A detailed aeromagnetic map at 1,700 ft. of 16 sq. mi. of the U-producing Blind River basin of Ontario, Canada, shows anomalies interpreted to originate from shallow basic intrusives and deeper basement rocks. The identification of a major basement fault from a basement anomaly is described after demonstrating the isolation of these anomalies from the intrusive rocks on the basis of calculations of their depths. An aeromagnetic map computed upward to 2,630 ft. and 2 second vertical derivative maps confirm the identification of the 2 categories of anomalies. Later drilling tends to confirm the existence of the fault. The entire study is considered applicable to much larger aeromagnetic surveys because all types of anomalies are present within this small area where they can easily be correlated with their origins.