The highly conductive structure of Ste-Mathilde (Quebec): Interpretation from magnetotelluric soundings

CIM Bulletin, Vol. 78, No. 881, 1985

MICHEL CHOUTEAU, Ecole Polytechnique

A very conductive zone was revealed in 1977 and 1978 by magnetotelluric (MT) measurements carried out near Ste-Mathilde, 6 km northeast of La Malbaie. In order to identify the structure causing the anomaly, thirty-five MT soundings were recorded in 1980 and 1981 on and around the conductor within a radius of 4 km. Results confirmed that the conductor is of limited extent (less than 2 km by 0.4 km). The proposed model is a highly conductive vertical slab, the top of which is within 200 m from the surface. At a depth of approximately 825 m this structure joins a conductive layer which extends to the northwest but not to the southeast. The large conductivities and the depths involved can only be explained by a fractured zone flooded with electrolytic solutions, by graphite veins or by metallic mineralization (sulphides). The former seems to be the most likely hypothesis because an EW fault running across this zone could have allowed percolation of water and the leaching of disseminated pyrite out of a mylolisthenite filling the fault.

Mots Clés: Mineral exploration, Technology, Magnetotelluric measurement, Electrical resistivity, Conductivity, Metallic mineralization, Electrolytic solution, Graphite veins.