The top of the cupriferous zone in the lower Nonesuch Shale of Northern Michigan is characterized by an abrupt zonation of Cu-Fe sulfides which represents a chemical and mineralogic transition into the overlying barren pyritic shale. This narrow fringe forms a blanket-like surface which cross-cuts bedding at shallow angles. Although the ore horizons show strict stratigraphic controls, the actual upper limit of copper mineralization in the White Pine area occurs at various stratigraphic levels 20 to 50 feet above the base of the Nonesuch. A strong inverse correlation exists between the height of the fringe surface in the Nonesuch Shale and the amount of copper in the lowermost shale unit. In local areas along fracture zones, a very minor amount of copper has been channeled upward along some fracture zones producing local anomalous elevations in the fringe surface. The White Pine fault is clearly a post-ore structure which offsets the fringe surface.The ore minerals of the Cu-Fe transition show a step-by-step replacement of ironrich sulfides by paragenetically later copper-rich sulfides. The sequence in ascending order is chalcocite, djurleite, digenite, bornitc, chalcopyrite and pyrite. Abnormal concentrations of Cd and Pb occur immediately above the cupriferous zone. These observations suggest, as others have proposed, that the mineralization of the basal Nonesuch was accomplished by the introduction of copper from the underlying Copper Harbor Conglomerate soon after deposition of the Nonesuch Shale. Copper deposition in the sulfur-rich reducing environment of the Nonesuch occurred in large part by replacement of original pyrite along an upward advancing mineralization front (fringe). Metals more soluble than copper were apparently swept upward before the copper front.Simple mathematical models of infiltration and diffusion processes were designed to test the above hypothesis. Each model results in a mineralization period of about 106 years. It seems entirely possible that these mechanisms could account for the transport of copper into the early unconsolidated Nonesuch sediment. Salt sieving and gravity stratification phenomena may have assisted these basic processes. The ultimate source of copper is open to speculation, but may lie in primary magmatic contributions or extraction of metals from the thick volcanic-sedimentary pile of the Keweenawan basin.

No abstract

No abstract

No abstract

No abstract