Stream placers . These represent the most important and widespread type. In the formation of the true placers transportation, sorting, and deposition of materials in flowing streams of water are important agents. Uplift may revive the forces of erosion and render these agencies repeatedly effective, which results in the re-concentration of the alluvial gold. Sorted stream placers may be further subdivided into hillside, creek and gulch, river bar, gravel plain, bench and high bench deposits.
As the products of rock decay are washed down the slopes into streams, the fine clayey material is carried a long distance, but the heavier particles, including gold, settle rapidly, the gold, on account of its higher gravity, usually collecting in the lower part of the deposit, or even in crevices of the bed rock. Even if it does not do so at once, agitation of the sediment may cause it to settle deeper, or even slowly migrate down stream as the sediment shifts. Coarse gold carried down by streams from higher levels, will settle with coarse sediment in the upper part of a stream's course, but very fine flake gold may be transported some distance farther down stream.
Running water is by far the most important agency in the formation of gold placers. First of all, attention must be directed to the high specific gravity of gold, which explains many of the puzzling features of the placers. Placer gold is six or seven times as heavy as the most common accompanying minerals feldspar and quartz and it settles to the bottom in flowing water with surprising rapidity. It is almost impossible to lose a particle of gold, of the value of 1/200th of an ounce, in a miner's pan; it sinks immediately to the bottom of the gravel and sand after one or two preliminary shakes in water. Once lodged at the bottom it stays there, in spite of shaking and rotating. This illustrates the fundamental fact that gold is mainly on the bed-rock. The ease with which some concentration, according to the specific gravity, is effected is shown by the well-known fact that in powdered samples of ore, as well as in dumps at the mine, a settling of the heavier ore particles toward the bottom can often be observed. Suppose we have a gold-bearing quartz vein deeply altered by rock decay; now let the region be raised say 500 feet by one of these slow oscillations which so commonly affect the crust. A river has excavated a valley to the corresponding depth in this elevated plateau, and this valley under the influence of a pause in the elevating movement becomes filled with gravels to a width of about 100 feet. Let a tributary gulch with steep grade be cut back into the plateau to the gold deposit; when the gulch reaches it the eluvial deposit will be carried down by sliding and washing; the clay and limonite are rapidly removed in suspension; the angular gravel of quartz and rock, grinding the fragments of gold between them and on the bed-rock, will be moved downward, the fine grains in suspension, the coarser ones dragging and rolling on the bottom. There is little deposition; the transporting power is great and in flood time the whole gravel mass, of no great depth, will probably be in motion.
Heavy gold nuggets may lodge in the lee of little ridges. The gold settles rapidly; most of it, "continually hammered'' and slowly shaping itself in flat, smooth grains, will be dragged down stream and finally reach the edge of the flood-plain in the river. At this place the larger part of the gold stops. It is not washed out with the sand and gravel but stays on the bed-rock near the margin. The finer particles will, of course, be carried out a little distance, but they soon sink into the water-filled gravel after the manner of grains of heavy ores in concentrating jigs. Just as in the gulch the whole mass of detritus is transported, so it is thought that in larger streams the body of water-soaked gravel and sand works downstream very slowly. During this process the lighter gold contained in the detrital material also works forward and downward, gradually joining the nuggets or coarser pieces, which have already reached their final resting ground. This mode of operation contains the key to the genesis of the placers. It is not to be expected that the coarse and ordinary fine gold will be carried out into the middle of wide flood-plains. As the flood-plain widens it will cover the accessions of gold along its margin, and the final result will be a streak of rich gold-bearing gravel, resting on the bed-rock and extending downstream deep underneath the surface. When this is traced upstream the primary deposit, the vein, will be found. The actual occurrences of course show infinite variation. Let us assume that, as happens in the Creswick district in Victoria, Australia, a broad stream with moderate grade crosses a deeply decomposed belt of soft slate containing an abundance of small veins or stringers of quartz with native gold, and that in addition a fair balance between transportation and deposition persists for a long time. The result will be a gravel deposit, only a few feet deep, but with an abundance of gold concentrated on the bed-rock over the whole width of the stream. Each freshet is sufficient to churn up and move forward the whole mass of gravel, continually adding to the concentrated gold on the clayey bed-rock.
Again, we may assume extremely active erosion, as is the case in the Sierra Nevada of California. Canyons several thousand feet in depth have been cut in an uplifted plateau, veritable trenches or sluice boxes, the grade of which is from 60 to 150 feet per mile. Stretches of wild gorges with polished bottoms alternate with stretches of less grade where shallow gravel accumulates. These canyons receive for long distances an abundant supply of gold of all sizes from older hill gravels or from decaying quartz veins. The result will be that but little gold will lodge in the gorges, while extremely rich shallow gravel bars will accumulate in the convex stream curves. Gradient, volume, and load usually vary in the same stream so that deposition may be going on in one part of its valley and erosion in another. Continued corrosion of the stream-bed results in deepening the canyon and leaving the bars as elevated benches. The miners of 1849 first found these bars and worked them. In searching for the source of the gold they soon found a trail of metal leading up the gulches to great masses of older gravels on the hills, 2,000 to 3,000 feet above. These gravels were washed by the hydraulic method; immense masses of tailings with a little gold were carried down to the rivers, totally overloading them. After the prohibition of hydraulic mining the streams gradually resumed active transportation. The whole gravel mass moved slowly downstream and a gradual re-concentration on the bed-rock took place. The tailings deposited became enriched and will ultimately be reworked.
