Principles of Mining: Valuation, Organization and Administration

old, Lead, Silver, Tin

ING, ASSAY PLANS, CALCULATIONS OF AVERAGES, P

zinc. The valuation of alluvial deposits, iron, coal, and other mines is each a special science

ne of the order under d

t may be won f

be derived from extension

or lower price of metal

the management d

ermed the speculative values, and are largely a matter of judgment based on geological evidence and the industrial outlook. The fourth is a question

above the minimum given represent various degrees of risk. Further, it would be but stating truisms to those engaged in valuing mines to repeat that, because of the limited life of every mine, valuation of such investments cannot be based upon th

ring and administration from an economic point of view an essential to any essay upon the subject. While the metallurgical treatment of ores is an essential factor in mine economics, it is consider

e fundamentals of valuation, such as ore reserves and average values, that managerial and financial policy may be guided aright. Also with the grow

hings generally. Despite the feeling of some engineers that the latter employment is not properly based professionally, it is an expanding phase of engineers' work, and must be taken seriously. Although it lacks satisfactory foundation for accurate judgment, yet the engineer can, and should, give his experience t

f mines involves a wide range of subjects. For purposes of t

Average Metal Con

on of Quant

ctive

Percentage o

of M

f Prod

rtization of Capi

Mines without

t of Examinati

AVERAGE METAL CON

etal content of standing ore are in use-Previ

f regularity of recurrence of these plums, dependence must necessarily be placed on past records, for if their reliability is to be questioned, resort must be had to extensive test-treatment runs. The Lake Superior copper mines and the Missouri lead and zinc mines are of this type of deposit. On the other sorts of deposits the previous yie

ty to fraud are usually prohibitive, even where the plant and facilities for such work exist. Even in cases where the engineer in management of producing mines is desirous of determining the value of standing ore, with the exception of deposits of the type mentioned

f small portions, that is samples, be taken from a sufficient number of points, their average will represent fairly closely the unit value

d of a surety lead to wholly wrong conclusions. Given a good sampling and a proper assay plan, the valuation of a mine is two-thirds accomplished. It should be an inflexible principle in examinations for purchase that every sample must be taken under the personal supervision of the examining engineer or his trusted assistants. Aside from throwing open the doors to fraud, the average workman will not car

inch would be more accurate than those from each alternate square inch. However, the accumulated knowledge and experience as to the distribution of metals through ore has determined app

distance along the strike either side of these cross-sections. In massive deposits, sample sections are taken in all directions. The intervals at which sample sections must be cut is obviously dependent upon the general character of the deposit. If the values are well distri

how the possible locus of values. The metal contents may be, and often are, particularly in deposits of the impregnation or replacement type, greater along some streak in the ore-body, and this difference may be such as to make it des

or lode is usually divided into two samples. If still wider, each is confined to a span of about four feet, not only for the reason given above, but because the more numerous th

writer had for some years the direction, and where sampling of mines was constantly in progress on an extensive scale, not only in contemplation of purchas

roken down upon a large canvas cloth. Afterwards it is crushed so that all pieces will pass a half-inch screen, mixed and quartered, thus reducing the weight to half. Whether it is again crushed and quartered depends upon what the conditions are as to assaying. If convenient to assay office, as on a go

ck, into which samples can be put underground and which is never unfastened except by responsible men, not only aids security but relieves the mind. A few samples of country rock form a good check, and notes as to the probable value of the ore, from inspection when sampling, are useful. A great he

are usually made of every sample and their average taken. In th

e preliminary plans of separate levels, winzes, etc., and to average the value of the whole payable widths on such plans before entry upon a longitudinal section. Such a longitudinal section will, through the indicated distribution of values, show the shape of the ore-body-a step necessary in est

if the value be confined to a few large particles they may find their way through all the quartering into the assay. Or the sample may actually indicate rich spots of ore; but in any event experience teaches that no dependence can be put upon regular recurrence of such abnormally rich spots. As will be discussed under percentage of error in sampling, samples usually indicate higher than the true value, ev

ary delineation of the boundaries of the payable areas on the assay plan will indicate the sections of the mine which are unpayable, and from which therefore sampl

ounces divided by the number of dumps. Likewise one sample over a width of two feet, assaying twenty ounces per ton, if averaged with another sample over a width of five feet, assaying one ounce, is no more twenty-one ounces divided by two samples than in the case of the two dumps. If common sense were not sufficient demonstration of this, it can be shown algebrai

ntity of the metal and volume of ore represented by each. The method of calculation therefore is one wh

sample can be considered as a prismoid, the

n feet of o

t of ore represen

ck to which values are

y als

of cubic feet

value of t

tonnage of th

total meta

e used, but it complicates the study unduly, and for p

their respective prismoids, divided by the total tonnage of these prismo

D1/C) + V2(W2 L2 D

1L1D1/C +

be redu

L1 D1) + (V2 W

L1D1) + (W2

ty of the same values through the block is a geological assumption. From the outer shell, all the values can be taken t

W1L1) + (V2

1L1) + (W

distances to the two adjacent samples. As a matter of practice, samples are usually taken at regular interv

1W1) + (V

1 + W

d by the width sampled.[*] It is by this method that all samples must be averaged. The same relation obviou

hod unless the two end samples be halved, but i

te narrow samples to a stoping width, a blank value over the extra width which it is necessary to include must be averaged with the sample from the ore on the above formula. Cases arise where, although a certain width of waste must be broken with the ore, it subsequently can be partially sorted out. Practically nothing but experience on the deposit itself will determine h

value and volume. It is but a basis for an estimate, and an estimate is not a statement of fact. It cannot therefore be too forcibly repeated that an estimate is inherently but an approximation, take w

he mines, the values indicated by sampling were 12% higher than the mill yield plus the contents of the residues. On the Witwatersrand gold mines, the actual extractable value is generally considered to be about 78 to 80% of the average shown by sampling, while the mill extractions are on average about 90 to 92% of the head value coming to the mill. In other words, there is a constant discrepancy of about 10 to 1

hen sampling, that is by the inclusion when mining of a certain amount of barren rock. Even in deposits of about normal stoping widt

ore, the discrepancy becomes very much increased. In mines where the range of values is narrower than the normal stoping width, some wall rock must be broken. Although it is customary to allow for this in calcul

material usually creeps in. This is true more in lead, copper, and zinc, than in gold ores. On several gold mines, however, tests on accumulated samples for their sulphide percentag

ed after arriving at calculated average of samples,-how much