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VIRTUALFIELD TRIP TO THE KEWEENAW PENINSULA, MICHIGAN
The Keweenawpeninsula contains one of the largest and most unique copper deposits inthe world. It is the only district where native copper was the primary mineralmined throughout the life spans of the mines. The preserved geologic historyconsists of rocks from the Precambrian (1.1 billion years) and Pleistocene ( 2million year) glacial deposits. Utilization ofthe copper from the glacial float as well as shallow mines occurred from atleast 6800years before the present. Native Americans may have mined between 100 and 500million pounds of copper from deposits on the peninsula and Isle Royale. The fissure veins were the first type of deposit to beexploited by the early prospectors in the 1840's to 1880's. The mines produced largemasses of nativecopper ( up to 520 tons), but were of irregular distribution, were difficult to find, and most did not pay backtheir investors. The greatest production of copper was from gas pockets in thelavas, interstitial spaces in the breccias developed on the top portions of the flows,and porosity between rock clasts in sedimentary rocks (conglomerates) derived from the volcanics. Morethan 11 billion pounds of copper have been produced in the district over the 150years of mining activity. Currently theonly mining that is occurring is in the Caledonia mine which is being mined forcopper and other mineral specimens. Several mines have tours which give a flavorof the conditions under which the miners lived and worked. Many of thewaste rock piles from the mines are being crushed and used as gravel in thelocal construction industry. The world's best specimens of native copper havebeen found in the mines in the district. It also has seen the production ofsilver crystals, copper included calcite crystals, chalcocite, copper arsenides,and secondary copper minerals. Lapidary material found on the peninsula has included datolite,chlorastrolite ( massive pumpellyite - the state gemstone of Michigan), and LakeSuperior agates. Copper Harbor from Brockway mountain (Copper Harbor and Lake Superior to theleft). Lake Fanny Hooe's (right) shape is controlled by the geology of thepeninsula. Erosion of softer rocks by the glaciers has resulted in the basinswhich are now filled by lakes. The more erosion resistant rocks to the right, form ridgesthroughout the peninsula. This area was one of the first locations on thepeninsula to bemined (most unsuccessfully) and was protected by Fort Wilkins (1844-1846, late1860's - briefly).
Lake Superior surrounds the Keweenaw peninsula. Rock outcrops on the northwest shorelineare composed of the Copper Harbor conglomerate. The Stop numbers in the following text refer to the locations in Bornhorst & Rose's"Self-guided geological field trip to the Keweenaw Peninsula,Michigan". [Include:'../../../inc/footer.html'] |
The Keweenaw peninsula is in Upper Michigan and extends into southern Lake Superior (theworld's largest fresh water lake). The peninsula is approximately 150 miles long and 50 miles wide. The Keweenaw fault extendsalong the length of the peninsula and marks the boundary between theMid-continent rift and continental rocks. The rift valley was filled with acombination of extrusive volcanic rocks with minor amounts of intrusive igneousrocks and sediments derived from the igneous rocks. The Greenstone flow is one of thelargest lava flows in the world. It can be mapped for 90 km on the peninsula andis also found on Isle Royale 90 km northwest on the other side of the rift valley. It has a thickness of up to 400 mwith an volume estimated to be between 800 to 1,500 km3of lava. Other flows on the peninsula can be mapped for lengths up to 160 km.
