Metalliferous Mining

Undercurrents are in some cases placed at an intermediate point or at the end of the main sluice to save fine gold. They are short sluices considerably wider than the main sluice, and are set on a steeper grade, generally at right-angles to the main sluice. A screen or grizzly in the main sluice lets the fine gravel, sand and gold pass through but retains the large rocks and suf ficient water to move them in the main sluice. The discharge from the undercurrent is either returned to the main sluice through an auxiliary sluice of flatter grade or goes directly to the dump.

Drift Mining

is the term applied to the working of placer deposits by underground methods of mining. It is classified under breast stoping. The paystreak is reached through a hori zontal opening called a drift or adit (q.v.), or through a shallow vertical shaft. Small cars are commonly used for transporting the gravel, which is removed in some system of regular cuts or sluices across the paystreak. Posts and headboards are used to support the roof, but if the gravel is loose a method of forepoling or tight timbering must be used. Frozen gravel must be thawed before it can be mined. Steam, hot water or cold water have been successfully used as thawing mediums, being forced into the gravel through pointed pipes. Drifting is more expensive than sluicing or hydraulicking and is used only for rich gravels. The thickness of pay gravel is generally between 2 and 8 feet.

As a separate method of mining this is applied to thin ore deposits that are either horizontal or dip at a small angle with the horizontal. Stoping is the term applied to breaking the ore from place by drilling and blasting. The excavation in which the miners work is called a stope. Thickness of deposits worked by this method is not usually over 12 f t., although in modified form this system is applied to beds up to 7o ft. in thickness, as in the Wisconsin zinc district. In thin deposits the entire thickness of ore is mined at one time, thus advancing the working in a vertical face or breast. In thicker deposits the ore is mined in a series of benches. Continuous horizon implies a long working face which is attacked simultaneously at a number of points.

In the Missouri lead and zinc region the roof is supported by pillars of ore from io to 4o ft. in diameter, left at irregular intervals. From io to 17% of the ore is left in the pillars, this amount depending upon the nature of the floor (hard or soft), the strength of the roof and total weight supported. The recovery of ore from the pillars is low, it being more economical to leave some of the ore than to use artificial means of support. To

reduce the cost of mining, hand shovelling is replaced wherever feasible by some form of machine loading. Small power shovels are used in some large stopes. Concrete discs and concrete col umns have proved a satisfactory means of support and in some places have supplanted wooden cribs filled with waste.

Room and Pillar.

This method applies to the systematic cutting of the deposit into regular lines of square or rectangular pillars, which later are removed as much as possible. At the May wood iron mine, Yorkshire, England, a deposit of iron ore 7.5 ft. thick is cut into pillars 54 to 6o ft. by drives 15 ft. wide, leav ing some 63% of the ore in pillars, which later are worked out in sections, using posts as temporary roof supports. Finally, the posts are pulled out and the roof settles down over.

As the name implies this is a method of working an ore body from the top down. Holes for blasting are drilled downward. Fig. 3 shows a composite section of underhand mining in a narrow vertical vein. In mining it is customary to concentrate the han dling of ore and waste and the distribution of supplies on certain horizons called levels, which are commonly spaced i oo f t. apart vertically, though this distance may be greatly increased in some mines. The ore mined between two consecutive levels is dropped through chutes to the level below for hauling to the shaft. On any level the passageways driven lengthwise of the vein are called drifts (a single drift may be called a level), and those driven across the vein from wall to wall are called cross-cuts. If the vein is inclined, the upper wall is the hanging wall and the lower wall is the footwall. Two levels are interconnected by verti cal or inclined openings called raises, if driven upward from a level, and winzes, if driven downward from a level. Thus the same connecting opening may be a raise or a winze depending upon which level is used as a reference point. The term back is applied both to the roof of an underground opening or to the entire body of ore between a level and the next one above. Since each main level must have a haulage system and a station must be made at the shaft, a large number of levels in a mine calls for a heavy development expense. To lessen this expense levels may be placed i,000 ft. or more apart, if the vein is fairly constant in dip and has strong walls, as in some of the Rand mines. Raises are spaced from TOO to 200 or even 500 ft. apart horizontally.