The bedrock north of Thunder Bay, Ontario is mostly the greenschist facies metasedimentary and metavolcanic rocks of the Shebandowan Greenstone Belt and the high grade metamorphic rocks of the Quetico Gneiss Belt. The boundary between the two is locally obscured in part by a zone of plutons extending west from Hwy 527 for over 45 km. The Barnum Lake Pluton occupies a central position within this zone, and is hosted entirely by metasedimentary rocks, mostly biotite schists. It has been classified compositionally as a porphyritic quartz monzonite, and is easily recognized on the regional aeromagnetic map as an elliptical positive anomaly.
A groundborne vertical component magnetic survey was performed over the area of the pluton and combined with the total field aeromagnetic data and pertinent structural measurements for interpretation. Strike and dip data from the surrounding conformable bedding suggests a cylindrical body dripping very steeply towards the north northwest. Preliminary modeling of the magnetic data shows good agreement between the aeromagnetic data and predicted profiles for a near vertical prism-shaped body of depth extent greater than 2 km.