A detailed three-dimensional (3-D) numerical electrical resistivity model of the Bathurst No. 12 deposit (New Brunswick) has been constructed using available geological and geophysical information. This model facilitates studies of the resolution capabilities of audio-magnetotellurics (AMT) at locating and defining mineral targets, and of methods of optimizing data acquisition. Different conditions were analyzed: presence of overburden, dimensions and positions of the ore body, and varying data sampling. The behavior of 3-D EM fields is compared with 2-D ones. The 3-D and 2-D responses are similar at high frequencies, but at low frequencies only those responses for current flow perpendicular to the body (TM mode in 2-D) are reasonably alike. The 2-D inversions show that the position and top of the 3-D ore body can be well resolved, but not the bottom and resistivity of the body.