Source: https://www.mindat.org/article.php/255/Geology+of+the+Keweenaw+Peninsula%2C+Michigan
Timestamp: 2019-04-25 11:55:30+00:00

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The Keweenaw Peninsula, located in Michigan’s Upper Peninsula, is home to the single largest concentration of native copper deposits in the world. The true size of the deposit is realised in the amount of purified native copper removed. Between 1845 and 1996 when the White Pine mine closed and ended copper mining in Michigan, the district produced approximately 15.5 billion pounds of copper. In 150 years of mining in the district, only about 40 percent of the total estimated copper contained in the deposit has been mined out. To understand how this area developed into a classic mining location and one of the great mineral collecting regions of the world, one only needs to study and understand the geology and mineralogy of “The Copper Country”.
The Keweenaw Peninsula lies on the southern flank of the Mid-Continent Rift system (MCR) of North America. The MCR extends for about 2,000 kilometres from Kansas northeast through Lake Superior, then southeast through the lower peninsula of Michigan where it ends abruptly near Detroit, Michigan. The MCR began forming around 1,100 million years ago (Ma) when a rising mantle plume came into contact with the Precambrian Superior Province. As the plume spread laterally along the base of the Superior Province, it caused the crust in the region to thin and pull apart, forming the rift. As the plume continued to rise, it began to depressurize and melt, producing rhyolite and basaltic magmas. Some of this magma found its way to the surface through faults and fractures formed from the thinning of the crust. On the Keweenaw Peninsula, over 200 individual lava flows have been counted, including the Greenstone Flow, said to be one of the largest, if not the largest, single lava flow on Earth. White (1960) suggested that the Greenstone Flow has an aerial extent of over 5,000 square kilometres and a total volume of 1,500 cubic kilometres. The Greenstone Flow is part of the Portage Lake Volcanics (PLV) which is exposed on the Keweenaw Peninsula and Isle Royale, 90 kilometres to the northwest. The PLV has a total thickness of about 10 kilometres in the center of Lake Superior (Cannon, 1992). On the Keweenaw, the PLV is about 5 kilometres thick. The PLV on the Keweenaw Peninsula erupted in a 2 to 3 million year span of time around 1,095 Ma (Davis and Paces, 1990).
Following volcanism, a period of sedimentation began that filled the rift with as much as 8 kilometres of clastic sediments (Cannon, 1992). As volcanism ended and sediments filled the area, the entire region began to subside under the immense weight of the overlying strata. This sedimentation is thought to have begun between 1,085 and 1,060 Ma. The clastic sediments consist of red coloured conglomerates (Copper Harbour Conglomerate) and red coloured sandstones (Freda Sandstone) with a thin layer of gray to black shaly sandstones (Nonesuch Shale). Late in the thermal subsidence of the rift, Cannon et al. (1989) propose that a mature red sandstone (Jacobsville Sandstone) was deposited over the entire basin.
The last phase of the MCR system involves the transformation of the original graben bounded normal faulting into high angle reverse faults. Today the main fault running along the spine of the Peninsula is the Keweenaw Fault (KF), a high angle reverse fault. Originally, this fault was a major graben bounding normal fault (Cannon et al. 1989). The KF has several kilometres of reverse displacement which caused additional steepening of already tilted strata. Additional faulting and fracturing occurred on the Peninsula in both basalts and sediments as a result of this compression on the region (White, 1968). Cannon et al. (1993) had determined that the reverse faulting occurred around 1,060 Ma based on Rb-Sr biotite ages within older Precambrian basement rocks. The most likely cause of the compression event is a continental collision on the east coast along the Grenville Front (Hoffman, 1989). The timing of reverse faulting in the Keweenaw coincides with the Grenville compression event as well. It is believed that the compressional phase began as early as 1,080 Ma (Cannon, 1994) and completed by 1,040 Ma (Price and Mcdowell, 1993).
After the completion of rifting and compression, the region experienced a quiet period. A large inland sea covered the region during the Paleozoic which deposited limestones and a few dolomites. These same rocks also make up the rocks of the Michigan Basin in the lower peninsula of Michigan. The Paleozoic rocks were removed from the Keweenaw by erosion and later by the great Pleistocene glaciation. The present day landscape of the Keweenaw is strongly influenced by the same glacial event.
As stated earlier, from 1845 to 1996, the district produced over 15 billion pounds of refined copper from 380 million tons of ore. Along with the copper, small amounts of native silver (around 11 million pounds) were also recovered (White, 1968). The major ore producing horizon was a strip of land 5 kilometres wide by 42 kilometres long between Mohawk and Painesdale. Up to 1929, this strip of land produced 95.4 percent of all copper mined in the region (Butler and Burbank, 1929). The most common host for native copper were brecciated and amygdaloidal basalt flow tops (58.5 percent of production) followed by interflow conglomerates (39.5 percent) and a cross vein system of fissures (2 percent). The four largest deposits produced 85 percent of the total district production at a grade of about two percent.
