LAKE SUPERIOR (EN: Lake Superior; RU: Верхнее озеро) is the copper ore region within the USA, within the Michigan state. This region includes the group of the deposits with the native copper (they are being developed since the 1845), and with the cupriferous sandstones (they have been discovered during the 1865, and are being developed since the 1915), with the total reserves of approximately 9 million tonnes of copper (1979). The Late pre-Cambrian volcanogenic-sedimentary formations of the Keweenaw peninsula system (the lavas, conglomerates, schists, and sandstones) form the southern wing of the syncline of the Lake Superior. The embedment of the rocks is complicated by the slightly inclined brachyaxial folds, and by the disjunctive dislocations.
The deposits with the native copper (the Calumet-Hecla is the largest deposit) are situated within the limits of the narrow (3-6 kilometres) zone, which may be traced for the distance of more than 120 kilometres. The mineralization is confined to the upper amygdaloidal parts of the individual basaltic lava covers, and to the layers of conglomerates (the Portage Lake series). The native copper with the admixture of chalcocite and native silver, together with other minerals (chlorite, quartz, calcite, epidote, and others), fills the pores within the cement of breccias and conglomerates. Copper forms the small grains, nuggets. The initial reserves of the native copper are 6 million tonnes. During the time of the operation of these deposits (1845-1968), there has been extracted up to 5.5 million tonnes of copper. The depth of the working is 1600 metres. Since the 1968, the mines have been mothballed, the remaining part of the reserves amounts to 500 thousand tonnes of copper, with the content of this copper within the ore of 0.8-1.5%.
The deposits with the cupriferous sandstones (the White Pine is the largest deposit) are situated at the distance of 100 kilometres to the south-west of the deposits with the native copper. The mineralization is confined to the Nonsuch schistose stratum, which has been formed with the interbedded layers of the gray and banded siltstones, argillaceous shales, and sandstones. The ore-bearing layers are concentrated mostly within the siltstones and carbonaceous-argillaceous mudstones of the basal pack with the thickness of up to 20 metres. At the White Pine deposit (which is being developed since the 1954 using the underground method), the ore bodies (the length along the dip is up to 900 metres, the thickness is up to 7.5 metres) are represented by the series of the adjacent copper-containing schists and mudstones, which are separated by the interlayers of the sandstone. The major ore minerals are chalcocite, bornite, chalcopyrite, less often the native copper and silver. The average content of copper is 1.2%. The system for the development is room and pillar, with the usage of the self-propelled equipment. The drilling of the blastholes is performed with the help of the carriages, the loading and delivery of the rock is performed using the diesel-powered machines, the roof support is performed using the anchorage. The delivery of the ore to the beneficiation plant is performed through the pipeline (the length is 8.2 kilometres), and within the transport vessels.
The annual production of copper throughout the entire Lake Superior (in terms of the metal, which is extracted) is 44.3 thousand tonnes (1979).
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