A magnetotelluric (MT) survey was carried out on the western flank of
the Sudbury Structure (Ontario, Canada) over a recently discovered ore
body to test the potential of the method as a reconnaissance tool for
deep mineral exploration. The observed responses exhibit a large phas
e anomaly (exceeding 75 degrees in some places) centered over the ore
body, which is interpreted as arising from a combination of 3-D induct
ion and current channeling. Impedance decomposition methods were used
to define frequency ranges in which the response is quasi 2-D with geo
logically plausible strike values. Two-dimensional rapid relaxation in
versions of the data, where appropriate, generated quantitative models
in good agreement with known parameters of the ore body. Two-dimensio
nal inversions: however, cannot replicate either the size or the pecul
iar nature of the observed phase anomaly. Therefore 3-D modeling trial
s were undertaken, which best explain the MT responses in terms of 3-D
induction within the Trillabelle body coupled with current channeling
through neighboring faults. An important conclusion of these trials i
s that isolated, deep, highly conductive ore bodies may not be observa
ble using MT. Prospects for their detection appear to improve when the
y are linked with regional, current gathering features such as conduct
ive faults, or a larger-scale, alteration or dissemination halo.