The Business of Mining Part 5

The following relative advantages of the several types of mine mouths are in addition to those already given and are worth consideration:

With an incline, the value of a tabular deposit is determined as work progresses; the course and dip of the body will be known at all depths along the incline; the body may be explored from the incline in both directions, simultaneously, with a resulting doubling of the development and production; all, or nearly all, the material removed is "vein stuff"

and its value may repay the sinking expenses; there is no losing of the ore body unless a geological fault is met.

With a shaft, more rapid hoisting is possible than with an incline; the timbering labor is less than in the case of an incline, but greater than in the case of an adit; with ground containing ore bodies in irregular ma.s.ses and at no uniform intervals, vertically or horizontally, stations and levels may be started wherever desirable; the crosscuts which are usually necessary to reach the bodies may disclose otherwise unknown bodies.

[Ill.u.s.tration: MILLS AND SHAFT HOUSE OF DALY WEST MINE, PARK CITY, UTAH.]

With a vein adit, the vein is prospected as work advances; the ore removed may pay its own way, as it were; the drainage is automatic; ore is transportable from the mine by haulage rather than by hoisting; the ore in place is above the level and will handle itself to the outgoing pa.s.sage by gravity.

With a crosscut adit, in addition to the last three advantages noted for the vein adit, there is bound to be exploration of the ground upon at least one side of the known body; there will generally be easier haulage because of the straighter track, since an adit driven along a vein will conform to the geological irregularities and the track is bound to be more or less crooked.

Without counting upon the doubtful success of the numerous propositions in tunneling machines, but judging only from past experiences, we may say that a shaft will cost about three times as much as a "tunnel" of equal transporting capacity. If the ground is wet, the discrepancy in first costs becomes much larger. In a remote region, with difficult transportation of machinery and fuel, it may be better to drive and use a long adit rather than a shallow shaft. An adit will transport more product than will a shaft of equal dimensions.

An adit may be driven to intercept a shaft and to serve as a sort of artificial surface, as it were, and thus save expenses in pumping and in hoisting up to the original collar of the shaft at the surface of the ground.

No matter how crooked an incline may be, it is possible to hoist ore in conveyances known as skips, although the hoisting may be necessarily somewhat slow. These same conveyances are useful for lowering and hoisting men, and the parody, "Men go down to the mine in skips," here finds its significance. The usual hoisting conveyances used in shafts are known as cages. They usually produce less friction than do incline skips. A skip in an incline must travel upon a track, while a cage, somewhat resembling a pa.s.senger elevator, has no wheels, but slides upon guides. However, an incline skip, because of the inclination of the pa.s.sage, does not exert the same dead weight upon the cable and hoisting engine and hence these parts of the equipment may be made correspondingly lighter. Skips for shafts are similar to cages in their lack of wheels.

Complete estimates of probable future requirements should be made before a shaft is sunk. When it becomes necessary to enlarge a single-compartment shaft to one with two compartments, the expense has been found to exceed one-half the original cost of sinking; while, to convert a one-compartment shaft into a three-compartment shaft costs fully three-fourths of the original sinking expense. Approximately the same ratios of cost will hold in the case of enlarging inclines.

Character of ore sometimes influences the selection of the kind of pa.s.sageway. Some high grade, brittle ores must not be dumped nor handled repeatedly, since values are lost in the "fines." Iron and copper ores will not probably be injured by any amount of dumping. Coal should be handled as few times as possible. In view of this fact, other things being equal, adopt that system that will injure the ore or coal the least.

As a rule, workmen are safer in tunnels than in shafts, since there is little danger from objects falling any great distance. Tiny bits of rock have been known to kill men in shafts. On the other hand, there is less liability of injury from falls of large rocks in shafts than in adits.

Roof falls are a very prolific source of mine accidents.

The workmen of neighboring mines will often be able to give much valuable information as to the proper procedure in opening a new property. For instance, water levels, amounts and kinds of gases that may be expected, the nature of the wall rocks, and other pertinent points may be learned by interviewing the men who are employed in adjacent mines. Still better information may be obtained by personal visits to the underground workings of the nearby mines. In this connection, one must not permit himself to be unduly influenced by the prejudices or hobbies of the neighboring operators or their employes if there is reason to suppose that such notions are contrary to good practice.

