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MODEL OF AN
ARIZONA GOLD MINE

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FIELD MUSEUM OF NATURAL HISTORY

CHICAGO
1922

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Field Museum of Natural History

DEPARTMENT OF GEOLOGY
Chicago. 1922

Leaflet Number 1

Model of an Arizona Gold Mine

This model represents a small gold mine of med-
ium richness in a vertical quartz vein six feet wide.
The vein and mine workings are presented in section,
that is, as they would appear if they were cut through
along the center of the vein and one-half removed.

This model is intended to illustrate the ordinary
features present in some form in the great majority of
all metal mines and to show the orderly manner in
which mines are developed and the ores extracted.

Great diversity in the nature and structure of ore
deposits compels a corresponding variety in the details
of workings by means of which the ores of these de-
posits are extracted.

However diverse these details may be, they all
may be grouped as parts of major features which are
common to nearly all metal mines.

In this respect these excavations resemble the
more familiar structures on the surface. As build-
ings, however much as they may differ in appearance,
are all built up of units, as walls, floors, windows and
doors, so these underground workings are composed
of groups of such units, as drifts, cross-cuts, stopes,
raises, etc.

There are some exceptions to this rule. The great
open-cut mines of the Great Lakes Iron Ranges, for
instance, are merely great quarries. In certain de-
velopments of caving systems of mining some features
of the ordinary type are nearly unrecognizable or
quite absent. Coal mines and mines worked along the

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K«tiirsl mtt»nr WSew3

2 FiKM) Museum of Natural History

lines of coal mines have developed a nomenclature of
their own for many of these features.

The mine represented in this model is a small one
in a vertical vein in hard rock. The rock is very firm
at the left near the shaft and becomes less solid to the
right, where the vein is intersected by another one in a
weaker and more broken rock which tends to cave and
fill any openings made in it. The mine is in a region
where timber is expensive and where there is but
little water. Electric current developed by water
power is available for power. The methods of mining
employed are those suited to these conditions.

The vein forms the entire vertical front of the
model. Its intersection with the surface of the ground
is its OUTCROP. Near the shaft and for the greater
part of its length, the outcrop is covered by soil.
Towards the right of the model the outcrop of the
vein appears above the soil in two places, where it can
be distinguished from the surrounding rock by its
darker color. This dark brownish-red color is due to
the presence of large quantities of oxides of iron which
are residues left from the destruction by weathering
of some of the original vein minerals. Such an iron-
stained outcrop is called an IRON HAT.

Not all veins develop an iron hat, but most of
them present at their outcrops color contrast with their
surroundings by which they may be readily traced
wherever they are not covered by soil.

THE ORE in the upper part of the mine is a
weathered and iron-stained quartz. The values lie in
minute particles of free gold disseminated through
this quartz. In the lower levels this ore is replaced,
as is usual in such deposits, by a cleaner quartz, which
contains no free gold, but encloses quantities of the
sulphides of the heavy metals, principally iron and
copper. The two principal sulphides in this vein are

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Model of an Arizona Gold Mine 3

the bright, yellow, glittering pyrite or iron pyrite
composed of iron and sulphur and the similar but
deeper yellow chalcopyrite, composed of iron, copper
and sulphur.

The gold in this part of the mine is thinly dissem-
inated in invisible form through the sulphides. The
quantity of gold is very small. On the average, two
ounces of gold will be scattered through a ton of the
sulphides. Any substance of ordinary value so thinly
distributed would be worthless, as the expense of
blasting and raising the ore would much exceed what
it could be sold for. The value of gold is so great,
however, that even the small quantity in a ton of
sulphides is worth forty dollars, a value that well
repays working.

THE GANGUE is the barren quartz which en-
closes the ore. In practice the terms ore and gangue
are somewhat loosely used. Thus one may describe
this material as sulphide ore in quartz gangue and
another may describe it as a quartz and sulphide ore.
Also the sulphide mineral with such quartz as is
extracted with it in the ordinary process of mining is
commonly referred to as the ore and only that quartz
which is extracted separately or is free from the sul-
phides is called the gangue. The gold is not equally
distributed through the sulphides. In some places the
sulphide will contain much more than the average
quantity of gold and in other places it will carry little
and in some places none. The sulphides without value
are not spoken of as gangue. They are called poor or
barren ore. In the upper levels, where the sulphides
are absent, the ore is that part of the quartz which
contains free gold, while the valueless portions of the
quartz form the gangue.

