3D conductivity model of the Lalor Lake VMS deposit using ground and airborne EM data

Abstract

Lalor Lake is a VMS deposit in central Manitoba, Canada. The deep ore body is buried under the cover rocks up to 1000m. Multiple EM data sets were collected to delineate the compact and conductive alteration zones and two data sets are available to us. The first is HELITEM, an airborne time-domain EM survey that covers the entire exploration area. The second is a ground loop EM data measured by SQUID magnetometers that have high precision at late times. The two data sets map the conductivity structures at Lalor Lake in different ways: the airborne survey covers a broad area but has limited resolving power at depth; the ground survey provides information about the deep targets through very late times but the measurements were made in a smaller area. Individual 3D inversions were carried out for both data sets assuming little a prior information. Both are able to recover the trace of the expected ore body, but the airborne model is smooth and the ground model contains highly conductive anomalies. Then we invert the ground data again with the airborne model as the reference model. The new inversion again confirms the existence of the VMS ore body but also rearranges the conductive material according to the constraints from the reference model. The new model differs significantly from the blind inversion model at the deposit scale. Based on the information from the inversion so far, we conclude both surveys have picked up signals from the ore body in different levels of detail. More analysis and further data are still required to better delineate the targetâ??s geometry.