Abstract

Economic native copper lodes in the Lake Superior district are confined to a 45-km-long portion of the Portage Lake Volcanics on the Keweenaw Peninsula, despite similar host rocks extending for more than 1,000 km around Lake Superior. The focus of ascending ore-forming brine into a narrow segment of Portage Lake strata may be attributed to upward thrusting on the Keweenaw fault during closure of the Midcontinent rift, forming anomalously hot, steeply dipping aquifers on the Keweenaw promontory, and to the development of a single, dominant thermal plume within the incipiently buoyant brine at the scale of the mine district on that promontory.

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