DELINEATING MINERALIZATION AND IMAGING THE REGIONAL STRUCTURE WITH MAGNETOTELLURICS IN THE REGION OF CHIBOUGAMAU (CANADA)

Tensor magnetotelluric (MT) soundings were carried out to delineate th e geometry of an extended shallow conductor detected by airborne elect romagnetic method in the mining camp of Chibougamau (Canada). Careful distortion analysis of the impedance tensors from eleven sites indicat e that the subsurface is reasonably 2-D within the 0.0025-10 s range, The recovered regional impedances are corrected for static shifts empl oying 2-D inversion for minimum structure, The modeled resistivity cro ss-sections show a far more complex conductivity structure than antici pated. The sheet-like shallow conductor lies near the boundary between the conductive southern section located in the Abitibi subprovince an d the resistive northern section located in the Opatica subprovince. I t extends to a depth of 250 m. Two other limited conductive zones are imaged respectively between 0.4 km and 3 km and between 1.5 km and 4 k m. We propose that the conductors are associated with sulfide minerali zation and graphite in shear zones, They are delimited by steeply sout h dipping thrust faults. The northern resistive section does not show clear resistivity changes up to a depth of 15 km, Gravity data collect ed along the MT profile have provided additional constraints. Both MT and gravity models are consistent with one another and with the known regional geologic structure. They indicate that the Abitibi greenstone belt is 3.5-4.4 km thick in the survey area and that the steeply dipp ing geology at the surface is flattening at depths of 4 km approximate ly.