Paleomagnetism and the age of copper mineralization in the Portage Lake Volcanics, Upper Peninsula, Michigan

In order to establish the relative age of Cu mineralization, 12 palaeomagnetic sites were sampled representing six mineralized flows and their associated massive interiors on the Keweenaw peninsula in Michigan. Three of the four less-altered flow interiors contained two components of magnetization: primary magnetite and secondary hematite. The hematite component was parallel to the single hematite component in the associated flow top, indicating that both components had a secondary origin. The low between-site dispersion of the hematite component and the similarity of the mean direction to primary currents of other rocks of the same age (1100 m.y.) suggest that hematization occurred over a period of time long enough to average out secular variation but before deposition of the overlying Nonesuch shale, extensive tectonic tilting, and significant polar wander. If Cu mineralization and hematization were contemporaneous than the relative age of Cu mineralization and the associated metamorphism has also been dated.