Paleomagnetism of the native copper mineralization, Keweenaw Peninsula, Michigan

Author:

Symons D.T.A.1,Kawasaki K.2

Affiliation:

1. Department of Earth and Environmental Sciences, University of Windsor, Windsor, ON N9B3P4, Canada.

2. University of Toyama, Toyama-shi, Toyama 930-8555, Japan.

Abstract

The age and genesis of Michigan’s world-class native copper deposits are poorly constrained. The copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 ± 2 Ma Keweenawan Portage Lake Formation. Progressive thermal demagnetization isolates stable hematite remanent magnetization components at 28 paleomagnetic sites. Paleomagnetic tilt tests show that magnetite in massive flow interiors is primary (1095 ± 2 Ma) and that hematite throughout the formation is syntectonic. The altered cupriferous deposits contain primary ∼1095 Ma and secondary ∼1053 Ma hematite in various proportions. The Caledonia Mine’s basaltic mineralization carries the ∼1053 Ma hematite dominantly whereas the Delaware Mine’s conglomeratic interbed mineralization carries the ∼1095 Ma hematite dominantly. The ∼1095 Ma hematite is attributed mostly to magnetite exsolution during flow extrusion and to weathering oxidation between extrusion events. An infusion of epigenetic hydrothermal fluids emplaced the native copper with additional hematite and polarity self-reversing titanohematite at 1053 ± 7 Ma. Importantly, paleomagnetic evidence supports a 1053 ± 7 Ma age also for the White Pine stratiform sedimentary copper mineralization, for the oxidation of the Oronto Group clastic rocks to red beds, and for the time limits of major tectonic uplift and deformation on the Keweenaw Peninsula.

Publisher

Canadian Science Publishing

Subject

General Earth and Planetary Sciences

Reference69 articles.

1. Books, K.G. 1968. Magnetization of the lowermost Keweenawan lava flows in the Lake Superior area. U.S. Geological Survey Professional Paper 600-D. pp. D248–D254.

2. Paleomagnetism of some Lake Superior Keweenawan rocks

3. Tectonic context of native copper deposits of the North American Midcontinent Rift System

4. Bornhorst, T.J., and Barron, R.J. 2011. Copper deposits of the western Upper Peninsula of Michigan. In Geological Society of America Field Guide 24. Edited by J.D. Miller, G.J. Hudak, C. Wittkop, and P.I. McLaughlin. pp. 83–99.

5. Bornhorst, T.J., and Whiteman, R.C. 1992. The Caledonia native copper mine. Society of Economic Geologists guidebook series, Michigan. Vol. 13. pp. 139–144.

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