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 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.