This report summarizes mining practices of a large number of metal
mines in the United States and foreign countries. The adaptation of
mining methods best suited to the various natural conditions in mines is
discussed, and comparative costs are presented. The basic data—gathered
in the field by Bureau of Mines engineers and consultants in
cooperation with mine operators—have been published in a series of
information circulars dealing with practices and costs at individual
mines and ore-dressing plants. The data have been made comparable by
adhering to a standard form of presentation in the individual reports.
The
first circular was issued, 250 dealing with mining and milling of
metallic ores had been published. Of this number, 164 deal with
operations at individual mines and plants—94 with mining only, 59 with
milling only, and 11 with mining and milling. Eleven circulars are
summaries, each covering a particular mining method, and 53 are
discussions of separate operations involved in mine operation or deal
with special phases of mining. Seven of them treat special ore-dressing
and allied subjects, and 15 arc a part of a mineral-industries survey by
counties or districts, in which many individual properties are
described and costs and other data are presented. Earlier bulletins
summarized the mining and milling of ores of the principal metallic
minerals,4 or specific problems 5 in the exploitation of metal mines.
The
purpose of this bulletin is to consolidate and analyze the information
contained in earlier reports on mining methods, practices, and costs.
Although
the exploitation of deposits of metalliferous ores cannot be discussed
comprehensively without considering milling and marketing of the ores,
this bulletin is concerned principally with mining or ore production. In
the following pages milling and marketing are discussed briefly only
when they have a direct bearing on some phase of mine exploitation.
The
circulars covering methods and practices at individual mines were
written for the most part by members of the operating staffs in
accordance with an outline prepared by the engineers of the Bureau of
Mines. The outline was designed to produce papers that would:
- describe
the characteristics of the ore deposits that governed the selection and
application of the mining methods employed in each case,
- describe the methods and practices employed, and
- present the costs of mining under the described conditions by the methods used, thus giving a complete picture.
The
outline was responsible for uniform presentation of comparable data
which it was possible to summarize and analyze in a series of reports on
different phases of mining practice.
The study of mining methods
and costs covered by the circulars and bulletins mentioned above would
have been impossible without the cooperation of the management and staff
of the many companies that contributed information and of the
executives of these companies who permitted the gathering and
publication of the information.
The authors acknowledge this
cooperation without specifically mentioning names. Original authors,
both of the information circulars and of articles in the technical press
from which data have been taken, are given in footnote references in
the body of the bulletin. If any have been omitted, the omission is
unintentional. These references not only serve as acknowledgments but
also assist the reader in running down details impossible to include
within the scope of the present summary.
Mining Definitions
The
terminology used in this bulletin is believed to be that employed and
understood by the average American metal miner, shift boss, foreman,
superintendent, engineer, or manager, to whom the bulletin probably will
be of most interest. This may vary somewhat from dictionary
definitions; hence, to avoid possible misunderstanding, a number of
terms are defined as they are employed in this paper.
Mine.—A
pit or excavation in the earth from which ores or other mineral
substances are taken by digging; hence, either an open-pit or an
underground working.
Prospecting—The search for outcrops or surface exposure of mineral deposits.
Exploration.—The
search for unknown ore deposits or the extension of known deposits,
including preliminary development and tests to determine probable value.
Exploration may be conducted from the surface or from underground
workings, by surface trenching, core or churn drilling, or by driving
drifts, crosscuts, raises, and shafts. The term is used here to cover
more comprehensive work than prospecting, and includes sampling and
other procedure for obtaining information upon which to base ore-reserve
estimates or plans for future mining operations.
Development.—The
preparation of a mine for ore extraction, including construction of all
openings required for ventilation, drainage, and transportation of
broken ore to the surface, such as shafts, tunnels, main raises,
crosscuts, and haulage drifts, skip pockets, and pump stations.
Stope Development.—The driving of subsidiary openings designed to prepare blocks of ore for actual extraction by stoping.
