CONCENTRATIONS OF PB, ZN, AND AG ASSOCIATED WITH NATIVE COPPER DEPOSITION, KEWEENAW MI

The Keeweenaw Peninsula is home to the world’s largest native copper deposits. Samples of native copper were analyzed from three distinct zones within the copper deposits including the interflow conglomerates, the brecciated/amygdaloidal flow tops of the Portage Lake Volcanics, and the fracture filling of the Nonesuch Shale. Data collected in this study, focused on Pb, Zn, and Ag concentrations associated with the native copper as analyzed using X-Ray fluorescence. Within the interflow conglomerates, chemical relationships showed an increase in Zn but a decrease in Pb and Ag with increasing Cu. The volcanic flow tops exhibited decreased Zn, Pb, and Ag with increasing Cu. In both of these sets, chemical ratios of Pb/Zn decreased with increasing Cu/Ag in a decay-like curve. Finally, the native copper as fracture filling in shale had a different chemical signature in which the Ag increased with Cu. Chemical data from this study may shed some light on the fluid composition associated with hydrothermal copper mineralization. In each case discussed here, native copper was deposited in stratigraphic horizons where permeability was high facilitating fluid flow. Based on previous studies, this cupriferous fluid migration was influenced by both structural limitations and impermeable geologic units associated with the long geologic history of the region. Stratigraphic and structural controls of copper mineralization have been linked to the 1.1 Ga Midcontinent Rifting event that dominates the geology of the Keeweenaw Peninsula and subsequent compressional stresses attributed to the Grenville Orogeny.