Collier's New Encyclopedia (1921)/Ore Deposits
ORE DEPOSITS. Any considerable accumulation of metal bearing matter in the crust of the earth. A metalliferous rock is spoken of as an ore when it contains metallic minerals (or in some cases non-metallic minerals, such as sulphur) in such quantities and mixtures as to make possible their profitable extraction. The metal contained in an ore is sometime found in its elemental form, as in the case of gold or platinum, but more frequently as a sulphur compound, or as an oxide carbonate or silicate. Often several different forms of one metallic element are found in the same deposit and several metals are often found together. Quartz, feldspar, hornblende, fluorite are the common gangue materials, and are sometimes so evenly mixed with the minerals that the ore must be crushed and metal picked out by magnets or separated by specific gravity devices. In other cases the gangue is in such a form that it can be avoided in mining. Some deposits were formed at the same time as the rocks by which they are contained, but in the greater number of cases, the mineral was deposited after the rocks were formed. The greater number of ore deposits are in igneous rocks and not infrequently near hot springs. It is believed that ground water, frequently hot, and sometimes under high pressure because of its depth and in vapor form carried many of the metallic elements in solution until a decrease in temperature or pressure caused the precipitation of the metallic salts they had been carrying in solution.
When metal bearing salts are deposited along a crevice or fissure in tubular form, the deposit is called a vein; and when the veins are parallel and closely spaced, they are known as a lode. A large pocket rich in ore is known as a bonanza. When the metal is found in the open mixed with gravel, as is frequently the case with gold and platinum, they are called placer deposits. The form of the deposit, the mineral contents or the origin of the ore body are the three most widely used methods of classifying ore bodies. The former is used by miners and the latter by geologists and mining engineers. The five main divisions under this method of classification are:
- Igneous, those which were formed with the rocks;
- Pneumatolytic Deposits, made by gases above the critical point;
- Fumarole Deposits, made by lava;
- Gas Aqueous Deposits, made by ground water at high temperature;
- Deposits by ordinary ground or surface water.
Ore deposits are found over a wide range of territory. They are found where there has been igneous activity, or where they have resulted from the work of meteoric waters.
Copper is found in the United States in Michigan, Arizona, Utah and Montana; zinc in New Jersey, and both lead and zinc are found in Missouri, Illinois, Iowa and Wisconsin; iron ore is found around Lake Superior and in parts of the Appalachian region, and gold and silver are found in Alaska, the Black Hills, and in the Cordilleran region.