Significance of magnetic susceptibility measurements from the Lac Du Bonnet Batholith, Manitoba, Canada

Petrophysical parameters including bulk magnetic susceptibility, anisotropy and natural remanent magnetization are used to characterize the lithology of rocks from the Lac du Bonnet batholith, Manitoba. Five boreholes measuring a total length of 2.8 km have been drilled. A relationship between the magnetic susceptibility and the lithology of the cores has been established. Distinct susceptibility patterns correspond to lithologic features: susceptibility lows (0–5·10−3 SI) characterize fracture zones (both open and closed) and associated altered rocks; susceptibilities in the order of 7–10·10−3 SI denote unaltered, non-fractured, homogeneous rock; and regions consisting of high susceptibility values (20–100·10−3 SI) and contrasting lows (0–5·10−3 SI) denote rock heterogeneities such as xenoliths containing mafic minerals. Correlation between boreholes has identified two major fracture zones within cores URL-1, −5 and −3 that are believed to be continuous and coincide with fracture zones defined by seismic reflection data. The anisotropy of susceptibility has been related to the petrofabric. Magnetic remanence measurements have not isolated a stable magnetic direction. The measurement of the magnetic properties of igneous rocks is an effective method for the characterization of such rock bodies.