Stoping.—The
act of excavating ore by means of horizontal, vertical, or inclined
workings in veins or large irregular bodies of ore, or by rooms in flat
tabular deposits, and also the mining of ore by caving methods. It
covers the breaking and removal of ore from underground openings except
those driven for exploration and development. In many of the information
circulars upon which this bulletin is based the term “mining” was used
synonymously with “stoping” and is often so used in practice.
Underground Transportation.—The
transportation of ore, rock, men, materials, and supplies through
shafts and haulage ways, including the loading of ore or rock into cars
and carrying it to the surface.
Mucking—Hand or
mechanical shoveling and power scraping of ore and rock. Because the
term covers both shoveling and power scraping and because it has become
thoroughly established in mining parlance through universal use, it is
employed freely here in this sense, even though its dictionary meaning
is something quite different.
Prospecting & Exploration
In
this presentation of the technology of mine exploitation, the various
operations will be discussed in the order in which they are ordinarily
performed in the discovery and exploitation of an actual mining
property. Thus, the search for valuable mineral by the prospector will
be discussed first.
In
North America the initial discoveries of valuable metal mines in a
district have seldom, if ever, been the result of organized exploration
of large areas by well-financed companies. Prospecting for metallic ores
has not usually been conducted as has prospecting for petroleum—by
test-drilling or “wild-catting” in areas indicated by structural data
(often gained from elaborate geological and geophysical surveys) to be
favorable for the accumulation of oil. Certain noteworthy exceptions are
found in the exploration for iron ore in the Lake Superior region and
for lead and zinc ores in the Southeastern Missouri and Tri-State
districts. Even in these districts, however, the original discoveries of
ore minerals doubtless were made by chance and at the outset
large-scale exploration was not carried on.
It is true that large
areas of North America have been mapped and studied carefully by
Government geologists, and the published results of their surveys have
been very useful in the search for ore. However, particularly in the
United States, these valuable reports have not been used by the average
prospector as freely and intelligently as they should have been.
With
the depletion of known and more easily discovered ore deposits and the
expected advance in geologic knowledge and application of geophysical
methods, prospecting for ore may some day be conducted on a scale and in
a manner analogous to petroleum exploration, covering wide areas that
the geologist has designated as favorable for ore deposition. In the
past, however, most original discoveries of ore in this country have
been the result either of accident or of the persistent efforts of
individuals or small groups of prospectors, often ill-equipped and
poorly financed.
Prospecting
In
the early days, the search for ore often was conducted in remote
regions, far from any base of supplies. Equipment and food had to be
packed on foot or on the backs of animals, often for long distances.
The
prospector was forced to provide his own shelter and prepare his own
food, perhaps to procure part of his food supply with rod and gun, and
to perform many tasks that absorbed much of the time and energy that
might otherwise have been spent in the actual search for ore.
The
life of the old-time prospector was an arduous and usually a lonely one,
and only those with natural physical endurance beyond the ordinary,
together with determination and extraordinary optimism, were fitted for
it. Relatively few prospectors have received financial rewards
commensurate with the efforts put forth and the privations suffered;
where they have persisted for years without success it has probably been
due to particular aptitude, a liking for the life, and the undying hope
of some day making a big “strike.”
Today, supply bases are closer
and more easily accessible in this country, due to railroad, highway,
and branch-road construction. In many remote regions in North America
and other continents modern means of transportation, notably airplanes
and motorized boats, have brought prospecting fields immeasurably
nearer, in point of time and effort, to bases of supply.
Prospecting
still requires special natural adaptability and, now more than ever, a
knowledge of the different ore minerals, the geological conditions
favorable for ore deposition, and the different rocks and rock
structures friendly to mineralization.
With the revival of
interest in gold mining brought about by the depression of the nineteen
thirties and the consequent resumption of prospecting for gold, many
classes for prospectors were established by Government agencies,
schools, and colleges throughout the world. This was a step in the right
direction and doubtless has resulted in more intelligent prospecting
than would otherwise be done, although classroom instruction alone
cannot engender the qualties so conspicuously displayed by the early
prospectors. Through long experience and observation the old-timer often
amassed a fund of information concerning the various minerals, their
mode of occurrence, and the rocks with which they are associated that
gave rise to that intuitive phenomenon popularly called “a nose for
ore.”