One of the objects of a good miner is to extract
as much of the good ore as possible and at the same

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4 FiF.i.D Museum of Natural History

time to leave as much as possible of the worthless
material behind, for it costs as much to mine and
raise gangue as it does to mine and raise ore.

If the sulphides are worth forty dollars a ton
and it is necessary to mine three tons of quartz with
each ton of ore, then the material mined is worth only
ten dollars a ton, a value that in so small a mine in
so remote a region will barely pay expenses of mining
and treatment, although larger mines find such ore
very profitable.

When, as is the case here, the ore can be so mined
that with each ton of sulphide only one ton of quartz
must be mined, the material hoisted is worth twenty
dollars a ton and yields a good profit.

As the ore is not distributed equally in all parts
of the vein, there must be certain areas where the ore
is richer than elsewhere. These areas are likely to
have some regularity of form and distribution. The
laws controlling their distribution are frequently very
obscure, so that in many mines the discovery of these
enriched areas before the workings actually cut them
is very difficult and often impossible. They frequently
assume very elongated forms, and are then called
CHUTES or SHOOTS or sometimes CHIMNEYS.
These may run in any direction in the veins. In some
mines the richer portions take the form of patches of
limited area called POCKETS. A very rich pocket in
a gold mine is usually called a GLORY HOLE. Often
ore chutes cannot be distinguished by appearance from
the rest of the vein and their location and limits can
be determined by assay only. There may be one such
chute or a number of them in a mine.

When the segregations of rich ore take an elong-
ated form, thick at the center and tapering toward
each end, they are called LENSES.

[4]

Model of an Arizona Gold Mine 5

This mine is on a large ore chute. After the vein
had been found, the prospector explored it along its
length until he found a place where it was unusually
rich. This was the outcrop of a chute. As the mine
grows deeper the bottom of the shaft, while still in
the vein, will leave the ore body. The working faces
of this mine at these deeper levels will be off to the
left and will be connected to the shaft by a long
gallery.

In the upper part of the vein, called the OXID-
IZED ZONE on account of the nature of the minerals
contained in it, the distribution of the gold is much
more irregular than it is in the SULPHIDE ZONE
below. In the oxidized zone the ore in the main ore
chute is again segregated into minor chutes. Three
of these are plainly shown by the worked-out areas
pitching do^\^lward and to the left which appear in the
upper part of the model.

Entrance to the mine is through the SHAFT
which appears at the left. This is a vertical shaft.
To suit other conditions, shafts in many mines are
inclined. Such shafts are sometimes called INCLINES.
In still other mines it has been found better to run a
nearly horizontal gallery into a hillside. This, which
is practically a shaft laid on its side, is a TUNNEL.
The shaft in the model is divided into two COMPART-
MENTS and is hence a two-compartment shaft. To
the right is the HOISTING COMPARTMENT through
which the ore is raised and through which entrance
and exit are ordinarily made. The smaller compart-
ment to the left is the LADDER-WAY. In this are
placed the various pipes and electric wires which enter
the mines, as well as ladders for emergency use. In this
mine the ladders are continuous. This is unusual. In
most mines the ladder-way is interrupted every thirty
feet or so by a platform to minimize the danger of falls.

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6 Field Miisettm of Natural History

In PROSPECTS, that is, preliminary workings
to determine the value and nature of an ore deposit,
the shafts usually have but one compartment and in
some districts of small mines one compartment shafts
are common. On the other hand larger mines usually
have more than two compartments. Three and four
compartment shafts are usual.

The shaft is strengthened by a skeleton lining of
heavy timber. Horizontal rectangular frames spaced
at frequent intervals are firmly held in place against
the rock by wooden wedges. These frames, called
SETS, are connected as a continuous framework by
upright posts at the four corners. Other uprights
mark the division between the compartments. Cross
timbers in each set complete the timbering and sepa-
rate the compartments.

Near the surface, where the shaft passes through
soil and broken rock, further protection is necessary.
This is provided by LAGGING, which is a plank lining
to the shaft.

