Most of the rocks in Northern Canada are part of the ancient Canadian Shield, and covered by a thin layer of soil. The Canadian Shield is one of the remaining pieces of the Earth’s original crust, dating over three billion years. The shield extends from the Arctic Islands in the north to Minnesota in the south and from Great Slave Lake in the west to the Coast of Labrador in the east. It is believed to have formed as the result of plate tectonic processes as smaller continental landmasses drifted together and collided over hundreds of millions of years.
Northern Ontario was once made up of a series of volcanic and plutonic rocks separated by oceans. When the belts finally collided, vast mountain ranges were formed and many of the rocks were deformed and metamorphosed at elevated temperatures and pressures. These conditions were ideal for mineralization. The shield is one of the world’s richest areas in terms of mineral ores. It is filled with substantial amounts of nickel, gold, silver and copper. The processes which metamorphosed the rocks are believed to have occurred between 4.5 and 540 million years ago. For example, in French River, the bedrock is of metamorphic origin and is from the Canadian Shield. Prehistoric alkaline intrusions of lower to middle Cambrian age produced rare minerals like sodalite and cancrinite.
Over hundreds of millions of years, the shield has been eroded to its current topographic appearance of relatively low relief. The majority of the shield which is revealed at the surface of the Earth today is composed of igneous rocks, formed by Precambrian volcanoes, and of metamorphosed rocks, as described above. A number of Canada’s major ore deposits are also associated with the Precambrian volcanoes.
Finally, there are economically important minerals of sedimentary origin. Iron formations can be found in some areas of the belt. From 1940’s to 1980, an open pit mine operated near Atikokan. To reach the hematite ore it was necessary to divert the Seine River and drain a large portion of the lake. The ore is generally composed of hematite (or magnetite), jasper and white quartz.
In Ontario, large quantities of amethyst formed during the faulting of the Lake Superior Basin, between layers of Archean and Proterozoic rocks. This process occurred before the glaciations of the Great Lakes basin 25,000 to 10,000 years ago, which depressed the land surface, scooped out thousands of lake basins, and carried away much of the regions soil. Formation of amethyst was restricted to near-surface conditions in veins and vugs (pockets within rock where trapped liquids and gases had created an environment suitable for crystal formation). The beautiful colour of amethyst is caused by iron inclusions. In Thunder Bay amethyst is found with other minerals such as pyrite, galena, sphalerite, chalcopyrite, baryte, calcite, fluorite and native silver.
The iron deposits north of Wawa Lake were discovered during the Mihipicoten gold rush of the late 1890’s. Helen Mine was the first mine to come into production. The main ores which have been extracted from the Helen mine include goethite, pyrite and siderite.