What is the line of value separating a low grade from a high grade of precious metal ore? There is no uniform practice along this line. One will notice that ores are nowadays spoken of as high grade that, before the practice of mining these described meagre deposits, were reckoned as low grade. This fact is due to two reasons, viz., the cheapening of metallurgical operations, and the greater respect that is entertained for ores of low metallic content. The Esperanza Mine, in Mexico, is called a high-grade gold mine. Its ore has averaged about $33 per ton and the profit therefrom about $19. The Oroya-Brownhill Mine, in western Australia, has had ore that carried a value of about $22 per ton and from it a profit of about $15 per ton was made. In the Cripple Creek District, ores that run above $30 per ton are considered high grade. This means that the average rich ore shipments of the district have a gold content of about 1-1/2 ounces per ton. The expenses of mining, freight, and treatment will probably total close to one-half the gross value, or about $15 per ton.
When one speaks of $30 or $40 ore as rich or high grade, it is not to be inferred that there is no ore in the shipments which is not worth a great deal more than this amount per ton. Such lots of ore will, no doubt, contain a great many chunks that would assay many times the average value. Such selected materials would not, however, be samples; they would be what are called "specimens." The specimen has its place in mine economic discussions because it furnishes the basis of operations for the ubiquitous "high grader" with which nearly every new and rich mining camp must contend.
Some writers claim that the high grader is a product of modern conditions; but we find that he has existed for such a long time that he was given mention by the Scotch historian and scientist Andrew Ure, who wrote of the precautions that were exercised in working the graphite mines of England, a century ago, to prevent the pilfering of even this comparatively low-grade material. Even the ignorant African natives of today cannot be trusted to wear clothing while working in the diamond mines. No, the cause of high grading is the innate greed of human beings and it has existed from prehistoric time and among all peoples.
In this discussion as to grades of ore, the question may arise as to what might be reasonably considered the most attractive kind of a mining proposition. This is too knotty a query to be answered in a few words. There are so many different phases that must be given due weight. Every mine is a problem in itself. The Minnesota mines afford the best examples of profitable iron mining. Under the classification of underground, tabular deposits such as veins or lodes, no matter in what metals their values are found, Mr. T. A. Rickard believes that the ideal mine would be one carrying ore worth $10 per ton, in a body five feet thick, with costs not exceeding $5 per ton, and so mined as to keep one million tons continually in reserve. According to these restrictions, he thinks the Robinson Mine, of Johannesburg, will about fill the bill as an ideal gold mine. It has a deposit of about the right thickness to avoid excessive timbering expense and this ore body is in such a vast, continuous sheet that its superintendent can depend upon maintaining a systematic development that will assure a constant supply of ore to the immense mill for ten or twelve years in advance. This same ore averages about fourteen pennyweights (approximately $14) per ton and upon this there is assured a profit of over five dollars per ton.
[XIII
VALUATION OF MINING
PROPERTY.]
Whenever a piece of mining property is to change hands, it is the proper procedure to employ an expert engineer to investigate the ground and the improvements and so arrive at some estimate concerning its intrinsic value. Nobody is infallible and it is a trite saying that "nobody can see into a mine farther than the last drill hole." But there is a great difference in the reliabilities of reports made by trained and by untrained men. A self-styled "expert" of the type which is so abundant in every new mining centre and about cities frequented by mining investors will probably not be able to comprehend anything beyond his vision; but the mining geologist and engineer—the man who has devoted the better part of his life to study and experience along these lines—will be able to reach conclusions upon which much reliance may be placed. This fact has come to be recognized by the men who exercise business judgment in their mining investments.
The sampling of mines has been studied and improved upon by succeeding engineers, until one may say that it is conducted along strictly scientific lines. The old method of taking a sample of a mine by scratching ore from the sides of a shaft from top to bottom and letting the collected material at the bottom represent a fair average of the ore body, has been relegated to its proper place in the evolution of mine valuation.
Without entering into a description of the methods now employed by the best examiners of mines, let it be said that every scientific precaution is taken to obtain representative portions of the ore bodies, at such intervals as seem best in each particular case; that measurements and assays are made for each and every sample taken and not for the aggregate of all the samples; that no opportunity is allowed unscrupulous persons to vitiate results in any manner; that a professional engineer will not hold nor acquire, in any way, an interest in any proposition which he examines; and that the report of a reputable engineer is equally acceptable to the seller and to the purchaser, no matter for whom the work is done.
Much discussion has prevailed as to the best means of estimating the amount and the value of ore in unbroken reserves. Associated with these beneficial disputes, there has been a further controversy as to the correct classification for reserves of unbroken ore. It is now conceded among mining men and engineers to be improper to longer make use of the meaningless but tongue-worn expression "ore in sight" as signifying any known or unknown volumes of ore in the ground. The only ore in sight is that which has been hoisted or which has been broken and stored underground. Well-known engineers have proposed the following expressions:
To denote the contents of ore bodies which have been exposed on four sides, we may say ore blocked out, positive ore or ore developed; for bodies exposed upon three sides, it is considered correct to describe the contents as ore partly blocked; for such bodies as are proved upon two sides only, the terms ore faces, ore developing or probable ore are appropriate; while in speaking of all ore that may be expected or suspected, but which is beyond the last exposures, we may use the expressions ore expectant and possible ore.