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<p>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, and the difficulty of relying on a definite EM signature to locate the economic sulfides. The Cu-Au zones showed a variety of EM responses with a maximum conductance of 100 Siemens and 2 ms time constant. The trends, sizes, shapes and conductances of the relatively strong conductors were identified with success and modeled using thin plates in full space. The vein direction in the weakly conductive zones were quantified from the x-component data. In only one instance was a TDEM response associated with mineralization interpreted to be chargeable. Petrophysical measurements and microscopic observations suggest complex interrelations between the amount of ore, the fabric of the rock, texture, porosity, mineralogical associations and impurities. This explains a wide range of bulk conductivity values from ~0.01 S/m to 4000 S/m measured on rock samples, and suggests that chalcopyrite might be a semiconductor at some locations at Opemiska.<br />
The magnetic viscosity effects observed at time scales between 0.01 and 10 ms at Opemiska are associated with magnetic grains of variable size in rocks. Recent observations made during a ground time-domain electromagnetic (TDEM) survey at Opemiska are consistent with four aspects of the spatial and amplitude characteristics of a magnetic viscosity response: (1) the ∂Bz/∂</p>