Abstract

Ten magnetotelluric (MT) soundings were recorded in the range 300 Hz to 200 s along an 11 km profile across a long, well-defined, east–west sub vertical conductor in the vicinity of Chibougamau. This conductor had been recognized by the airborne transient electromagnetic technique and confirmed by other ground electromagnetic methods. The primary purpose of the MT survey was to further understand and constrain the structure of the conductor, especially its extent at depth. This study is the first to demonstrate the utility of the MT method for mineral exploration in Abitibi subprovince.The subsurface of the survey area can be roughly divided into a resistive northern section, a conductive central section (containing the conductive sheet), and a resistive southern boundary. Although some data are distorted by static shifts or three-dimensional effects, there is evidence that the data from the central conductive section are relatively free of static distortions. Therefore, taking into account existing geological and geophysical information and with extensive two-dimensional modelling, it is concluded that a conductive sheet with conductivity of 2 S∙m−1 and a width of 25 m should extend to a depth of no more than 750 m; an additional conductive block is required below this sheet. Cores from boreholes have shown that the conductivity of the near-surface sheet is mainly caused by sulfide minerals and graphite. It is also believed that graphite in the metasedimentary rocks under the central section may be responsible for the conductivity at depth.There is a clear boundary in terms of geoelectric characteristics between the northern part of the MT profile which is located in the Optica subprovince and the southern MT profile located in the Abitibi subprovince. Under the Optica subprovince, there are no structures other than a very resistive block with a thickness of at least 20 km. In contrast, conductive layers are found in the upper crust in the western and central parts of the Abitibi subprovince.

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