Prospecting For Placer Gold: Part I

A prospector hunting for a gold placer follows up the water channels in which he finds specimens of all the rocks in the neighborhood. In Australia, the prospector looks amongst these to find samples of granitic, porphyritic and quartzose rocks or clay-slate as likely signs, and also pieces of quartz honey-combed and rusty, which we have described before as "float or blossom." Plenty of broken up quartz he considers a good sign, but very pure, hard, dull white quartz is generally considered as "hungry" or " barren" the size of the fragments denotes his nearness or otherwise to the reef, ie., the vein.

A prospector examines closely the fine sandy matter of the stream bed especially where eddies and backwater have been formed. A likely deposit should be scraped up, even down into every crevice and depression in the bed rock or solid rock bottom over which the river, modern or ancient, has worn its channel. This material should be panned. Gold, too, is often found on points and slopes of the bed rock as well as in the deepest portion. Nuggets found on high reefs above the level of the stream, imply that their weight enabled them to remain in their position, during the deeper erosion of the neighboring streams, and that the original vein from which they came, is not far off. As a rule, large nuggets and coarse gold are found much nearer to the source whence they came, than fine or "flour" gold, which is often carried to unlimited distances away out on the plains.  



The character of quartz veins and of their enclosing rocks in the immediate vicinity, decides the character, too, of gravels derived from them, hence sometimes a peculiar pebble may be traced up to the peculiar rock whence it came, and the gold vein be found near it in place. It has been observed that “leads" following the course or lines of a gold-bearing reef, maintain a more continuous yield than those crossing a number of gold reefs at intervals. Gold occurs in pockets and "shoots" at intervals, with barren portions between, which accounts for what we have stated above. In a country where the gold quartz veins are small, though rich at wide intervals, the gravels will also be small. In very deep ground where the "wash" is very heavy a series of borings or even shafts are made to test the quality of the bank. The following points have been observed as worthy of note in prospecting for gold placers.

1. Streams crossing the lamina or stratification planes of gold reefs at right angles are likely to be richest.
Gold is rarely found plentiful where there are indications that the current was strong, but rather in the lee under projecting points of rock, where beaches are usually formed and the water was slack.
Gold in streams is deposited in crevices of the "bed rock," which should be laid as dry as possible and picked up to such depths as the sand descends between the laminations.
Terraces are shelf-like excavations and deposits upon hill slopes above valleys, and are the remains of old glacier or river beds. The prospector should discover the inlet and outlet of the terrace and examine the gravel. The "wash" sometimes contains gold in layers one above the other.
Whilst working up stream attention should be paid to the banks on each side where sections are exposed so that no outcropping vein be overlooked.
Alluvial gold should if possible be traced to its source whence the "float" came. When the gold is large and plentiful and the boulders large and angular the reef is likely not far distant.
Sometimes there is a distinct peculiar feature in all the veins of a district, such as a peculiar band of a definite color.
Coarse alluvial gold is not always incompatible with fine reef gold as a source, because the reef gold may be so fine in general as to lend itself to very wide distribution when once it is liberated, while the rarer coarse grains would not be transported far.
Alluvial placers are richest where the current of the stream is interrupted by diminution in fall, by sudden change of direction, or by entrance of a tributary, also by reefs, bars, eddies, etc. Absolute richness depends upon local circumstances and the size and weight of floated masses.
Creases, holes and fissures of bed-rock over which the stream passed are favorite places.
The lowest layers of each separate period of deposition are the richest. Sometimes several different periods of deposition have succeeded each other.
The courses of present streams and of ancient channels are placers.

Loaming is a form of prospecting. It is preliminary to such prospecting as cutting experimental trenches, or sinking trial shafts or boring. It consists in washing surface prospects from the bases and slopes of the ranges, until specks of gold, or specimens are found to be obtainable with tolerable frequency, within certain limits. The prospector then proceeds to trace the gold up hill to its source, narrowing the limits of his work as by patient search he approaches the vein, whence the gold has been derived. When he can obtain surface prospects of gold up to a certain point, or line, but no farther, he then proceeds by means of trenching to search for the gold vein. The prospector has often to work along a steep scrubby mountain side selecting his prospects, numbering them, and placing samples in his "loam bag." If he discovers prospects of gold, he finds his way back to the spots the samples were taken from, so as to continue his up-hill search, and trace the gold to its source or vein. Sometimes there is no indication of a vein, soil and bushes and debris covering its out-crop, but by learning, the prospector ascertains its position, so as to expose it by a trench not many feet in length. We remember an ingenious way in which a valuable and long sought for vein was at last discovered. Prospectors had long found very rich "float" at the base of a hill whose surface was so deeply covered with loose debris that no trace of the vein could be found. A prospector found a small lake on top of this hill, and conceived the idea of cutting a trench from this body of water to the edge of the hill, and by damming up the trench, and then suddenly letting out the water to full force, it cut a deep trench through the loose debris down to bed rock and the vein was discovered. This process is called "booming."