Equipment
The
equipment required varies with the type of prospecting to be done, the
climatic and physical characteristics of the country, mode of
transportation, length of the campaign, and the funds available. Gardner
has discussed and listed the usual requirements in the western United
States for transportation, camps, tools, cooking, provisions, clothing,
and first-aid supplies. For the purpose of the present discussion, it is
sufficient to note that all these different types of equipment and
supplies must be considered.
Mining Laws
Knowledge
of Federal and State or Provincial laws relating to locating, staking,
and recording claims on public lands in the countries and areas in which
the prospector plans to work is essential.
Obviously, prospecting
on private lands can only be done with the consent of and by
arrangement with the owner. The reader is referred to Bulletin 94 for a
discussion of the United States mining laws. Reference is also made to
Circular 1278, United States Department of the Interior, General Land
Office. A summary of these laws as they apply to location of lode claims
is given in Information Circular 6843 (revised). A summary of the
placer-mining laws is given in Information Circular 6611R.
The Bureau of Mines has issued a number of circulars summarizing the mining laws of other countries.
Reporting On Prospects
Usually
it is necessary for the prospector to obtain financial assistance for
further exploratory work before his property can be brought favorably to
the attention of exploration and mining companies whose business it is
to develop and operate mines. More often than not, such companies, the
larger ones especially, are interested only in properties where ore is
actually proved, and they will not go to the expense of an examination
of a “raw” prospect.
This situation often leaves a gap between the
preliminary work of the prospector who makes a discovery and does only
the small amount of work upon it that can be done with his own hands or
his own meager capital, and the work of the development company.
This
gap frequently is filled by work paid for with funds supplied by local
merchants and professional men or by a small syndicate, whose aim it is
to block out enough ore to warrant forming an operating company or to
interest the larger companies in exploitation of the property.
There
are probably many instances in which examination of prospects of merit
has been refused because of failure of the prospector to present
essential facts.
Capital is risked in mining ventures to make a
profit from operation. (This bulletin deals only with legitimate mine
exploitation and not with promotional ventures, the main object of which
is to make a profit from the sale of stock.) Hence, the first
consideration is ore, if ore be defined as a body of mineral of such
grade and occurring in such quantities that under the physical
conditions affecting the cost of operation and in the locality in which
it occurs it can be mined and sold at a profit.
Therefore, any
report, to arouse the interest of mining capital, must present facts
that indicate the existence of ore or the likelihood of its existence.
Misrepresentation of facts by the prospector will not profit him, since
such misrepresentation will only lead to condemnation of his property by
an examining engineer who may be led to visit it; and this, in turn,
might easily make it impossible to obtain a later examination.
Furthermore, misrepresentation in a report can often be detected by an
engineer because of the very means employed to cover the real facts.
If
the prospector can afford to employ a reliable engineer to examine and
report on his property, it is advisable to do so. Even though this
engineer may be unable, because of the amount of work done, to make a
favorable report, he may be able to indicate the best procedure for the
prospector to follow in order to explore his ground further. In
selecting the engineer, it is advantageous to employ one who is not only
able and reliable but who also has the confidence of mining capital.
If
unable to employ such an engineer, the prospector himself may be able
to prepare a satisfactory report, if he will bear in mind the
essentials. The importance of a map and the information that should be
placed upon it have been mentioned already. To this should also be added
any information the prospector can give concerning the mineralogy of
the lode; the kind and condition of the wall rocks; relationship to
intrusive rocks; structural features such as folding, shearing, and
faulting; and surface weathering. There should also be given information
on factors that would affect operating costs in the district, such as
mode and cost of transportation to and from the property, climate,
topography, vegetation, fuel and water supply, availability and type of
labor, and other local conditons. Self-deception is a real hazard to a
man preparing a report on his own property, and its possibility must be
recognized and forestalled.
If ore is actually blocked out the
prospector may include an estimate of its tonnage and grade; but if maps
(in some instances cross sections also may be necessary) are included
showing the essential facts as previously outlined, the examining
engineer can and will make his own deductions regarding ore blocked
out. The essentials for a preliminary report have been discussed in more
detail by Wright.