Due consideration must always be given to the selection of some method of opening up what might be supposed will never amount to a great mine, so that, should subsequent disclosures exceed expectations, enlargement of the scale of operations can be advantageously effected. Always bear in mind that legitimate mining is just as much a commercial enterprise as is any other kind of business. The utmost concern for financial showings must be constantly borne in mind. Select a scale of operations consistent with the known--not the hoped-for--bodies of coal or ore; but have a certain feature of elasticity about the plans that may take care of future increase in business if found desirable. Do not "over-plant."

Never plant, at all, _prematurely_. It is better to postpone the installation of the equipment until some specific facts are available.

Many companies have met defeat in the exhaustion of capital through the purchase and installation of elaborate plants which were never warranted.

After a mine is once opened and preparations have all been perfected to operate upon a certain scale of output, it is quite essential that exploitation and production be maintained without material fluctuations, if the greatest economy is to be attained. Exploitation, _i.e._, development work, must be kept well in advance of actual mining operations to a.s.sure plenty of working s.p.a.ce for the extraction of the normal output.

XI

TYPES OF ORE BODIES.

It has been necessary, a number of times in this discussion, heretofore, to make mention of kinds of ore bodies. It is well, at this time, to get some fixed ideas concerning the leading types of bodies of minerals which are extracted as ores.

Because of the laxity in type differentiation which has prevailed among miners and writers, the same geologists who have framed definitions of ore, have also defined the various types of ore bodies. The definitions, having been accepted by the leading mining geologists and engineers of the present day, it is well for us to fall into line and to agree with the authorities in such matters.

A _vein_ is a _single, ore-bearing fissure_, generally, though not necessarily, with at least one well-defined wall.

When we run across a tabular-shaped deposit of ore that looks as though it may have been put into a pre-existing fissure or chasm, the chances are that it is a vein. But a vein must not be confounded with a dike. A dike is a filling that has been injected, while molten or fluid, into an open pa.s.sageway or rupture across rocks, or into an opening which it created for itself. A little examination of the material should tell, to even the novice, whether or not the substance is of plutonic origin. The filling of a vein is not eruptive, at all. Veins have been filled from circulating aqueous solutions, by slow depositions, that have occupied very long periods.

A vein may be any thickness, since a fissure may have been opened to any width. Hence, a vein may be as thin as a sheet of paper, or it may be a hundred feet across. However, it is true that some wide veins have resulted by a sort of enlargement from original thin seams. Very few of the notable wide veins of the world are believed to have been created by the filling up of chasms originally as wide as the present ore bodies.

But, in all cases of real veins, there were original fissures, fractures or crevices which acted as channels for circulating solutions that contained the materials which were left to make the vein matter.

A _lode_ is an _a.s.semblage of veins_ so closely s.p.a.ced that the ground between the veins becomes, in places, ore-bearing, and the entire width of the aggregation becomes an ore body.

A zone of sheeted rocks like schist or slate, if sufficiently mineralized to warrant mining, would be a lode. Sometimes, in certain districts, the earth's crust has been subjected to many approximately parallel, closely-s.p.a.ced fractures, and by the subsequent filling of these cracks, with the accompanying corrosion of the walls and their replacement by ore, extraction of the entire ma.s.s of rocks across a considerable distance will be found to yield a profit. Any such body is a lode.

In the Cripple Creek District, the ground is criss-crossed in every direction by tiny fissures which have resulted from the contraction of the country rock, just as a bed of mud is fissured in the process of drying up after a rain. Wherever these fissures are found in aggregates that are closely s.p.a.ced and in which a majority of the cracks have a general trend so that the whole a.s.semblage can be readily worked as one ma.s.s, this whole body of fractured rock may be found worth mining and it will then const.i.tute a lode. It may be mentioned here that the so-called ore of this district is not really ore according to the accepted definition. The true ore, the filling of these innumerable, tiny cracks, really const.i.tutes but about five per cent. of the material that is shipped as ore, but which is princ.i.p.ally the "country rock" broken down with the small volume of ore.

In _legal_ phraseology, the word lode has come to include all sorts of ore bodies. When the word is thus used, in a legal sense, it should not be confused with the strictly technical meaning.

It has been the fashion for prospectors to dilate upon the fact that they have located "true fissure veins." This expression, formerly on the tongues of most mining men in districts possessing veins at all, is now obsolete and hence should be placed in the discard. There can be no such thing as an "untrue" vein nor an "untrue" fissure. Neither can there be any vein without a fissure. Therefore, if there is any vein, it must be a real or true vein. Accordingly, the verbiage is to be discouraged. The intention of a miner, in using this pet phrase, has been to convey the impression that his vein extended downward, indefinitely; there having arisen a notion that some veins are rather superficial and liable to "peter out" at slight depths, while others--the kind he invariably has located--persist both in size and value to extreme depths.

