Gravity studies of the Coldwell alkaline complex show that the complex is associated with a positive gravity anomaly. The anomaly indicates that a significant volume of dense material occurs below the current levels of exposure of principally felsic rocks. A new high-resolution gravity profile used to model the infrastructure of the complex shows that the felsic rocks form a 3–5 km thick layer over a differentiated basic intrusion. This intrusion is interpreted to consist of a layer of gabbro 3–5 km in thickness underlain by 3 km of peridotite and (or) pyroxenite. Intrusion of the basic magma was associated with Keeweenawan continental rifting. The presence of large volumes of basic magma supports the hypothesis that some of the Coldwell felsic rocks were derived by differentiation of a basic parent.