Mineral Guide > Mining of Metals

Mining of Metals

Mining of MetalsOur knowledge of what is underground, so far as it is based on actual observation, has been chiefly obtained from mines. Natural caves, of course, are at times extensive, and the visitor may wander through miles of passage-ways; but caves seldom reach to any very great depth beneath the surface, and they are practically limited to one kind of rock, limestone. Mines, however, have been sunk in all kinds of rock, and to very considerable depths; for while, as a rule, they vary from a few hundred to a thousand feet down, yet some have gone much lower.

The shafts in the silver mines at Prizbram, in Bohemia, are nearly four thousand feet, and the great shafts of the copper mines on Lake Superior are already between four and five thousand feet, and will go deeper. Ten thousand feet is probably about the limit of practicable excavation, and very rich ore, it must be, which will tempt the daring miner even to this depth.

All our observations go to show that the rocks at the depths thus far reached are essentially like those on the surface, except that they are hotter; for we now know that the temperature of the earth's interior gradually and regularly rises as we descend.

When an observer unfamiliar with mining stands on the surface, near the mouth of a shaft, and looks down into the dark, misty hole, he probably feels as if it were something uncanny, and as if it led to mysterious, if not dangerous, depths. If he descends with a guide, and is taken through the winding passages and up and down the ladders, he feels confused and at a loss. He probably concludes that the work is all a haphazard enterprise, and that the operators chance upon the ore as a matter of good luck. And yet the mining and explorations are conducted, when under good management, in accordance with some reasonable plan, and almost always because the superintendent has formed an opinion of the way in which the body of ore was formed in the regular processes of Nature, and by this opinion, combined with sound engineering practice, he is guided in developing the workings. As fundamental to all mining, therefore, it is necessary to have some good grasp of the geological processes that lead to the formation of ores. All bodies of ore have such definite histories, the result of the labor of many close observers and acute thinkers, that the man engaged in mining can now be placed in command of much useful and sound scientific theory.

For our present purposes we may briefly classify orebodies, as (1) those formed in loose surface deposits, and (2) those deposited underground. The first class consist chiefly of heavy minerals, such as native gold and tin-stone, which become concentrated, because they are heavy enough to sink in swift streams and currents of water, while the light sand travels on. Their mining is, as a rule, simple, and does not always require even shallow, underground work.

But if a flow of lava, or the accumulation of heavy beds of overlying gravel buries them up, they must be followed below the surface. The second class of deposits are the chief basis of deep mining. They are mostly produced by the circulation of ore-bearing solutions along cracks in the earth, which are either of large size or are individually small, but are grouped in series. In these cracks, usually called fissures, the ones crystallize out together with barren minerals known as gangue. By this process a vein or group of veins is produced, which outcrops on the surface, where it may be discovered by some prospector. The way in which the original crack, or series of cracks, has been formed by earth movements, and the direction from which the ore-bearing solutions have come are the chief points which the operator considers when he decides how he shall direct his excavations.

There are other forms of ore-bodies, besides the ones that fill these long and relatively narrow fissures; but, except in the case of iron ores, the veins, as defined above, are the ones most commonly mined, and when once located much the same plan of developing them is adopted, whatever the kind of metal. A moment's reflection will show that when a mine is begun upon one, the superintendent has before him the problem of taking out of the ground a long and relatively thin body of valuable mineral that may run down into the earth at a steep or a flat angle, that may swell and pinch, and that may be rich in some places, and poor or " lean " in others. At the same time he must keep the workings free from water, for a mine, after all, is a huge well; he must support with timber the overlying wall-rock, from beneath which the ore has been taken; he must either treat the ore himself to get the metal from it, or he must sell it to smelters; and all the while he must appreciate that he is " eating his cake and having it too," for his ore will not last forever, and before it is gone he must get back all the money that was paid for the property, and a good interest rate in addition.

In order to make clear the general method of procedure, let us take an instance in gold mining in California. The early "forty-niners" found the yellow metal in the stream gravels. They dug these over in a few years, and washed out the gold dust at first with pans, like milk pans; later with a rocking, canvas-covered device, suggesting a baby's cradle, and called a" cradle "; still later, in a swift current of water, in troughs of wood, that had cross-cleats called " riffles," the troughs themselves being called sluices. In time these rich gravels gave out; attention was directed to the veins on the mountain slopes, whence the gold had come.

Imagine a prospector wandering over the sides of the Sierras. In the soil he notices stray fragments of rustylooking quartz. Suspecting the presence of gold he pounds it up fine and washes it in his pan, obtaining perhaps a small scale or two of gold, the so-called " color." Immediately he searches up the hillside for the outcrop of the vein that furnished his loose "float," and perhaps finds it projecting somewhere, as a band of rusty quartz, a foot or two wide, or perhaps he digs a ditch to bedrock and uncovers it. At once he stakes out his claim, fifteen hundred feet along the vein, and one hundred and fifty feet each side of it, sets up corner posts and center posts, and writes a notice of his discovery, which he tacks up. Blasting out some of the quartz, he makes his discovery shaft, so-called, packs up his samples to be assayed, and returning to the nearest settlement has them tested, and registers his claim. Let us suppose that the assays show good values, and that the property is sold to some one with the necessary capital to develop it. The outcrop is tested for the richer places, and on one of these the shaft is sunk, at first with a bucket and windlass ; later with a bucket and a small hoisting engine, which are replaced in time, perhaps, with a larger engine and a platform elevator, called a "cage." If the values prove good, probably at a depth of about fifty feet, a tunnel, called a" drift " or " level," is started each way from the shaft along the vein. If the owners have confidence in the claim, they may wait for a greater depth before starting the level, as the ordinary distance is from sixty feet to one hundred feet. While the level is advancing in each direction, the shaft is being continually sunk, so as to begin a new level, called the second, and then a third, and so on. In these blind drifts and in the shaft the air gets bad quickly, and some sort of a blower is needed to renew the supply. Water is met, necessitating the installation of a pump. For a supply of fresh air by natural draft the early levels are driven ahead as rapidly as possible, and a new shaft is begun, perhaps a hundred yards from the first one, or a short connecting shaft, called a "winze," may be sunk between two levels. With a vein two feet wide considerable wall rock is, of necessity, removed, in order to give the miners elbow room in which to work, and this if barren is thrown away on the dump. The shaft is timbered and lined with boards to keep loose rock from falling down, and props and cross-pieces are placed in the levels to prevent caving in.

