Stratigraphy and Metamorphism in a Part of the Northern Black Hills and the Homestake Mine, Lead, South Dakota

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