The Lady A (C zone) deposit is underlain by cherty sediments of the Upper Devonian McLaughlin Ridge Formation, Sicker Group. Locally the sediments strike northwest and dip to the northeast at 50 to 60 degrees. Intruding the strata are dykes and sills of gabbro and diabase (informally called the Mount Hall Gabbro) that are coeval with the Upper Triassic Karmutsen Formation, Vancouver Group.

The deposit is a stratabound taconite lens composed of grey chert and red jasper hosting bands of very fine-grained magnetite with minor hematite. The deposit outcrops along strike for a distance of 42 metres, having an apparent thickness of 12 metres. Drilling has revealed a downdip extent of at least 48 metres. A "conservative" estimate of reserves based on four drill holes put down in 1953 was reported to be 2.15 million tonnes grading 18 per cent iron (Buckham, 1953).