Keweenaw Peninsula, Michigan, USAi

Regional Level Types
Keweenaw Peninsula	Peninsula
Michigan	State
USA	Country

This general locality has been created so people who have copper, silver and other specimens from the Keweenaw Peninsula 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.

The Keweenaw Peninsula lies on the southern flank of the Midcontinent Rift System. The rift system extends some 2,200 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 percent 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 1,109 to 1,087 Ma, with the PLV being erupted during a 2 to 3 million year span around 1,095 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 1,060 ± 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.

Fossils

There are 3 fossil localities from the PaleoBioDB database within this region.

These data are provided on an experimental basis and are taken from external databases. Mindat.org has no control currently over the accuracy of these data.

Occurrences	3
Youngest Fossil Listed	444 Ma (Silurian)
Oldest Fossil Listed	450 Ma (Late/Upper Ordovician)
Stratigraphic Units	Unit No. Occurrences Age Waynesville 1 449.5 - 443.7 Ma (Paleozoic)
Fossils from Region	Click here to show the list.
Accepted Name Hierarchy Age Strophomena vetusta species Animalia : Brachiopoda : Strophomenata : Strophomenida : Strophomenidae : Strophomena : Strophomena vetusta 449.5 - 443.7 Ma Paleozoic Tetraphalerella neglecta species Animalia : Brachiopoda : Strophomenata : Strophomenida : Strophomenidae : Tetraphalerella : Tetraphalerella neglecta 449.5 - 443.7 Ma Paleozoic Holtedahlina sulcata species Animalia : Brachiopoda : Strophomenata : Strophomenida : Strophomenidae : Holtedahlina : Holtedahlina sulcata 449.5 - 443.7 Ma Paleozoic
Fossil Localities	Click to show 1 fossil locality

Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/Keweenaw_Peninsula
Wikidata ID:	Q12430

Mindat Articles

Early Exploration on the Keweenaw Peninsula, Michigan by Paul Brandes
Geology of the Keweenaw Peninsula, Michigan by Paul Brandes
Porcelaneous Datolite of the Keweenaw Peninsula, Michigan by Paul Brandes

Localities in this Region

Other Regions, Features and Areas that Intersect

References