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Systems of Mining

gold, mines, ore, steam, placer, shovel and sluice

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There are two classes of mines, namely: (a) Open-pit mines, which are operated with or without steam shovels, and (b) underground mines, which are entered by a shaft, drift, entry or slope, and in which the mining is done under cover of rock.

This class of mines is becoming more and more important since the development of the steam shovel. In the iron mining districts of Minnesota, it is not un common for mines to be operated where an overburden of 100 or 150 feet is stripped and carried a distance of three or four miles in order that large ore bodies may be exposed. After the overburden has been removed the operation of the steam shovel is continued and the iron ore loaded into railway cars which are provided for it in the bottom of the pit which in many cases is 80 acres, or more, in area. Steam-shovel mining is the principal 4v...44NA SIM 5 lllll and is rapidly coming into favor for mining copper ores in Arizona, Utah and Nevada. The steam shovel is becoming an important factor in coal mining where an overburden of 15 to 50 feet may be removed from a four- or five-foot coal bed, and the coal loaded by steam shovel into rail road cars. The open-pit mines op erating without steam shovel are usu ally small and in this connection the overburden is removed by drag line scrapers, wheel scrapers, as in the case of railway excavations, or by the use of wheel-barrows and hand labor. This work is confined largely to the less important minerals, as barite, small iron-ore deposits, phos phate deposits and to shallow coal beds.

Placer Mining.—Another class of open-pit mines is known as pla cers nr placer mines where gold bearing sands and gravels are mined for the gold which they contain. In the earliest stages of placer min ing the gravel was washed in pans, the gold collecting at the bottom, and saved as metallic gold. This was followed by mechan ical devices such as the rocker, long torn and sluice. The sluice consists of a long wooden trough or box, or channels cut in the bed rock, provided with riffles in which the gold collects as the gold-bearing sands are washed through the sluices. Mercury is spread in the bottom of the sluice so as to catch the fine particles of gold, forming a gold mercury-amalgam. The "clean-upp takes place at stated intervals when the amalgam is col lected and the mercury is driven off by heat and the gold collected. The large placer oper

ations are known as hydraulicing and dredg ing. Mining by hydraulic methods consists in using water under various pressures from 250 to 400 pounds per square inch, which escapes through a nozzle and is directed at the gold bearing gravel banks; as the water carries the finer gravels and sands to the sluices the gold is collected in the manner described above under sluicing. The latest commercial development in placer mining is dredging, whereby it is possible to recover sands and gravels containing as low as five or six cents in gold per cubic yard. The powerful dredges scoop up the sand and pass it through screens, the finer material being washed through sluice boxes provided with riffles and mercury traps and the gold is recovered by cleaning out the sluice and retorting the amalgam.

Underground method of mining in which the ore is excavated from a vein by driving horizontally upon it a series of workings, called stopes, one immediately over the other or vice-versa. (See Fig. 1). A stope is an excavation from which the ore has been extracted, either above or below a level, in a series of steps and is usually applied to highly-inclined or vertical veins, or an open place in a thick ore body. The term is fre quently used incorrectly as a synonym of room, which is a wide working-place in a flat mine.

Each horizontal working or stope (probably a corruption of step), when a number of them are in progress, assumes the shape W of a flight of stairs. When the first stope is begun at a lower corner of the body of ore to be removed, and, after it has advanced a con venient distance, the next is commenced above it, and so on the process is called overhead stoping. When the first stope begins at an upper corner and the succeeding ones are below i it, it is underhand stoping. The term stoping is loosely applied to any subterranean extrac tion of ore except that which is incidentally performed in sinking shafts, driving levels, etc., for the purpose of opening the mine. This method probably originated in the Cornish tin mines and is applicable to vertical or highly inclined ore bodies; also to massive deposits which may have great thickness.

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