Magnetotelluric delineation of the Trillabelle massive sulfide body in Sudbury, Ontario

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 phase anomaly (exceeding 75 degrees in some places) centered over the ore body, which is interpreted as arising from a combination of 3-D induction and current channeling. Impedance decomposition methods were used to define frequency ranges in which the response is quasi 2-D with geologically plausible strike values. Two-dimensional rapid relaxation inversions of the data, where appropriate, generated quantitative models in good agreement with known parameters of the ore body. Two-dimensional inversions: however, cannot replicate either the size or the peculiar nature of the observed phase anomaly. Therefore 3-D modeling trials 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 is that isolated, deep, highly conductive ore bodies may not be observable using MT. Prospects for their detection appear to improve when they are linked with regional, current gathering features such as conductive faults, or a larger-scale, alteration or dissemination halo.