The Business of Mining / A brief non-technical exposition of the principles involved in the profitable operation of mines

by Arthur J. Hoskin

No matter how or where found, placers were all originally of surface deposition. They are now found in gulches, cañons, valleys, ocean and lake beaches, glacial drifts, and sometimes beneath eruptive flows. Such placers as occupy the courses of streams are spoken of as gulch, valley, bar, and bench placers. The meanings of the first three names are obvious. By a bench placer is understood a deposit that was originally the bed of a stream, but which, in the course of time, has been cut down, or through, in such a manner as to leave a shelf or bench of the "wash" hanging up some distance above the present base of the gulch or valley.

When such deposits that have been covered by lava flows are disclosed and worked, they go by the name of "buried placers." They are, by no means, uncommon, and typical "drift mines" of this sort are operated in California and New Zealand. They present the novelty of working alluvial deposits under cover of solid rocks, and they thus conform to one of the early definitions of a mine, as previously given. Since the workings of such subterranean placers are generally confined to an approximately horizontal zone, the mine passages, to a certain degree, resemble those of a coal mine.

Placer deposits, being of a secondary nature, the materials are not in the place nor form of the original components. The gravels and sands, together with the valuable contents, probably originally existed in some solid forms such as rocks or massive minerals. The primary structures, in the course of ages and by atmospheric agencies, have been disintegrated and carried by gravity and flowing water to lower levels. The finer the decomposed material, the further it has been transported.

If the original rocks carried gold, the flakes of the metal, being of high specific gravity, would tend to settle to the bottom of the channels and to be carried shorter distances than would the lighter, non-metallic particles. The finer the gold, the more evenly will it be distributed in the bed of gravel. Likewise, placers near the heads of gulches, as a rule, carry coarser gold than those farther down stream.

The valuable materials found in placers must, of necessity, be those that possess the property of resisting corrosion and disintegration. The minerals and metals are, therefore, of a very permanent character.

Every find of "values" in a placer is unquestioned evidence that somewhere, above the present deposit, there originally existed primary depositions containing the valuable metals or minerals. The trail can frequently be traced back to them. These so-called "mother lodes" are not necessarily rich. In the case of gold, for instance, these original deposits of ore may not carry the metal in coarse enough particles to be visible and yet the placers may contain nuggets. There are numerous theories proposed to account for this observed phenomenon, but we will not discuss them here. The fact remains that nuggets have been actually produced artificially in flowing water under conditions similar to Nature's.

The methods of prospecting and working placer ground have undergone many improvements, but there are still many men practicing the primitive ways of a generation ago. The use of devices of simple construction and for operation by muscular effort is still familiar in many regions; and there are good miners who cling to such practice in the belief that it is the cheapest and truest way in which to ascertain the values of wash deposits. Also, there are many placers of limited areas and irregular shapes that cannot be well handled in any other manner.

With a "pan," a man can wash, in ten hours, not over one cubic yard of dirt; and to accomplish this amount of washing the ground must be very loose and favorable. An ordinary ten-hour day's work is about 100 pans. This is equivalent to about one-half of a cubic yard, which is the unit of volume in all placering operations. One may thus readily arrive at the cost of carrying on operations in this way. A cubic yard of ordinary placer dirt is the equivalent of less than two tons. A batea is the Mexican equivalent for the American iron gold pan. It is a sort of broad, conical, wooden bowl and its capacity is not equal to the pan.

A "rocker" or "cradle" is a trough on rockers somewhat like the old-fashioned child's cradle. In using it, a stream of water is caused to flow into the device which has been nearly filled with gravel and the miner gives it a rocking motion that causes the contents to classify or stratify according to the laws of specific gravity. The valuable particles, being the heaviest, will settle to the bottom, whence they may be subsequently removed. A "long tom" is an inclined, narrow box set stationary with a constant stream of water entering at the upper end. Gravel is also shoveled into the device at the same point. The process is more continuous than the preceding ones, the values accumulating at the bottom of the lower end, while the upper layers of gravel are carefully removed by skimming with shovels. The work will keep two men busy and the capacity is correspondingly greater. With a long tom, two men will ordinarily handle about five or six cubic yards in ten hours.

Whenever deposits of a broad area, with considerable and uniform depth, are thought to be valuable, it has become a practice to prove their value by "prospect drilling." This is a mechanical method and one form of apparatus employed is of the churn-drill type common throughout oil and coal regions. With these portable machines, holes are put down to bed-rock at intervals across the ground. As they are sunk, the holes are cased with iron pipes, the drillings are carefully saved and washed, and the values are estimated for each foot of descent. From the summation and averages obtained from all the holes, a very fair knowledge of the ground's worth can be obtained.