The gold in the placers and paystreaks has usually been deposited where the current of a stream that has been checked. A broad basin above a steep-walled canyon is more likely to carry gold than the valley below the canyon, provided the bed rock source of the gold is above the basin. Coarse gold is more likely to be found at the head of a filled basin than near its outlet. The same holds true of a stream that debouches on a coastal plain which will deposit the coarse gold it may carry near the head of its delta. Gold in stream placers tends to concentrate in certain areas known as “paystreaks”.
The change in grade of most streams traversing auriferous areas is usually considerable. Where the grade is steep, only the coarser pieces of rock which find their way into them that find a place to lodge, the finer and lighter particles being carried down-stream to a point where the grade and current velocity is less, unless they are checked by some especially favorable condition. The tendency then is to sort the materials entering the streams, each particular stream varying that action with its varying conditions of grade, width, depth and character of channel. Such variations are constantly happening at any particular place owing to changes in shape of channel and seasons. It is evident then that there will be a considerable overlapping of the grades of sorted products, including the accumulations of gold and associated minerals.
Disintegrated fragments of quartz veins entering a stream may be comparatively large fragments, pebbles or sands, which continue to be reduced in size by abrasion, thus gradually approaching the normal specific gravity of the gold. The movement then of such a fragment under the action of running water becomes more and more retarded until freed of all adhering gangue it attains a maximum specific gravity, and a minimum volume, when it stops and is added to the accumulation in the bed of the stream, while the gangue passes on. There is little question that the character of the bed-rock exercises an important influence upon the nature of the deposit. Slate forms one of the richest bed-rocks, as the gold accumulates behind the natural "riffles" or checks produced by its edges. In some of the chief diggings in Victoria, it has been noticed that hard sandstone bottoms were the richest; and in many cases, the gold has been found to penetrate as deep as 3 or 4 ft. in narrow crevices. The slates, and decomposed mudstone and clay bottoms were less rich in comparison. In one case, the gold in the deep ground was quite black, while that from the shallow parts was clean and bright. The goal which has hitherto guided the operations of the placer-miner has almost always been to take the cream, if one may use such an expression, and to leave the skimmed milk, to hurry through the ground, taking out the bulk of the gold with the greatest possible economy of time and labor, and to let the rest go.
The gold lies sometimes, but rarely, in a thin layer of sand or pipe-clay on the surface of the bed-rock; more generally, in the crevices of the rock itself, often more or less rotten, which is broken up to a depth of 12 to 20 in., and is occasionally found in what are termed "pot-holes," from the fact that they are of the form and size of a camp pot, say 15 to 18 in. in diameter and 6 to 10 in. deep. In the latter instance, the bed-rock is either a very hard blue schist or soft rotten granite. It is a curious fact that deep pools under waterfalls in auriferous streams seldom contain an appreciable quantity of gold. Many such have been worked out in expectation of a rich harvest, but with disappointing results. All large auriferous rivers show analogous cases; in them the gold is always found on the bars or points, and not in the deep pools or falls. The gravel material in flowing streams is agitated and shifted downstream during times of flooding, and any gold present tends to work its way down and settle to bedrock. As a result, in gold bearing gravels the gold is generally greatly concentrated on bedrock. Dredging or sluicing operations normally yield the greatest profit from the material lying within a few inches of the bottom of the gravel bed. It has been observed that coarse gold tends to be concentrated on soft bed-rock in preference to hard. The reason for such occurrence is that the soft bed-rock wears away much more rapidly than the hard and therefore the disintegration of the quartz-veins is more rapid at such points and consequently more gold is freed and in larger pieces than would be the case were the wear less rapid. Further, the base level of a stream in a soft country-rock is more quickly attained and the deposits of gravel are therefore more uniform.
In the larger streams, when the current which transports gold along with other detrital material suffers a decrease of velocity, and hence of carrying power, some material may be dropped to the bottom, while the water flows on more slowly and gradually becomes comparatively clear. The rate of cutting and consequently the load carried by a stream has much to do with the deposition of the gold carried. A greatly over-loaded current will deposit too rapidly to admit of the concentration of the gold and especially the fine gold. Evidently then there is a certain relation between the velocity of current and the load of debris carried that is of considerable importance in determining the character and location of the pay streak. Further, this condition of affairs varies considerably with the position of deposit with respect to the banks there may be an unloading on the outer rim and an overloading on the inner. It thus happens that in long rivers which flow through auriferous formations or whose feeders cut auriferous rocks there are at intervals deposits of fine gold in gravelly accumulations. Such accumulations of detrital material are known among the miners as bars, and when sufficiently rich in gold to repay working, these are called “bar diggings.” Bars are formed wherever the curves of the channel are such that the current flows off at a tangent to the bank, thus producing slack water and often back water or an eddy in the immediate vicinity of the bank. Thus the most common loci for bars are points immediately below where a pronounced curve changes suddenly for another. Slightly concave irregularities in an otherwise straight channel also produce the conditions necessary for these deposits, since such concavities will not ordinarily influence the course of a moderately swift current."
The character of the bed-rock when not covered with gravel is largely responsible for the nature of the deposits and may contain within itself the larger part of the gold. In comparatively soft bed-rock the gold may be found in paying and often large quantities for a depth of 18 to 24 inches, while hard bed-rock, especially when fissured and cut by cleavage planes, may hold the largest values. Subsequent deposition of gravel may form a deposit of many feet depth throughout which much gold may occur, thus forming an exceedingly rich gravel bed. When a stream crosses a formation consisting of stratified rock more or less tilted so that the upturned edges of the strata form riffles on a large scale, or when under similar conditions hard and soft strata alternate one with the other, exceptionally favorable conditions exist for the collection of both coarse and fine gold. The position of such riffles with respect to the stream has much to do with the collection of gold on the bed-rock and in the overlying gravels. To illustrate this point the bend of a stream cutting upturned strata may be taken. If the case is chosen where the course of
the stream directly at the bends is at right angles with the stratification, there will be no other point in the stream's course except at the bends where the same conditions exist. Now, the effect of riffles placed normal to the current of a sluice (the same being applicable to streams) causes a uniform settlement of the transported gold, but on changing the direction of the riffles with respect to the current it is found that there is a deflection of the sorted material, which occurs on the side of the larger or obtuse angle. It is evident then that on approaching such a bend, with the current, the pay streak of gravel beginning on the left hand side approaches the middle of the stream at the bend, after which it shifts to the right side and ultimately hugs the bank only to be again thrown across the stream by a reverse bend. With streams of variable cross-section the above-described method of deposition of gold may be somewhat modified.
The bed-rock may be soft and decomposed, or hard, as granite and slate, but in nearly all cases it is usually uneven and full of crevices which form the riffles that have caught and held the gold. In most of the largest and most prominent camps the richest spots are generally those on hard rough bottoms, where the gold has been picked out by hand from the crevices and with small tools, as knives and spoons. Montana Bar, Confederate Gulch, near Helena, Montana, is said to have yielded four men $1,200,000 from three quarters of an acre in 100 days, the bed-rock being tough, hard and ragged. The extensive distribution of gravels in benches on hill and mountain sides may be partially due to river action but not entirely so in some cases at least. Where there are evidences of recent submergence, as in Alaska, it is probable that the origin of the deposits of gravel is marine, or that they have been redistributed by marine action. However, the lower terraces or benches are probably remnants of older river-beds left by the down-cutting of the present streams.
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Paystreak Formation – Part
II
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All About Placer Gold
Deposits
