surface. If it be driven along an ore body, as a vein, it is properly
called a _vein adit_; if it is driven _across_ barren country to
intercept presumed or known bodies, it is spoken of as a _crosscut
adit_. All adits must be given a small amount of grade for drainage
necessities.
Before getting underground we should consider what is required in the
way of opening our mine; what is positively known about our body of coal
or ore; and what conditions are liable to confront us later on. We must
consider the type of ore body; character of material to be extracted;
average thickness and hardness of the body; desired tonnage; power
facilities; probable surface and underground drainage to be maintained;
and dozens of other things which only the experienced man will think of
and appreciate. The right kind of a manager will know that he cannot
afford to overlook such points.
Every case involves different contingencies, and therefore extreme
forethought must be given to the subject before deciding upon any
particular kind of an opening into the ground for mining purposes. This
remark does not apply to such openings as prospect drill-holes,
openings which are not for mining purposes, but for exploitation.
Assuming that sufficient data are known concerning the property to
warrant the expenditures incident to the making of a mine, the question
remains as to the best way of proceeding.
It is a well-established fact that it is much cheaper to drive an adit
than to sink a shaft of equal transporting capacity. It is also cheaper
to drive an adit than to sink an incline. If the topography is such that
an adit can be driven into or beneath an ore body and thus expose it
from a low elevation, the temptation is strong and along lines of good
practice to do so. If the country is quite flat or nearly so, or, if the
surface is such that, while rough, an adit of reasonable length cannot
be driven to tap the valuable mineral and handle it economically, then
it is good practice to decide upon a shaft mine.
An adit will not only be cheaper, foot for foot, than a shaft or
incline, but, if given the proper, slight grade, it will afford a
natural drainage outlet for all subsequent workings above its level.
The cost of pumping, as already suggested, may be a considerable item
and it may be a deciding factor in favor of an adit when this form of
opening is possible.
Furthermore, an adit will obviate the installation and use of hoisting
machinery, and thus there may be maintained a greater efficiency in the
operating expense of the mine than would be possible with a shaft.
Again, it is a simpler and cheaper matter to maintain a mining tunnel in
working shape than it is a shaft, particularly in bad ground. By the
settling or "working" of the ground, a shaft may be thrown perhaps but
slightly out of alignment and annoying interferences will be experienced
in hoisting, especially when rapid and uninterrupted hoisting is
necessary to maintain the desired output. While the same amount of
disturbance does take place in an adit, it is an easy matter to readjust
track grades while continuing regular haulage operations.
The timbers, in the case of either a shaft or an adit, will require
occasional renewal, but the expense of such repairs is less in adits
than in shafts or inclines, while the delay to other operations of
mining, in the case of the adit, will be inappreciable.
Topography has been referred to above, but it must be again briefly
mentioned. There are some places in which ore bodies extend to, or exist
at, such depths that adits could not be projected to get beneath enough
of the ore to warrant their construction. An adit mine is not a
practicable thing in a flat country like Nevada or the Rand, but in the
rough country of the San Juan it is the customary kind of a mine. In the
very early days of Comstock Lode mining, shafts were sunk by each of the
hundreds of companies. Before a great while, the advantages that would
accrue from having a deep "tunnel" became evident, and the famous Sutro
Tunnel, with its historic, checkered career, was driven. Although it
loomed up like a gigantic undertaking for that period, the immense
prospective or future value of it could not be denied.
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
masses 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.
[Illustration: 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
passage 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 passenger elevator, has no wheels, but slides upon
guides. However, an incline skip, because of the inclination of the
passage, 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
passageway. 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 assure plenty of working space 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 passageway 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 _assemblage of veins_ so closely spaced 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-spaced 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 mass 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 spaced and in which a majority of the cracks have a
general trend so that the whole assemblage can be readily worked as one
mass, this whole body of fractured rock may be found worth mining and it
will then constitute 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 constitutes but about five per cent. of the material that is
shipped as ore, but which is principally 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 Bassick Mine is a
conglomerate of rounded boulders of all sizes cemented together,
somewhat like concrete, by the materials which really carry the values.
This mass 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 _mass_ is a deposit whose irregularity of shape is so great that it
cannot be recognized as belonging to any of the types already mentioned.
Masses 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 substitution 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 masses which are believed
to have accumulated 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 masses 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.
[Illustration: Shaft No. 3, TAMARACK MINING COMPANY, CALUMET, MICHIGAN.]
[Illustration: 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.
The "reefs" or bankets of the Rand are so termed because these ore
bodies were undoubtedly ancient coast beaches or sea placers. The
gravel, sand, and gold particles were cemented together into a
conglomerate, then covered with many later sedimentaries, and finally
the continent of Africa was so raised or altered in some manner as to
bring these gold deposits into their present inland and tilted
positions.
In veins or lodes, it is not supposed that ore-making minerals could
have been precipitated from solutions travelling either upward or
downward and obeying chemical laws if the depth were sufficient to
furnish great temperature or high rock and hydrostatic pressures.
Therefore minerals which were deposited from aqueous solutions rising
from depths, for example, must have retained their dissolved condition
until they ascended to horizons in which both pressure and temperature
were low enough to permit the precipitation and crystallization that
create ores. Contrarily, descending solutions must have given off their
contents before reaching the deep zones of heat and pressure, or not at
all.
It is a quite common phenomenon to observe that the richest _gold_ ore
in a mine is found close to the surface, if not actually at "grass
roots." The explanation is simple. The gold, being the most stable of
the aggregate of minerals composing the original ore, has the better
resisted the corrosive attacks of atmospheric agencies and has remained
nearly intact, while its associated minerals have been dissolved or
altered and carried away. The same amount of gold remaining with a
diminished quantity of the worthless, non-metallic minerals--the
"gangue"--inevitably renders the ore richer per unit of weight (such as
a ton), although per unit of volume the value remains constant, or
nearly so, so far as the gold is concerned.
But with other kinds of ore, as, for example, copper, the best grades
are found, not close to the surface but some two hundred or more feet
down. The explanation is that the minerals of copper are considerably
more soluble than the ordinary gangues and therefore the weathering and
oxidation that takes place in the upper horizons of ore bodies will
dissolve out the cupriferous compounds and thus deplete the
superficial ore. But, by the flowing of the copper solutions to a lower
zone, there occur certain reactions that reprecipitate the salts of
copper upon compounds of the metal already formed and we have instances
of the phenomenon known as "secondary enrichment."
[Illustration: CONCENTRATOR DIVISION, WASHOE REDUCTION WORKS OF THE
ANACONDA COPPER MINING CO., ANACONDA, MONTANA. Largest Copper Works in
the World.]