*Please note that much of the information and all of the images below were provided from a book written in the 1920's. It was very up-to-date at the time when the book was published, but is not the most current information. It has been greatly simplified.
Discovering a deposit
On August 7, 1903, two railroad workers –J.H. McKinley and Ernest Darragh –found pliable, silvery flakes of metal on the shores of Long Lake. The town of Cobalt came into being almost overnight. The formation of the town was very much unplanned, and came into existence simply due to the lure of this “silver rush”. Wherever a flat outcropping was big enough to support a building, a miner built a home.
The largest silver vein was yet to be uncovered. In 1904, a party of four men discovered a massive native silver vein along a trail. The vein was named the “Lawson Vein.” Due to a disagreement over the ownership of the vein, mining did not begin until 1908. When mining finally began, it was discovered that the vein, which reached 0.5 metres across, and 100 metres long, also continued to a depth of 60 metres below the surface. Needless to say, all men who discovered the vein retired very wealthy. The vein soon began to be known as the Silver Sidewalk, and was the richest silver vein of its time.
Rocks of Cobalt
In the image gallery below, there are various geologic maps of Cobalt to help you visualize the arrangement of the geologic formations when reading about the geological history (which is found underneath the image gallery).
A geologic map of Cobalt
Legend accompanying the preceding map
Part 1 of geologic cross-section of Cobalt
The legend for the geologic cross-sections
Cobalt’s geological history
The rocks which make up Canada’s gorgeous landscape contain segments which represent the ocean floor, deep crustal sections, ancient mountain chains, deep sea sediments, and both ancient and modern glacial deposits. The lowermost rocks in Canada are called the Canadian Shield, and they are believed to have formed as the result of plate tectonic processes as smaller continental landmasses drifted together and collided, hundreds of millions of years ago. The Shield extends from the Arctic Islands in the north, to Minnesota in the south; from Great Slave Lake in the west, to the Coast of Labrador in the east. The geologic history Canada dates back over 3.25 billion years.
Cobalt is a small mining town in Ontario which is well-known for its large deposits of silver, and has a very interesting geologic history. The lowermost rocks in Cobalt belong to the Canadian Shield, as described above. The rocks which rest upon the Canadian Shield in Cobalt are known as the Keewatin. These rocks were formed by basaltic lava flows, interbedded with sedimentary beds of “iron formation” consisting of slate, chert and greywacke. These Banded Iron Formations, BIFs, formed when the Earth was very young and had little to no oxygen in its atmosphere, over 2,700 million years ago. When oxygen producing photosynthetic bacteria evolved they gave off a great deal of oxygen as a waste product. This oxygen built up in the atmosphere and dissolved in the sea water. When iron, produced mainly from huge submarine volcanoes, came in contact with this oxygen, it precipitated as iron oxide in the sea floor and resulted in iron-rich and silica-rich bands, which are now classified as banded iron formations.
The Timiskaming series was laid down atop the Keewatin layer. It is made up of conglomerate, slate, greywacke and quartzite. The Keewatin and Timiskaming series were intruded upon by a great mass of pink granite of Algoman age (2,700 to 2,500 million years). The area was greatly folded and faulted during a period of mountain building.
Following this active period was a prolonged period of weathering and erosion. The surface rocks were worn down to a surface probably not unlike the surface existing in Northern Ontario at present time.
Great amounts of faulting of the rocks mentioned above occurred throughout geologic history, and cannot be confined to any one period. This faulting left fractures in the existing rocks which was key to the formation of the silver deposits.
The greatest geological event of economic importance to the Cobalt area took place between 2217– to 2210–million years ago; the intrusion of an enormous sill of diabase about 1,000 feet thick, which has since been named the Nipissing diabase. The diabase penetrated both the sedimentary rocks and the Keewatan basalt. Veins of ore-rich rocks formed through the faulted fractures, especially in the areas near Cobalt. The silver veins were deposited from mineral solutions which were given off by it during the time the diabase was cooling and for some time after it had solidified. The veins of deposit average three or four inches wide and the silver occurs mainly in its native form.
During the Pleisocene epoch, 2.588 million to 12,000 years ago, great sheets of ice known as glaciers spread out in all directions across Canada, sweeping the rocks clear of their weathered and decomposed surfaces. The upper parts of ore-bodies were carried away, lakes were formed, and Canada’s landscape as we know it today began to be formed.