Author Keywords

Keweenaw Peninsula, Midcontinent Rift, Nonesuch Shale, Portage Lake Volcanics, White Pine Copper mine, Precambrian Supereon, Mesoproterozoic, Superior Craton, Great Lakes Tectonic Zone, Marquette Supergroup, Penokeon Orogeny, Bovine Igneous Complex, Duluth Complex


The Keweenaw Peninsula of northern Michigan is home to the largest accumulation of native copper in the world. Native copper deposits are hosted in the 1.1 billion-year old midcontinent rift which extends from Kansas, up through Lake Superior, around into lower Michigan, but is only exposed in the Keweenaw region. Native copper is found in the vesicular and brecciated flow tops of the volcanism associated with this rift, within the interflow conglomerate that developed in-between individual flows, and also minor native copper deposits are found with the copper sulfide deposits in the Nonesuch Shale. The purpose of this study was to examine the concentrations of zinc, lead, and silver associated with native copper in each of these depositional environments in hopes of gaining an understanding of the geochemistry of the native copper in this area. Eleven copper samples were examined using X-Ray fluorescence spectroscopy. Three geologic areas of copper deposition studied were: the brecciated and amygdaloidal flow tops of the Portage Lake Volcanics (PLV), conglomerate layers in between PLV basalt flows, and fracture fillings of the Nonesuch Shale, as seen in the White Pine Copper Mine. The volcanic flow tops exhibited decreased Zn, Pb, and Ag with increasing Cu. Within the interflow conglomerates, chemical relationships showed an increase in Zn but a decrease in 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. 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 help in our understanding of fluid composition associated with hydrothermal copper mineralization.