Knowledge of the stratigraphy has provided the surest guide in iron ore exploration and development at Steep Rock Lake. Most and possibly all of the 33 million tons produced has come from a single stratigraphic member.

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The Steep Rock group lies unconformably on a granite complex. The basal unit is a conglomerate succeeded by the Dolomite, Orezone and Ashrock formations. Intense deformation is indicated by steep dips, brecciation, shear folds, and numerous faults and dikes, but thermal effects are negligible, and Precambrian limonite and bauxite have remained unaltered.

The Orezone formation has three members. The lowermost of these, the Manganiferous Paint member, is up to 1,000 feet thick, is mainly below ore grade, and represents a residuum derived from the Dolomite formation which it overlies disconformably. The middle, or Goethite, member is up to 300 feet thick, is dominantly goethitic iron ore, but includes minor aluminous and cherty sediments and, in a few places, lenses of ferruginous bauxite. The uppermost, or Pyrite, member occurs sporadically along and near the contact of the Orezone with the overlying Ashrock formation. Microcolloform structures in the pyrite, paucity of trace elements, and association with carbon and banded chert indicate a sedimentary origin for this member.

Valency changes in iron and manganese within the Steep Rock group suggest that deposition spanned a critical period in atmospheric evolution about 2 billion years ago.

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