Abstract

Alternating field and thermal demagnetization studies of a thin lava flow unit within the Copper Harbor Conglomerate, Michigan Native Copper District, show a westerly increase in chemical remanent magnetization (CRM) overprinting along a 50 km strike length. The CRM may have formed while secondary minerals, including native copper, were being generated in the underlying Portage Lake volcanics, and when a rhyolite plug was injected into nearby siltstones of the overlying Freda Formation.The direction of magnetization during Copper Harbor time as given by the flows is virtually indistinguishable from that found from the Portage Lake volcanics, thus supporting previous geological observations that the two formations are closely associated in time.

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