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

Abstract

The close association of hematite with native copper in the flow tops of the Portage Lake Volcanics invites the opportunity to establish the relative age of copper mineralization. For this purpose, we sampled 12 paleomagnetic sites representing six mineralized flow tops and their associated massive interiors on the Keweenaw Peninsula in Michigan. Three of the four less-altered flow interiors contained two components of magnetization: one was carried by primary magnetite, and therefore the other, carried by hematite, must be secondary. The hematite component was parallel to the single hematite component in the associated flow top, indicating that both components had a common secondary origin. The low between-site dispersion of the hematite component and the similarity of the mean direction to primary directions of other rocks of the same age (1100 Ma) 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 apparent polar wander. If copper mineralization and hematization were contemporaneous, then we have also dated the relative age of copper mineralization and the associated metamorphism. In contrast, paleomagnetic results from the two extensively altered flows (from below the Greenstone flow) are consistent with the mechanism of deuteric or auto-oxidation.