Slike strani
PDF
ePub

up to the next higher level and finished. In the Treadwell and Ready Bullion mines the slate-horse forms a natural division between the stopes of the north and the south ore-bodies. The walls of the ore-body are supported by vertical pillars, or ribs, 15 feet thick, and from 200 to 300 feet apart. For means of communication and ventilation, man-way raises are put up in these pillars and connected with the levels. At intervals of 25 feet, short drifts are run in opposite directions from the man-way raise; so that, as the working floor of the stope advances, each of them is used successively when the workings connect with the main raise, and in turn abandoned and closed up as connection is made with the next higher one. The levels are protected by horizontal pillars from 20 to 30 feet thick. Heretofore these pillars have been left at each level, but from now on only the pillars at every other level will be left in place; yet even with this saving, fully 20 per cent of the ore must remain in the mine in the shape of pillars and ribs to support the ground and prevent caving."

South Dakota: the Homestake Mines. Not unlike the AlaskaTreadwell mines both in character and extent of deposit and magnitude of operations is the Homestake mine of South Dakota. A special method of mining has been evolved and successfully employed in this mine. It is known as the "Homestake System of Stoping."

Levels are formed in the lode which are laid out in squares or blocks, and sill timbers are placed. Three lines of track are then laid running in the direction of the lode, and are connected by such cross-tracks as are deemed necessary. Sets are then placed and securely lagged above, and on the sides of the car-ways in order to protect the tracks from materials loosened by the mining operations carried on above, while spaced lagging is placed on the sets between the car-ways. It is then evident that with the breaking down of ore the space not occupied by the car-ways will be filled, none of it is removed, however, until the filling has been completed. The lagging serves as a false floor upon which the miners work, and is removed on the completion of the filling of the sets. This filling of ore is not disturbed until during the next operation of cutting out the stope it may be necessary in order to make room for the miners.

This method is conducive to rapid work owing to the fact that little or no attention need be paid to the timbers, and therefore large charges may be fired knocking down large masses of rock and ore.

Two or more sets are placed next to each wall of the stope, keep

ing pace with or in advance of the stoping operations, which are kept open and serve as ladder and pipe-ways, also as a means of facilitating ventilation.

Ore-chutes may or may not be employed depending on the size of ore produced in blasting. With softer ores the size may be such as to warrant the use of chutes, but with hard ores the large masses broken down do not permit of the use of chutes. The ore is then usually shoveled from the floor on a level with the tracks in the car-ways, there being as many places to shovel from as there are spaces between the posts of the sets.

When a height of some 85 feet has been attained in stoping, the stope worked and the level above are connected by raises which are to serve as means of introducing filling into the stope. The work of removing the ore is begun, the walls being examined in the meanwhile in order that serious falls may not occur. Having emptied one end of the stope the floor is lagged, and the filling from the stope above is run in. As the work progresses a stope will be partially filled with broken ore and filling, but occupying opposite ends of the stope. The walls are thus well supported, and the miners are provided with support for drills and staging upon which to stand.

To effect a still further saving in timber the above described method has been modified by employing only two lines of track in the stopes, one being in the middle of the stope while the other is laid in a drift cut in the pillar to the side of the stope. No sets are used between these two lines of car-ways as was formerly the practice.1

Idaho: the Coeur d'Alêne Mines. The lead-silver ores of the Coeur d'Alêne region occur in typical fissure-veins and are associated with considerable quantities of siderite. The fissures occupy fault-planes which have suffered more or less displacement. As a usual thing there is only one principal plane of fissuring in each vein which generally occurs near the middle of the vein, but often forms one of the walls.

The following brief description of the methods of mining is taken from a paper by J. R. Finlay."

"At least 70 per cent of all the ore thus far mined in the Coeur d'Alênes has been extracted through tunnels without hoisting or

1 Min. and Sci. Press, Vol. 88, pp. 177, 178, 1904. For details of stoping in the Homestake mine see pp. 165–166.

2 T. A. I. M. E., Vol. 33, pp. 250-252, and 255.

pumping. Of the remaining 30 per cent, which has been hoisted, at least two-fifths has been hoisted through underground shafts, to be subsequently hauled out through tunnels. The Tiger-Poorman is the only mine which has always been operated by shafts from the surface. This large proportion of tunnel-work has been a great advantage to the district. ...

"Three methods of mining are used in the district: (1) backstoping and timbering; (2) back-stoping, timbering and filling; and (3) back-stoping and filling without timbering. . . . At the Bunker Hill and Sullivan, the most interesting feature of the underground mining is the extraction of wide bodies of low-grade ore by stopes which are filled, as the work progresses, with waste rock sorted from

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

Mining with Square Sets and Filling, Bunker Hill Lode, Coeur d'Alêne Region.