In the hoisting compartment is the CAGE. This
is an elevator much like the elevator of an office build-
ing. On this cage, the cars of ore are raised and
access to the workings is provided for supplies and
men. Over the top of the shaft is the HEAD-FRAME.
This carries at its summit a pulley over which the
cable from the cage passes to the HOISTING EN-
GINE housed in a building at the rear. At the bottom
of the model the shaft is continued to form a kind of
cistern called the SUMP in which the mine waters
collect. On the lower level of the mine to the left of
the shaft is the PUMP driven by an electric motor.
This takes the water from the sump and delivers it to
the COLUMN, a large pipe in the ladder-way, through
which it passes to the surface.

[6]

Model of an Arizona Gold Mine 7

THE LEVELS. When the shaft had been sunk
fifty feet a small chamber called a STATION was
excavated at the right. From this station a long
gallery called a DRIFT was excavated along the length
of the vein. A drift differs from other passage ways
in mines in that it always runs lengthways of the
deposit.

This drift and the workings connected with it
constitute the FIFTY FOOT LEVEL, so called be-
cause its drift is fifty feet below the surface.

When the shaft had been sunk a hundred feet at a
point near the base of the model, another station was
cut and another drift started. This drift and its
workings constitute the HUNDRED FOOT LEVEL.

On the hundred foot level to the right of the shaft
is a gallery running directly into the model. This is a
CROSS-CUT, so called because it cuts across the de-
posit. This cross-cut runs through the vein and into
the country rock behind. It was driven to find out if
there was another vein parallel with the one worked.

From the lower level two shaft-like openings
extend downward. From the upper level a similar
opening extends down to the left. These are WINZES
which were driven where some features of the vein
suggested that exploration in this direction might be
of value.

Near the right of the model a vertical, shaft-like
opening extends from the lower level to the surface.
This is a RAISE, so called because it was excavated
from below upwards. The reason for driving it from
below is that in mining it is much cheaper to work
from below than from above. This raise is lined with
CRIBBING, that is, logs built up log-house fashion.
The raise has two objects. By providing a second
opening to the surface it permits a circulation of air
and furnishes much needed ventilation. It is also the

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8 Field Museum of Natural History

final stage of blocking out the ore that lies between it
and the shaft. It will be seen that the block of ore
lying between the hundred and the fifty foot levels
and between the shaft and the raise has four of its
six faces open for examination. Experience has sho'w^l
that when four faces of a body of ore can be examined,
a very fair estimate may be made of the quantity and
value of the ore. Such ore is said to be BLOCKED
OUT or ORE IN SIGHT. On this model two such
blocks are shown, one from the surface to the fifty
foot level and the other from the fifty foot to the
hundred foot level. The mine workings described,
shafts, drifts, cross-cuts, raises, etc., constitute DE-
VELOPMENT WORK which must be done before the
extraction of the ore in a large way can be undertaken.
It is customary to carry development considerably
ahead of the mining proper so that it may be known
what may be expected of the mine for some time in
the future.

Some ore is extracted during development but at
a cost much greater than that of the average cost of
mining.

The larger part of the ore is taken from chambers
called STORES.

The principle on which mining is carried out in
this mine and in the great majority of all metal mines,
is to so conduct the operations that the ore always
moves downward to the shaft and no lifting of the ore
is performed except at the shaft. Ore moves down-
ward by its own weight, but to raise it, effort, human
or mechanical, must be employed. It is economical
to concentrate all this application of power in one
operation at the shaft. Stopes worked along these
lines are called OVERHAND STOPES.

The beginning of a stope is shown about midway
of the lower level, where a small opening appears

t8]

