Sulfide-mineral zoning in the basal Nonesuch Shale, northern Michigan

Pyrite is the characteristic sulfide mineral throughout most of the Nonesuch Shale, which is 400 to 600 feet thick. In a zone 1 to 50 feet thick at the base of the formation, the dominant sulfide is chalcocite. The top of this cupriferous zone is marked by the upward mineral sequence chalcocite-bornite-chalcopyrite-pyrite. The boundary between the zones transgresses stratigraphy, and has the geometry of a simple, rather smooth envelope. Its regional configuration suggests that the copper is not syngenetic, but that it is related instead to a zone of wedging in the underlying Copper Harbor Conglomerate. Copper, introduced from below, may have replaced syngenetic iron-sulfide.

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