Native copper occurs as fine dissemination, vesicle fillings, and masses. A “typical” brecciated flow top is 3 to 5 metres thick and 2 to 11 kilometres along strike. The down dip length is usually 1.5 to 2.5 kilometres (Butler and Burbank, 1929). While the flow tops had the most production, one of the interflow conglomerates, the Calumet and Hecla Conglomerate, was and still is the largest single native copper lode in the world. The conglomerate produced 4.2 billion pounds of copper over a strike length of 4.9 kilometres and 2.8 kilometres down dip, and an average bed thickness of 6 metres (Weege et al. 1972). The third type of deposit on the Keweenaw Peninsula are the copper bearing veins, also known as fissure veins. The first mines of the district, the famous Cliff, Central, and Minesota mines, worked these fissures. The copper formed in open space, filling fractures that cut the basalt beds at high angles. Some of the largest masses of copper ever found (up to 520 tons) in the district were found in the fissure mines, yet the veins only produced about two percent of the total copper mined in the district (Broderick, 1931).
Native copper makes up over 99 percent of all metallic minerals mined on the Keweenaw Peninsula, making it the largest accumulation of native copper on Earth (Bornhorst and Lankton, 2006). Other metallic minerals include silver, which make up only about 0.1 percent of copper production. Most of the native copper also carried a small amount (less than 0.5 percent) of arsenic and silver (Broderick, 1929). Copper nickel arsenides are also present occurring in veins in a flow top near Mohawk, the type locality for Mohawkite (now referred to as Domeykite) (Stoiber and Davidson, 1959). Chalcocite was also found in association with native copper, but is insignificant on the Peninsula (It should be noted that White Pine mined chalcocite almost exclusively and with great success, producing approximately 4.5 billion pounds of refined copper in 43 years of operation).
Native copper is found accompanied by various gangue minerals (Butler and Burbank, 1929). Associated with copper are more than 100 hydrothermal minerals on the Peninsula (Bornhorst and Lankton, 2006). A close relationship in time exists between native copper mineralisation and other hydrothermal minerals which fill vesicles and veins throughout the district.
Much research has been done on when native copper was emplaced in relationship to other events on the Keweenaw Peninsula. White (1968) interpreted the age of native copper mineralization as younger than the rift filling sediments (Freda and Copper Harbour), but older than the Jacobsville. Weege et al. (1972) has pointed out that a close relationship exits between copper emplacement and deformation of the Keweenaw Fault. Based on field relationships, native copper mineralisation is younger than rift filling volcanics and sediments and synchronous with reverse faulting and the very beginning of rift flanking sedimentary deposition. Bornhorst et al. (1988), using Rb-Sr methods, determined the age of mineralization at between 1,060 and 1,047 Ma, some 30 my after rift filling volcanism, but contemporaneous with reverse faulting along the Keweenaw Fault.
Genetic models for native copper emplacement favor two theories. One proposes that evidence of native copper in other areas of the MCR suggests that widespread regional hot mineralising waters flowed through the region. The second suggests burial metamorphism of the basalts at temperatures of 300 to 500 degrees Celsius would generate copper rich ore fluids (Jolly, 1974). Conclusive evidence for or against either hypothesis does not exist, but a growing amount of evidence, though not conclusive, favor the burial metamorphism of rift rocks. Dissolution of only a few parts per million of copper from the basalt yields plenty of copper-rich solution. Research suggests that a sufficient amount of copper existed on the flanks of the rift locked in basalt. (Bornhorst and Rose, 1994).
As the lavas were erupted, it is thought that degassing removed large amounts of sulphur from the basalts and thus the ore fluids had a very low sulphur content (Cornwall, 1956). Movement of these fluids was accomplished through the many faults and fractures formed during regional compression and subsequent deformation. Since the age of native copper emplacement is nearly contemporaneous with compression and faulting of the PLV (around 1,060 Ma), this provided the necessary plumbing system to allow ore fluids an easier path to their final deposition place (Bornhorst, 1997). Without sulphur in the system, the copper was deposited as native metal instead of copper sulfide. The major compressional event late in the MCR system may have provided the difference in the genetic model to generate the world class native copper deposits in the Keweenaw Peninsula and nowhere else (Bornhorst, 1997).
Despite over 150 years of mining and research conducted on the Keweenaw Peninsula and its native copper deposits, many questions are still unanswered. One of the many questions is why large amounts of native copper deposits were emplaced on the Keweenaw Peninsula and no where else along the MCR when it is known that basalts are present all along the rift, based on drill core data. Another key question is why the MCR is host to such large native copper deposits when other flood basalts around the world are not. These and other important research questions will likely keep professionals and amateurs alike thinking for years to come.
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