As soon as the ore is blocked out by one or more levels, the miners begin to blast it down between the levels, or between the first level and the surface. This is called "stoping," and may be carried on in two ways. Usually it is by " overhand stoping." A miner blasts down the ore over the level until he gets a perpendicular working face of six or eight feet. Then, propped up on timbers, he drills horizontal holes in this, so that the powder will throw the ore down to the level, and as he advances he takes off a horizontal slice parallel with the level, and just above it. Meantime when he has gone twenty-five feet or less, another man may come along behind him and take off the next higher slice, and so on, as many as there is room for, or as many as the hoisting machinery can take care of. But as each blast would block the level, it is customary to set timbers across it, at about six or seven feet from its floor, after the first stoper has gone along, and these timbers are called " stulls." tough boards or poles called "lagging," are laid on them, making a roof over the level, onto which the stoping miner throws down his ore. At intervals openings are made in the lagging, through which the broken ore can be tapped or shoveled into wheelbarrows or tram-cars on the level's floor. In this way work proceeds on all the levels, and the ore is systematically removed, timbering and waste rock being inserted to support the walls when necessary. And hence, what may seem to the chance visitor a haphazard excavation in the dark, is really a carefully planned and systematically executed operation.

The other method of stoping is called the "underhand." In it the miner begins next a winze, and drills holes downward either in the floor of the level or in the sides of the winze, and blasts the ore outward and down the winze to the level below, through which it is removed to the shaft. As the miner advances downward a vertical prism of ore is taken off, and then another, and so on. Underhand stopes are far less frequent than overhand, and are seldom so desirable.

In accordance with this systematic plan underground mining proceeds in nearly all metal mines based on veins, that are not too wide. Lean spots, too poor to mine, are left untouched, and exploring shafts and drifts are kept ahead of stopes, so as to have reserves blocked out. Almost always the rich portion runs diagonally down the vein, and is called a ""shoot" or "chute." When this is located and understood, the work is planned accordingly. Cross-tunnels, called "cross-cuts," are often run into the walls in search of parallel veins, and every endeavor consistent with prudent management is made to keep fully informed about the location of the profitable ore. The difficulties lie in meeting weak walls, that cave in and crush supports; in wet spots with influx of water, and in fires that make sad havoc among the timbers; but the worst mishap of all is the pinching out of the ore.

In many mines the ore body proves too wide for the methods just described for narrow veins, and while thick it may lie in the rocks at too flat an angle to enable single timbers to span the gap from wall to wall. In such cases it is usual to adopt a system originally invented for the great bonanzas on the Comstock Lode, and known as Comstock "square sets." Heavy timbers, each six feet long, are mortised together so as to build up a series of hollow cubes, of which the timbers form the edges. Each single stick is an edge for the four cubes adjacent to it, and in this way a huge hollow crib-work arises, which may be filled in with loose rock, and which supports a vast weight. The plan has proved of priceless service in many great mining districts, for without it much valuable ore would of necessity be left to support the overlying rock.

A mine is often popularly regarded as a sort of hidden treasure, which a lucky discovery brings to light, and which is a source of profit to its owner, far beyond that of ordinary business. In instances this is undoubtedly true, but it is also true that in most cases mining requires the same special knowledge of the business and the same special training that are essential for success in mercantile affairs, or in the practice of medicine or law. The development of a large mine is an engineering enterprise quite as difficult as the building of a railway, or the construction of a bridge, and calls for the same degree of skill and ability. Many chances and uncertainties are involved, and when persons unfamiliar with mining make investments in mines they should realize their dependence upon their advisers, and that this very fact makes them an easy mark for a plausible and unscrupulous promoter. Yet the magnitude of the interests involved in mining is enormous, and he who ably and honestly practices the art adds to his country's wealth, just so much that she did not possess before.

Iron and copper pyrites look so much like gold that they are often mistaken for gold by inexperienced persons. Quartz may contain gold when none is visible to the naked eye. In Paraguay, pieces of gold weighing from one to fifty pounds were taken from a mass which fell from one of the highest mountains. The largest piece ever found in the world was found in California, yielding one hundred and nine pounds eleven ounces of pure gold, and sold for twenty-seven thousand six hundred and sixty dollars.

Silver ore usually contains lead and other metals, and is hard to separate.

Copper occurs in northern Michigan in larger quantities than in any other part of the country. Specimens weighing two tons have been found there.

Tin is found in the Black Hills of South Dakota, but it is in such thin scales in the rock that it is unprofitable to work it out. Most of the nickel produced in America is from Sudburg, Ont. Nickel has become very valuable from being used in the manufacture of armor plate for war ships.

Lead is found in immense quantities in Illinois, Wisconsin, Missouri and Arkansas. Three million pounds have been taken from a spot fifty yards square.

Conservation of Iron Ore
Minerals Containing Carbon
Process of Obtaining Coal
Sources of Coal
Uses of Coal