The torrential floods of the canyons scarcely permitted the lodgment of fine gold. This was swept out through the narrow portals into the Sacramento Valley, where the grade of the streams suddenly diminishes. The most minute particles may have been carried as far as San Francisco Bay, but the bulk of the fine gold lodged in the flood-plains within a few miles of the mouth of the canyons. Easily caught upon the clayey "false bed-rock" of a volcanic tuff, this gold, the average particles of which are about 0.3 millimeter in diameter, formed meandering pay streaks at the base of a sandy gravel bed from 10 to 60 feet in depth. Such deposits are now worked by dredging. By an odd paradox, gold is at the same time the easiest and the most difficult mineral to recover. It is divisible to a high degree and owing to its insolubility the finest particles are preserved. A piece of gold weighing 1/200th of an ounce is without trouble divisible into 2,000 parts, and one of these minute particles can readily be recognized in a pan. In extreme subdivision the gold acquires a scaly, flat form, being known as flour gold or flake gold, is carried away very readily by water, and does not sink easily in sand or gravel. In part the flour gold is suspended by air films, and it can be carried away in rivers of moderate grade for hundreds of miles. The gold occurring in the sand bars of Snake River, Idaho, is a good example of this. It will settle in thin pay streaks at bars and other favorable places, but the next freshet will probably destroy the sand bars and sweep the gold away. This accounts also for the distribution of fine gold in great masses of gravel beds for example, in the wash 600 feet thick deposited by glacial streams at Tacoma and other places on Puget Sound. Almost every pan of this gravel will show a "color," but the material contains only a fraction of a cent per cubic yard. The fine colors along the Columbia River in northeastern Washington range in value from less than 1/1000 th of an ounce.
The much-discussed concentration of gold on the bed-rock seems, then, to be due partly to the natural jig-like movement in moderately deep gravels, during long-continued conditions of fair balance between loading and erosive power; partly to slow forward and downward motion of heavier gravel masses, of which exact measurement as yet is lacking, and last and largely to the fact that heavier gold will not be carried out into the gravel flats of rivers of gentle grade the only ones that have extensive flood-plains but is immediately deposited on the marginal bed-rock of the gradually deepening and widening gravel The best conditions for the concentration of gold are found in moderately hilly countries where deep secular decay of rocks has been followed by slight uplifts. Subsequent slight elevations would easily produce re-sorting and enrichment of the gravels.
In regions of gold placers the richest material is usually produced by repeated reworking of gold-bearing gravels by nature. Each reworking increases the richness of the gravels, eliminates easily decomposed pebbles, and finally results in a gravel of the hardest, most resistant rock quartzite or quartz. Quartz is the common gangue mineral in gold regions; hence the prevalence of "white gravels" or "white channels," almost exclusively composed of white quartz pebbles. Examples of present gulch, creek, and river gravels are not difficult to find; they occur in all gold-bearing regions where erosion is active and where precipitation is abundant enough to cause the sorting and carrying forward of the gravels in the stream beds. In the upper parts of the stream courses the gravel will be coarse and semi-angular; in the lower parts the sands increase and the pebbles are smoother. Where the rivers emerge from their narrow valleys and spread with gentle grade over flood-plains, more extensive sand and gravel beds will accumulate, generally, however, with less gold than in the more confined part of the course.
Local enrichment of gravels may take place below the junction of two auriferous valleys, or, as is often the case, or below the outcrop of gold-veins crossed by the stream. A general relation between the coarseness of gold grains and of gravel may be made out for most regions, fine gold occurring, as might be expected, with the finer gravel ; it is possible that examination might establish a fairly constant ratio. For the same reason "black sand," containing magnetite, ilmenite, garnet, and other heavy minerals, is a common associate of gold in gravels. Ideal rivers for the concentration of gold are those in which natural conditions approach most closely those of a long sluice; they therefore possess even, flowing waters, have a moderate gradient, and are at times subject to floods. Deposition of gold in these regions takes place mostly during flood periods, when the rivers are raging and when the boulders and rocks are in suspension. Gold moves quickly downward and is concentrated in paystreaks in favorable places along the river. The greatest gold deposition occurs at the heads of gravel banks along the inner sides of the river curves, under and behind boulders and in cracks and crevices in the river bedrock.
Continue on to:
Types of Placers – Part
I: Residual Placers
Types of Placers – Part III:
Marine and Old Placers
Return To:
All About Placer Gold
Deposits