There are districts in which are found short fissures, generally confined to certain horizons in sedimentary rocks, such as the limestones of the great Mississippi Valley, from which are mined lead and zinc ores. These are called "gash veins." These are always readily recognized and there is not the slightest excuse for confusing them with the fissures which are common to other kinds of rock formations.

A _bed_ or _blanket vein_ is the term applied to any nearly flat deposit conforming to the bedding. Such a body of ore must be in a sedimentary series of rocks. Coal bodies are all of this type. Many bodies of iron ore are also of this type.

A _chimney_ is an ore body which has not the tabular form of a vein but is rudely elliptical in outline, horizontally, and with a very considerable vertical extent. A _stock_ is a similar body but it is of still greater irregularity of boundary.

These bodies are usually the filling of extinct volcanoes or geysers, and therefore they are presumed to extend to very great depths. The diamond mines of Kimberly, Africa, are of this type and the ore is a sort of hardened geyserite or mud in which are enclosed the precious gems. In Custer County, Colorado, the ore body of the Ba.s.sick Mine is a conglomerate of rounded boulders of all sizes cemented together, somewhat like concrete, by the materials which really carry the values.

This ma.s.s occupies an ancient volcanic neck or throat of a geyser, probably the latter. The main portion of the Cripple Creek District is the crater of a great prehistoric volcano. It might be called a great chimney, but custom seems to limit the use of the word chimney to a smaller body such as might be included in a single mining property.

A _ma.s.s_ is a deposit whose irregularity of shape is so great that it cannot be recognized as belonging to any of the types already mentioned.

Ma.s.ses conform to no rules as to shape or size. They are usually the result of a chemical dissolving of the original barren rocks with a simultaneous or subsequent subst.i.tution of valuable materials. There are many instances of ores that have been deposited, molecule by molecule, replacing equal volumes of the previous rock, much upon the order of the petrifaction of wood. Again, there are immense ma.s.ses which are believed to have acc.u.mulated in caves already dissolved out of the containing rocks.

While recent geological study of the districts in which such ore bodies abound have disclosed numerous facts about their occurrence, there still remains much conjecture concerning their origins, and we may still believe that they do not conform to any rules as to regularity or size.

The ore bodies of Leadville are of this type, and they may be described by the homely similes that they are as like and as unlike, and their occurrences are about as regular, as potatoes in a hill. The potato-tops give the farmer a suggestion as to where to dig. So, also, do certain geological relations guide the miner. And yet a shaft may be sunk hundreds of feet down among ma.s.ses and not happen to penetrate a single one.

There are numerous recognized types of ore body not enumerated here; but it is sufficient for the average layman in mining matters to understand these few distinct types and to believe that all other types are rarities, and are, as a general thing, but intermediate forms of those defined.

[Ill.u.s.tration: Shaft No. 3, TAMARACK MINING COMPANY, CALUMET, MICHIGAN.]

[Ill.u.s.tration: SMELTERY OF THE BALAKLALA CONSOLIDATED COPPER CO., CORAM, CALIFORNIA.]

XII

THE QUESTIONS OF DEPTH AND GRADES OF ORE.

The prevailing belief of a few years ago that ore bodies always improve with depth has been discredited. Not a single mining geologist will longer maintain such a notion. The evidence of many thousands of mines has refuted this older belief and it has been proven that quite the opposite view is the correct one concerning changes of value with depth.

Values, instead of getting better, do actually, in the majority of cases, grow poorer as depth is gained.

President C. R. Van Hise, of the University of Wisconsin, was among the early expounders of the newer theories to account for this fact. The writer heard him state, years ago, before a scientific gathering (which, at that time, was not quite ready to agree with him), that if he were given his choice, he would much prefer to own the upper thousand feet of the earth's crust than all the rest of the globe. In this remark, he was referring only to mineral values, of course.

This belief that the best values are to be found not far from the surface has since become popular, for it is based upon proven facts. It is not claimed that values are never mined below an elevation that is a thousand feet from the surface. There are many mines, and great ones, too, that are operating at depths greatly exceeding this distance; but in these same mines there will be found valid reasons for not applying the general statement to their particular cases. For instance, the great copper mines of the Keweenaw Peninsula are productive at depths of a mile or more from the surface; but we believe that here the ore must have been originally deposited at, or near, the surface, that it was then overlain with rock strata; and subsequently steeply tilted by earth movements which carried some of the ore bodies down to the depths where they are now found.