Sedimentary deposits such as those of Bissel, California, and near St. Thomas, Nevada, are by their nature more regular in occurrence, and their tonnage can be estimated from the outcrop in natural exposures and prospects.
Replacement deposits like those in Washington and Quebec are not so regular as the sedimentary deposits, but are more regular than the veins, and tonnage estimates may be based on the surface exposure and an assumed depth of 50 to 100 feet.
GEOGRAPHICAL DISTRIBUTION
The known distribution of magnesite deposits is as follows:
- North America.
- Canada: Quebec, British Columbia and elsewhere.
- United States: California, Washington, Nevada.
- Mexico: Lower California on Santa Margarita Island.
- South America.
- Venezuela: Island of Margarita.
- Europe.
- Austria, Hungary, Germany, Greece, Italy, Macedonia, Norway, Sweden, Russia.
- Africa.
- Transvaal, Rhodesia, Portuguese West Africa.
- Asia.
- India, in Madras and Mysore.
- Australia.
- Queensland, New South Wales, South Australia, Tasmania.
- Oceania.
- New Caledonia.
The following description by countries is in the order given above:
North America.
Canada.
—The principal magnesite deposits in Canada are in the Grenville district, Argenteuil County, Quebec, where the mineral is associated with serpentine, dolomite, and other minerals. The magnesite in the Grenville district is a glistening cream-white to milk-white or gray material that occurs in extensive masses associated with bands or lenses of dark green to light-yellow serpentine. Throughout the great mass of the deposits the magnesite and dolomite are so similar in appearance that the detecting of dolomite is difficult. There is considerable positive evidence in support of the hypothesis that the deposits have been formed by the solution and replacement of crystalline limestone through the agency of magnesia-rich solutions. Outcrops of the deposits are up to 1,000 feet long and 300 feet wide. It is estimated that there are in sight 686,900 tons of magnesite containing less than 12 per cent. CaO and 483,700 tons of magnesite-dolomite containing more than 12 per cent. CaO.
In the Atlin mining district, in British Columbia, both magnesite and hydromagnesite have been noted, but the extensive masses of hydromagnesite near the town of Atlin are the most important. These deposits are superficial beds of fine powdery white hydromagnesite 6 to 8 feet thick, that cover areas up to 18 acres in extent. Two groups of these deposits are estimated to contain 180,000 tons of hydromagnesite.