(From Mines and Minerals.)

the broken ore. There is usually more than enough of such material to keep the stopes full, and provision has to be made for tramming the surplus waste away. Sometimes this back-stoping is done without any timbering, other than an occasional prop to support a suspicious-looking piece of ground in the roof; but more commonly the stopes are timbered with light square sets.

"In stoping the ore at the Standard it has been found necessary to fill up the stopes with barren material from the vein. This is done by the simple process of sorting out part of the waste rock

from the barren streaks and from the walls, and throwing it down among the timbers below. It is quite easy, in most parts of the mine, to secure in this way enough filling to keep the stopes full within two or three floors of the back. As the levels are 200 feet apart, it is necessary to build massive cribbed chutes up through the timbers.... In the Mammoth mine, the ore almost invariably lies in a single streak in immediate contact with the fissure. The shear-zone is much narrower than elsewhere; and the mining is simpler, in that no filling on an extensive scale is required. . ..

"At the Tiger-Poorman, Hecla, Frisco and Morning mines, all the stoping is done with stull-sets, about as in the Mammoth. Very little effort is made to sort the ore before concentrating, or to fill the stopes systematically."

For a further detailed account of the mining practice in this region the reader is referred to a paper by R. N. Bell in the Seventh Annual Report of the Mining Industry of Idaho 1905.1

2

Bedded Deposits.- Utah: the Mercur mines. The essential features of the mining practice in the Mercur mines as given herein are taken from a paper by Geo. H. Dern: "The ore-veins of Mercur, Utah, .. consist of parallel blanket deposits, dipping at a slight angle from the horizontal. The upper vein is usually from 12 to 20 feet thick, though often much thicker. The middle vein is situated about 40 feet lower, and is from 12 to 30 feet thick. The mineralization sometimes extends throughout the entire intervening space, forming a continuous ore-body, from the floor of the middle vein to the roof of the upper vein. The dip varies from nearly horizontal to 25 degrees. . . . The veins outcrop on the sides of Mercur and Marion hills. Naturally, the original prospecting and development work consisted of driving tunnels on the veins, following the ore on its strike. The tunnels were close enough together to make it convenient to stope out the ore between them. . . .

"The original method of mining consisted of advancing large stopes, leaving pillars of ore to hold up the ground, and supporting the roof by means of stulls. The pillars were subsequently withdrawn, so far as practicable. Usually this method admitted of clean mining though sometimes cave-ins caused loss of a good deal of ore. As a general rule, it was "good ground" and there was little difficulty in holding the roof and keeping the stopes from caving, which was at that time not desired. In the Marion mine, which has not been 'Mining Magazine, Vol. 13, pp. 306–307.

2 Mines and Minerals, Vol. 25, pp. 1-3.

operated for several years, and in the old workings of the Sacramento mine, there are still vast chambers from which the ore has been extracted, which show no signs of closing, and in which the timbers, when any are present, have taken very little weight. ... With increased output, and enforced economies due to lower-grade ores, the mining methods were necessarily changed and improved."

The following description of methods is that of the Consolidated Mercur Gold Mines Company in the Mercur and Golden Gate mines. "In the Mercur mine, the hanging-wall is usually very hard and firm, so that it does not cave readily.

"In fact, it is often difficult to make a stope cave in, even when such action is desired. Hence the method of mining hitherto employed in this mine differs somewhat from the more typical caving system worked in the Golden Gate mine. The latter system, however, is now being more generally adopted in the Mercur mine on account of its superiority.

"Let us assume that there is a certain body of ore to be stoped out with a definite boundary, caused by a fault, a lean streak, or an old stope. A drift is driven into this body until the boundary, or end of the values, is reached. Then stoping is commenced by 'swiping' on both sides of the drift. An open stope is thus formed. This is supported by stulls so long as necessary for the safety of the men. The stope is drawn backward toward the place where the drift started. The men work for the most part in the drift, and the 'back' is blasted down in front of them. Of course with the width of such a stope limited, and if the block of ore be large, the same operations must perforce be carried on in two or more drifts, so as to draw back all the ore. As the stope is drawn back a large chamber is left, which is expected to cave in.

"The advantages of this system are that the place where the ore is coming from is in plain view of the miner, and the ore is always mined clean up to the hanging-wall. A disadvantage is that, with an inclined vein, the drift cannot always be kept upon the foot-wall, hence there is danger of leaving a layer of ore in the floor of the stope. Also, a premature cave often causes the loss of valuable pillars of ore. This can hardly be called a true caving system, because the only caving about it is that the stopes are allowed to cave in after the ore has been extracted. In the Golden Gate mine, the ore itself is partly caved, at least enough to crush it and minimize the amount of explosives required to mine it.

"In working the latter system, the extent of the ore-shoot is first

« PrejšnjaNaprej »