Exploring for copper–gold deposits exhibiting a wide range of conductivities with time–domain electromagnetics at Opemiska, Canada

Abstract

Finding and delineating new economic Cu-Au ore zones corresponding to poorly conductive disseminated mineralization and narrow massive chalcopyrite veins in the Chapais-Chibougamau mining district of Québec is a challenging exploration problem. The site of the former Opemiska underground mine was the location for conducting an experimental ground time-domain electromagnetics (EM) survey for mapping the conductivity, the anisotropy of the conductivity, and the chargeability estimated from shape reversals. Measurements at fourteen different sites confirmed the variability of the EM response. The trends, sizes, shapes and conductances of the relatively strong conductors were identified with success and modelled using thin plates in full space. The vein direction in the weakly conductive zones was quantified from the x-component data. Petrophysical measurements and microscopic observations suggest complex interrelations between the amount of ore, the fabric of the rock, texture, mineralogical associations and impurities. This explains a wide range of bulk conductivity values ~0.01 S/m to 4000 S/m measured on rock samples and also suggests that chalcopyrite might be a semiconductor at some locations at Opemiska.