Methods of Mining Metals

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:

1. describe the characteristics of the ore deposits that governed the selection and application of the mining methods employed in each case,
2. describe the methods and practices employed, and
3. 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.

Placer Deposits

Lode Deposits

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.