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Keweenaw Peninsula, Michigan, USA

This page kindly sponsored by Paul Brandes
Dipping beds of the Copper Harbor Conglomerate

Keweenaw Peninsula, Michigan, USA
On top of Brockway Mountain

Keweenaw Peninsula, Michigan, USA
Dipping beds of the Copper Harbor Conglomerate

Keweenaw Peninsula, Michigan, USA
On top of Brockway Mountain

Keweenaw Peninsula, Michigan, USA
Dipping beds of the Copper Harbor Conglomerate

Keweenaw Peninsula, Michigan, USA
On top of Brockway Mountain

Keweenaw Peninsula, Michigan, USA
 
 
This general locality has been created so people who have copper, silver and other specimens from the Keweenaw peninsula but with no specific known locality can associate images with this locality and not have to take a guess and hope that they come from say Keweenaw County. The specimens have a much better chance of having been found in Keweenaw, Houghton and Ontonagon counties than just taking a guess at one of the more specific localities.
[Rock Currier 2009]

The Keweenaw Peninsula lies on the southern flank of the Midcontinent Rift System. The rift system extends some 2200 kilometers from near Detroit, Michigan northwest through lower Michigan, curving through Lake Superior then arching back southwest through Wisconsin, Minnesota, Iowa, and Kansas. Rift filling volcanic and sedimentary rocks on the Keweenaw Peninsula dip moderately toward Lake Superior and the rift, which today is buried under Lake Superior (Bornhorst, 1997). The Portage Lake Volcanics (PLV) are the host rock unit for both native and sulfide copper deposits. The total thickness of the PLV is about 10 kilometers, of which 5 kilometers are exposed on the Keweenaw Peninsula (Cannon, 1992). The Portage Lake Volcanics consist of over 200 subaerial flood basalt lava flows with occasional interflow sediment layers (Bornhorst, 1997). A typical lava flow is about 10 to 20 meters thick and consists of a massive interior and the uppermost 5 to 20% being a vesicular flow top. About one-fourth of the individual flow tops are brecciated (White, 1968). Interflow sandstones and conglomerates, with a thickness of up to 40 meters, make up less than 5 volume percent of the PLV (Merk and Jirsa, 1982). Both the tops of lava flows, especially brecciated zones, and the interflow conglomerates host native copper ore deposits. Massive interiors do not contain copper or secondary minerals except along fractures and faults (Bornhorst, 1997).

Major magmatism and extension along the Midcontinent Rift occurred as a result of rifting over an asthenospheric mantle plume (Hutchinson et. al, 1990). This magmatism occurred from 1109 to 1087 Ma with the PLV being erupted during a 2 to 3 m.y. span around 1095 Ma (Davis and Paces, 1990). On the Keweenaw Peninsula, the PLV is overlain by Oronto Group sediments, including the Copper Harbor Conglomerate, Nonesuch Shale, and Freda Sandstone. The base of the PLV is cut by the once normal, graben bounding Keweenaw Fault, now a major high angle reverse fault with several kilometers of displacement (Bornhorst, 1997). The age of this changeover from normal to reverse motion is about 1060 ± 20 Ma based on Rb-Sr biotite ages (Cannon et. al, 1993). This age corresponds to the Grenville Orogeny on the eastern margin of the continent, where major compression from a continental collision is postulated to have inverted the Keweenaw Fault (Cannon, 1994). Many smaller scale faults and fractures trending along and parallel to the more major Keweenaw Fault are most likely the result of the same compressional event that produced the reverse motion on the Keweenaw Fault.

The Jacobsville Sandstone fills a rift-flanking basin on the southeast side of much of the Keweenaw Peninsula. It was deposited during reverse faulting (Bornhorst and Rose, 1994). The bedrock strata of the Keweenaw Peninsula are, in most places, capped unconformably by unconsolidated Pleistocene glacial deposits consisting of sands and gravels.

Many of the waste rock piles are still accessible to the collector; however, most of the piles are also privately owned, so permission must be obtained before entering. The Keweenaw Peninsula offers collectors a full range of minerals from micromounts to the very large. Although nicknamed "The Copper Country", the Peninsula offers many other minerals besides copper, including silver, datolite, calcite, prehnite, pumpellyite, analcime, and many others.
[Paul Brandes 2011]

Mineral List



19 entries listed. 14 valid minerals.

The above list contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.

References

Bornhorst, T.J., and Rose, W.I., 1994, Self-guided geological field trip to the Keweenaw Peninsula, Michigan: Institute on Lake Superior Geology Proceedings, 40th Annual Meeting, Houghton, MI, v. 40, part 2, 185 p.

Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Boulder, Colorado, Geological Society of America Special Paper 312.

Cannon, W.F., 1992, The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics, v. 213, p. 41-48.

Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline – A 35 km thick cross section of the Midcontinent Rift in northern Michigan and Wisconsin: Tectonophysics, v. 12, p. 728-744.

Cannon, W.F., 1994, Closing of the Midcontinent Rift – A far-field effect of Grenvillian contraction: Geology, v. 22, p. 155-158.

Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54-64.

Heinrich, E. W. and Robinson, G. W., 2004, Mineralogy of Michigan, 2nd Edition, 252 pp.

Hutchinson, D.R., White, R.S., Cannon, W.F., and Schultz, K.J., 1990, Keweenaw hot spot: Geophysical evidence for a 1.1 Ga Mantle plume beneath the Midcontinent Rift system: Journal of Geophysical Research, v. 95, p. 10869-10884.

Merk, G.P., and Jirsa, M.A., 1982, Provenance and tectonic significance of the Keweenawan interflow sedimentary rocks: Geological Society of America Memoir 156, p. 97-105.

White, W.S., 1968, The native copper deposits of northern Michigan, in Ridge, J.D., eds., Ore Deposits of the United States, 1933-1967 (the Graton Sales volume): American Institute of Mining, Metallurgical and Petroleum Engineering, New York, p. 303-325

Rosemeyer, T. 2011 New from the Keweenaw: Part 4 - Recent Mineral Finds in Michigan's Copper Country. Rocks & Minerals 86:205-227

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