The Cost of Mining and Treatment

The cost of mining and treatment is a factor as important as grade of the ore, and is probably more subject to the personal equation than any other branch of an examining engineer's work: individual judgment based on experience is the final guide in an estimate of costs. This subject, which applies rather to the examination of developed mines than to prospects, is most thoroughly gone into in Mr. J. R. Finlay's book on "The Cost of Mining."

The results of past operations must be considered in the light of future probabilities, and it should be borne in mind that mining costs and selling prices of metals vary greatly with time, and, in some cases, with the seasons.

The larger the property and the more complete its development the easier becomes the problem of estimating costs. Those properties that most nearly approach the character of a manufacturing enterprise are the easiest with which the engineer has to deal.

In the examination of an isolated prospect having no ore reserves, a detailed discussion of probable costs is out of place, as the costs will vary according to the tonnage developed. A tentative estimate based on experience is the best that may be offered in such cases.

The regularity of a deposit and its absolute size are important factors in the cost of mining; the angle of inclination of the deposit with the horizon is important, also, as determining whether the ore when broken in the stopes will run or whether it will have to be shoveled.

All the other attributes of a prospect must be considered in the light of its situation with respect to transportation; excessive distances, or a rugged topography without roads or trails, may as important as that of the average grade of the ore; in all cases render valueless a property that would be valuable if better situated.

Water is a necessity for camp use and for metallurgical plants, and if there is no visible source of water near a prospect, the question at once becomes grave. Some gold mines in the Altar District of Sonora, and in many other desert regions, are commercially impossible on account of scanty water supply. Water in old workings should be investigated, and its source determined — whether it is due to seepage, or whether it comes from an underground channel. The amount of flow of water is, of course, of vital importance, and the question as to whether the flow comes from an imderground reservoir which may ultimately be drained, or from a regularly flowing channel, is important. Examples are numerous where large flows of water have rendered impossible the mining of otherwise valuable ore-bodies.

Climate and altitude are not usually controlling factors in the United States, but become such in the high latitudes, or in the fever belts in the tropics. The high altitude has been a great detriment to the development of certain districts in southwestern Colorado.

Wages and supply of labor are factors for careful consideration; fuel and motive power also are factors of prime importance. It is often advanced that a company, through large expenditure for equipment, may greatly better the working costs of a property that has been operating on a small scale; this is rarely true with small high-grade mines, where the small owner can secure working costs comparing favorably with anything that a large company can accomplish. This is the reason that small high-grade mines are usually poor purchases; their owners demand all that they are, or may become, worth. Good examples of this condition are offered by many mines in Mexico that produce ore as cheaply with primitive methods as could a large company with expensive installations.

The cost of equipment must finally be charged off against the tonnage mined, and for this reason large low-grade properties are likely to offer the best opportunities to the average investor.

In determining the probable cost of mining, the development cost must not be forgotten; the footage of shafts, drifts, raises, and so forth must be considered in connection with the amount of ore developed, and the probable cost of future development must be included in the estimate of the mining costs.

Finally, the purchase price is a charge of which each ton must bear its share. These factors are often forgotten in America, where mining risks are too often accepted in a gambling spirit.