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The study is a result of a continuing surface-mapping program that was begun in 1953, with additional data provided by the drilling and underground activities of mining companies. The Archean rocks are Keewatin greenstones intruded by Algoman granites, for which the geological age has been determined as about 2,500 million years. These granitic rocks consist of gneissic granodiorites and massive, slightly radioactive quartz monzonite. The Archean complex was eroded to a peneplain with valleys in the less resistant rock types. The Lower Huronian formations are a sequence containing a great variety of sedimentary rocks such as conglomerate, arenites, argillite, siltstone, greywacke, limestone, and quartzite. Thickness and facies changes indicate a northwesterly source, nort erly overlap, and deposition in shallow water controlled by basement topography. The Lower Huronian formations unconformably overlie the Archean rocks and in turn are unconformably overlain by the Middle Huronian formations. The Middle Huronian rocks consist of the Gowganda and Lorrain formations of conglomerate, greywacke, quartzite, and arkose.
Age-dating methods give the age of the Nipissing diabase as 1,950 million years and of the granite at Cutler as 1,750 million years (Penokean orogeny). A few dikes of Keweenawan olivine diabase have been tentatively dated at 1,000 million years.
Copper mineralization is associated with the Nipissing diabase. Uranium ores in oligomictic conglomerates are post-Archean placers modified at a later date. Uranium production to the end of 1962 consisted of 44,937,871 tons of ore grading approximately 0.1% U3O8, valued at $944,373,250.
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