Model of an Arizona Gold Mine 9

above the drift. Here a narrow niche has been exca-
vated to the side of the drift and a raise has been
driven a few feet into the ore of the roof. A miner
may be seen blasting out the beginning of a stope above
this raise. Between this and the shaft appear two
stopes in operation. These two have in the progress
of the work run together, making one large stope.
There are many ways of conducting mining operations
in a stope and the methods employed must be adapted
to the conditions in the mine. The method employed
here is called shrinkage stoping and the stope is a
SHRINKAGE STOPE. The ore is blasted from the
roof and allowed to lie on the floor of the stope. As
the broken ore occupies more space than it did when
unbroken, if all were allowed to remain it would more
than fill the stope. It is therefore drawn off from
time to time from below but leaving enough behind so
that miners standing on it may reach the roof con-
veniently. Standing on this broken ore they drill
holes in the ore above them, using for the purpose a
special type of air drill, called a stoper. The com-
pressed air for these drills comes from an air com-
pressor on the surface in a small building behind the
shaft. It is led down the shaft and along the drifts
in metal pipes. From these pipes it is conducted to
the drills in the stopes by rubber hose. When the
proper number of holes are drilled they are loaded
with dynamite and after the men have left, the holes
are fired, breaking more ore. In a small stope on the
lower level, the holes have been loaded and a miner is
preparing to light the fuses. The excess ore is then
drawn from below and the operation repeated. Access
to the stope is by a manway with ladders which is kept
open through the broken ore by timbering. At the
bottom of the stope near the entrance of the manway
is a chute equipped with a gate through which the ore
is drawn off into ore cars in the drift below. These

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10 Field Museum of Natural History

ore cars run on a tramway to the shaft. At the shaft
tliey are run on to the cage and taken to the surface.
The broken ore in the stope not only provides a work-
ing platform for the miners but also serves in place of
timbers to keep the sides of the opening apart. At the
upper left corner of one of the stopes a small raise has
been driven to the level above for ventilation.

When all the ore has been broken, the continued
withdrawal of the broken ore from below would leave
the stope empty. Left empty it would in time cave
and the motion of the rock thus produced might dis-
turb other workings of the mine. To avoid this after
the stope has been completed, as fast as ore is drawn
from below, waste rock is brought from the surface
to the fifty foot level and fed into the stope through
this ventilation-raise. The stope will thus remain full
even after all ore has been withdrawn. On the upper
level there are a number of worked out stopes which
have been filled from the surface. Not all stopes are
thus filled however, since in many cases conditions are
such that caving will do no harm, and there are some-
times other reasons for leaving them empty. In some
systems of mining caving is even invited and is so
controlled as to be of great assistance to the mining
operations.

On the lower level and against the raise are two
stopes where on account of the value of the ore, none
has been left to form a floor. Here the timbering of
the drift has been made heavier and a floor of poles
laid over it to hold the broken ore.

In one of the open stopes of the upper level the
walls incline to bulge inwards so that broken ore left
in the stope would bind and could not be drawn ofl".
In this stope ore has been removed as fast as broken.
The walls are kept apart by round cross timbers called

[10]

Model of an Arizona Gold Mine 11

STULLS. The miners work on temporary wooden
platforms laid on the stulls.

The drifts are made more secure by TIMBERING.
As the rock is firm and as these levels are so near the
surface that there is little pressure, not many timbers
are necessary. For a long- way from the shaft the
drifts have no timbers whatever. Towards the right,
where the rock is more broken, sets of timber designed
to hold the roof of the upper level consist of two
POSTS, one on each side supporting a CAP.

The corresponding place on the lower level is
provided with full SETS of timber consisting of two
POSTS, a CAP and a SILL.

Near the right of the model is a vein of another
kind which runs at right angles to the face of the
model. This vein carries silver and lead. In the
upper part the ore is an impure carbonate of lead
stained with iron oxides. In the lower part, it is
the dark-colored sulphide of lead, galena. The silver
content of the ore is disseminated in invisible form
through the lead-bearing minerals. This vein DIPS
to the left. The left wall overhangs the vein and is
the HANGING WALL. The wall to the right is the
FOOT WALL. This vein and its country rock are so
broken that the method of mining employed on the
gold-bearing vein cannot be used and exploitation
must be by more costly methods.

On the upper level the stope runs upward from
the drift and will eventually break through at the
surface. The stope is left empty and is kept open by
numerous cross timbers (STULLS). At the lower
level the tendency to cave is too great to be resisted
by stulls and an elaborate system of timbering called
SQUARE SET is employed. This, which is often em-
ployed in deep mines and where the ground has much

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12 Field Museum of Natural History

tendency to move, consists of a continuous system of
hollow squares framed from heavy timbers. In this
mine these squares are ten feet on edge but under
worse conditions, they have to be made much shorter.

Henry W. Nichols.

This model is exhibited on the second floor of the Museum in
an alcove in Frederick J. V. Skiff Hall.

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