smaller deposit north of Midwest). Resistivity sections derived from the airborne electromagnetic data do not always show these features clearly. The HFEM sections tend to show the lakes and possibly the alteration, while the FTEM results show the lakes, the bedrock conductor and the conjectured paleoregolith. There is a hint of the alteration at Midwest and Mae on the FTEM on-time data but this might also be a response associated with the lakes. Modelling shows that the alteration response at Midwest is subtle and unlikely to be detected: at early time the response is dominated by the overlying lake sediments, at late time by the basement conductor. The modelling suggests that alteration could be detected at delay times less than 0.2 or 0.3 ms but only if there is no near-surface conductive feature like overburden, lake sediments, etc.