Measured susceptibilities of >3500 samples from a 500¿500 km area of the Minto block, Superior province, Quebec, have an average value of ~0.03 SI (equivalent to induced magnetizations of ~1.5 A/m). Charnockitic lithologies (igneous orthopyroxene-bearing granodiorite, granite, diorite), that constitute large parts of the Minto region, yield the highest susceptibilities. The variations in susceptibility show a good correlation with the variations in the observed magnetic field, including those at long wavelengths (>100 km), suggesting that charnockitic and related granitoid rocks could be responsible for the long-wavelength anomalies. Modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the Minto block supports this contention. Charnockitic lithologies are not usually considered a possible sources of long-wavelength anomalies: mafic rock types are generally favored. The dominant Minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron. ¿ 1999 American Geophysical Union