VII Placering
Different writers hold the following slightly different definitions of a placer: One says, “a placer is a surface accumulation of minerals in the wash of streams and seas,” while another writes that a placer is “a place where surface depositions are washed for valuable minerals, such as gold, tin, tungsten, gems, etc.” One definition conveys no notion of the operations of mining, but is merely geological, while the other involves the thought of the recovery of values.
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.
Intensive placering is now the order of things and the marvelous increase in the use of dredges attests the success which these “gold ships” have attained. It is very interesting to watch the operations of these huge boats loaded with ponderous machines, especially when they are installed in inland regions or up in high mountain gulches. Yet numbers of them are thus in steady use. Wherever suitable beds with a tolerably uniform size of boulders and gravel are found, dams are built to retain the flows of streams until ponds are created of sufficient size to contain and float the barges.
Dredges of Yuba Consolidated Goldfields, Hammonton, California.
Continual improvements are being made in the construction of these mammoth machines with a view to economy in operations that will result from greater capacities. All costs of placering are reckoned per cubic yard washed. Costs have been rapidly dropping during the past decade until now some companies, with extensive operations, are handling dirt at not to exceed three cents per cubic yard for excavating, washing, wasting the refuse, maintenance, repairs, labor, taxes, interest on investment, and the depreciation of equipment. Such figures will hold good only under very favorable natural conditions of ground and climate such as prevail in California; they have not been attained in the frigid regions of Alaska nor in the torrid South American interior. In view of the wonderful improvements brought forth by mechanical engineers, it is improper to deny that the future will bring still further reductions in placer costs. On the contrary, the signs are good for material reductions.
Dredges are very costly in their installation. They are usually designed to handle so many thousands of cubic yards per day. It has been stated, as a fair but rough rule, that “bucket” dredges will average, in initial cost, one dollar for every cubic yard the boats will handle per month. Thus, if a dredge of this type is built to treat fifty or seventy thousand cubic yards in a month, working steadily, the costs will be respectively $50,000 or $70,000. Other types of dredges, known as the “dipper” and the “suction,” will cost less than the bucket type, but have not gained general usage.
“Hydraulicking” is extensively practiced. This term signifies the working of placer deposits by water which is conducted through flumes and pipe-lines and, by means of nozzles called “giants” or “monitors,” is directed, in huge jets, against the banks of gravel. These banks or walls are thus torn down and, by the same water, the loosened, disintegrated materials are caused to flow into and through long, wooden, box-like troughs known as “sluices.” The floors of these sluices are paved with ribs, cleats or other obstructions termed “riffles” whose function it is to retard and collect the heavy particles which may, later, during the process of cleaning up, be removed as the valuable product. The word “sluicing” is frequently used quite synonymously with hydraulicking.
Costs of this latter sort of placering are considerably higher than those of dredging; but there are many deposits not adapted to dredging operations that may be nicely worked by sluicing, so that there will always be a field for this scheme. Average costs are difficult to obtain since it happens that most of the companies now operating hydraulically are secretive in their accounts. More labor is entailed, more time is required, greater delay is occasioned in cleaning up, and the amount of water used is much greater. Where water is abundant, this last item need not be considered. It is well to remember that even a very large dredge, while requiring a continual and large flow of water through its devices, can still operate with just the water in which it floats, this water being pumped and used repeatedly; whereas, in the case of hydraulic mining, the water may be used but once and, consequently, there must be a large supply and at a good head or pressure.
But, in spite of these disparaging points, we find instances in which, under peculiarly favorable conditions, hydraulicking has been carried on at very low figures. E. B. Wilson says: “The yield of the gravel at North Bloomfield was 7.75 cents per cubic yard; the cost of mining, 4.1 cents per cubic yard. The yield per cubic yard of gravel at La Grange was 10.19 cents, the cost of mining, 6 cents. The costs of mining at these two mines would analyze about as follows: Labor, 60 per cent; supplies, 17 per cent; water, 13 per cent; office, 10 per cent. Ground carrying but 3.99 cents per cubic yard has been worked at a profit at the first mine. With such a small margin to work on, it is evident that skill and executive ability must be provided from the pipemen up.” It is claimed that an Idaho mine was worked profitably with less than two cents value in the dirt, but this is to be regarded with some doubt.
The Snowstorm Placer, Fairplay, Colorado.
A typical Hydraulic Mine.
There are large deposits in the arid portions of the globe where water for working is not obtainable. To meet such conditions, numerous inventions continue to be placed upon the market. These devices are all planned in such a way as to use very little or no water. If water is required at all, the machines are expected to use it repeatedly. The machines are built to effect the segregation of the precious contents gravitationally, electrostatically, pneumatically, and by amalgamation with mercury. It is too early to say how successful such devices will prove in commercial operations. Because some of them have not “made good” does not mean that genius will not yet cope with the situation; and we look into the future to see large operations efficiently and economically conducted by dry placer machinery. There are now no authentic figures obtainable upon this question of dry placering costs.