Exploration
Assuming
that prospecting operations and preliminary exploration have revealed
the existence of a lode or body of ore-grade material under geological
conditions favorable for extensive deposition of ore and that money has
been provided for a more thorough exploration program, the procedure to
be followed will then depend upon a number of things, some of which
already may have been partly determined by the preliminary work.
Thus,
the type of deposit, its shape and dip, its relation to the surface
topography, and the nature of the topography, and the character and
condition of the lode and wall rocks are important considerations. The
amount of money available for exploration, physical aspects of the
property, accessibility for men and supplies, and fuel and water supply
also may influence the procedure.
To direct exploration
intelligently, careful records should be kept of results obtained,
especially of all geological information, as the work progresses, in
order that each step may be guided by the information obtained earlier.
It is of great advantage to be able to obtain prompt assay returns on
samples, and, except for small-scale operations within easy reach of a
commercial assay office, it is advisable to have a laboratory and assay
equipment on the job.
As in prospecting work, all ore exposures
and mineralized material should be sampled and assayed systematically as
the work progresses.
The object of an exploratory campaign may
vary from one designed merely to prove enough ore to warrant starting a
productive operation on a small scale to one aimed at determining
approximately the total amount and grade of ore available as a basis for
planning large-scale mining operations. In general, the former is
applicable to relatively small high-grade deposits, whereas the latter
applies to large deposits of low- or medium-grade material, including
large low-grade placers.
Exploratory Workings
It
has been stated already that in prospecting, work should be confined as
much as possible to the vein or ore body with a view to obtaining the
maximum amount of information about it at a minimum cost. This applies
also to exploratory workings, especially in the earlier stages before
the existence of enough ore to justify mining operations or the
continuity of the ore bodies has been demonstrated. Too often long
crosscuts have been started, designed to tap anticipated downward
extensions of a vein exposed at surface, only to find that the ore was
not where expected, whether because of petering out, faulting, or change
in direction or dip.
If considerable is already known about the
boundaries of the ore, exploratory workings may be planned with a view
to their utility and adequacy for future use during the productive
period of the enterprise. Sometimes, however, it is possible to give too
much weight to future usefulness of exploratory openings, particularly
if it has not yet been demonstrated that deposits of economic importance
are present. It may be advisable, then, to ignore the future usefulness
of these openings and locate them to obtain maximum information at
minimum cost, even though it may mean that for efficient mine operation
new shafts, adits, and other main openings will have to be excavated
later on at considerable cost. Thus, an inclined shaft may serve to
develop a deposit dipping at a low angle much more quickly and at much
less cost than a vertical shaft, although a vertical shaft might be
better adapted to operating purposes later when the mine enters
production.
Therefore, an engineer who employs an inclined shaft
in the vein or other main entrance for exploratory purposes, poorly
planned from an operating viewpoint, should not necessarily be
criticized if a mine is proved up and these openings have to be replaced
later by others better suited to an operating property. However, in a
producing mine or partly developed property, exploration work should be
done with a view to utilizing it later for development and production to
save duplication of expenditures.
The cost per foot of
exploratory work at an undeveloped property is generally at least twice
as much as at producing mines, for several reasons. In the first place,
supervision, shop, camp maintenance and operation, other general
charges, pumping, hoisting, power, etc., are all chargeable to the
exploratory workings instead of being prorated between production,
exploration, and development, as at an operating mine. Furthermore,
exploration work often is conducted in localities where transportation
and power costs are high and where living quarters must be provided and
maintained, all of which add materially to unit costs.
Table 3
presents data on costs of transporting men, equipment, supplies, and ore
in a number of mining districts, and figure 2 shows costs of trucking
ore plotted against length of haul. The transportation of equipment and
supplies constitutes a somewhat different problem from that of
transporting ore, but the latter is indicative of the variations in
costs in different localities and under different conditions and should
approximate the costs of the former. Table 4, taken from Gardner, gives
some costs of packing ore in the Western States and Canada.
