The pre-Cambrian rocks of the northern Black Hills which have been mapped and studied by the staff of the Homestake Mining Company comprise sedimentary formations with a total stratigraphic thickness of about 20,000 feet. The oldest formations, the Poorman and Homestake, are dominantly ankeritic carbonate and iron-magnesium carbonate respectively. The younger formations, the Ellison, Northwestern, Grizzly, and Flag Rock, are dominantly argillaceous rocks with some quartzites and other rock types. Pre-Cambrian intrusive rocks, originally gabbroic, cut the pre-Cambrian sedimentary rocks; and the whole succession is overlain unconformably by the Cambrian Deadwood formation, and everything is cut by Tertiary intrusive rocks.
Regional metamorphism of pre-Cambrian age, presumably related to an undisclosed granitic intrusion somewhere northeast of the district, has developed metamorphic zones of the Scottish Highlands types. Three metamorphic zones are developed in the argillaceous rocks: a biotite zone; a garnet zone; and a staurolite zone. The iron-magnesium carbonate, sideroplesite, the essential mineral of the Homestake formation, is converted to the iron-magnesium silicate, cummingtonite, during this metamorphism. Some of the mineralogic changes that took place during this regional metamorphism suggest that there were changes in the bulk composition of the rocks whereby potash migrated outward from zones of higher metamorphic grade ahead of the advancing isothermal surfaces.