The cleavage of quartz is said to be freer, sharper and better defined, in gold-bearing quartz than in that which is barren. Pyrite is a good indication. A soft, fatty clay or gouge often flanks the vein in its gold-bearing portions. The mountain spurs should first receive attention for veins; if the quartz is hard, it stands up, if soft, as it more commonly is, it will leave a streak-like depression. On finding such, the prospector should first wash out some of the decaying rock. If only a trace of gold is found in the quartz, there is probably a gold vein in the neighborhood, and trenches should be dug and exploration systematically followed up. Gold is generally near one wall of a vein, seldom all through the stone. Quartz gold occurs in "shoots" with barren spaces. Before setting a valuation on a discovery, the facilities for working the mine, such as we have alluded to, should be considered. Placer mines as well as other mines are often supposed to be "worked out." These are sometimes well worth investigating and examining by cross-cuts or other means. Sometimes it happens that more gold is obtained from "leader" veins that had been overlooked, than from the main worked vein.

Quite commonly, especially in the lower part of a placer, the pebbles and sand are firmly cemented together into a coarse conglomerate by infiltration of iron oxide and clay. This may consolidate into a false-bottom and not be true " bed rock." Generally two or three such false-bottoms, with intervening strata of greater richness, alternate with barren ones. So, many old diggings, thus supposed to have been exhausted, may be worked again, the true bottom not having been reached. These conglomerate bottoms may lie just upon bed-rock, with a white clay rich in gold beneath them. Gold occurs also in the conglomerate and must be stamped out. Modern rivers frequently cross in their course old river courses, and redistribute their golden sands. Placers are richer in their richer parts, than the veins from which their gold was derived. When shallow placers are due to the wearing down of quartz veins, no placer will be found above these veins, or above the point where the vein crosses the placer. In the Sierra Nevada there is but little alluvium, the gold comes from veins near by. Gold placers may sometimes occur below silver mines. Thus. the Comstock vein was discovered by following up placer gold to its source. This vein has produced a gold bearing silver-ore, the silver rapidly disappearing and leaving the gold behind.





It follows in order to speak of gold placers, because these are derived from, the former by the agencies of water, either in the form of glaciers of old, or of ancient or modern streams. The glaciers in olden times heavily mined the rocks and the veins, by cutting broad gashes through them, thus originating the canyons.  In this way millions of tons of rock were mined, together with the gold bearing veins in them, and also the precious metals minutely diffused and scattered throughout their masses. After the glaciers, the rivers took up the work, deepened the canyons, broke up the boulders and sorted them, setting free the gold and other metals they contained, and again sifted and sorted them and deposited them along their banks and in their beds. Of the various metals thus handled by nature's jigging process, many were dissolved and destroyed by various acids in the waters, and by acids of vegetation and iron salts percolating through the placer dumps after they had been laid down. So with the exception of a few very hard minerals, such as magnetite, diamonds, garnets, rubies, etc., little remained in the placer but the imperishable gold, and even that appears to have been refined of its alloy of silver which it contained in the original vein, for placer gold is generally much purer and more valuable than that in the original vein.

In the past some geologists believes that the fine gold disseminated through the placer appears to have been acted upon by certain salts, such as the salts of iron, and concentrated and amalgamated into large nuggets. Some contend, however, that these nuggets are only water worn pebbles of gold, brought direct from the vein, the result perhaps of concentration there of the contents of large masses of gold-bearing pyrites; it is to be noted, however, that whilst gold-bearing nuggets of various sizes are to be found, not uncommonly in gold placers, they are very rarely found in gold veins. With the gold in placers, is commonly found what is called "black sand," which is composed of grains or pebbles of magnetic iron ore, relics of the old gold-bearing pyrites chemically changed. Being near in gravity to gold, and originally associated with it, the two are generally found together in a placer, and a prospector in surveying a bank of placer-material made up of sand, pebbles and boulders, generally looks for a streak of "black sand" as a likely place for gold. Also by reason of the gravity of gold he is inclined to look for it more down on bed-rock than in the upper looser strata.

Ancient river beds as well as those of modern rivers may be found gold-bearing, rivers that have long ceased to flow, by reason perhaps of change in the configuration of the country. In California and Australia many of these ancient fold-bearing river-beds have at a period not long distant, having been deluged and covered by lava, and the gold is extracted by tunneling beneath the lava-sheet or by shafting down through it to the gravel below. These are called deep leads whilst the ordinary uncovered gravels are called "shallow placers." Almost anywhere along ancient or modern water courses not far from mountains, a prospector by panning, can get colors of gold even on the pebbly "wash" covering the surfaces of large portions of our plains, or even on the tops of table lands that once were plains, over which broad rivers and glaciers and large bodies of water distributed their debris, but as a rule it will only pay to work where the "wash" or "drift" or "alluvial" matter is plentiful and thick, and more than this, only where water is accessible to the work.

Continue on to:
Placer Prospecting: Part I

Placer Prospecting: Part II
Placer Prospecting: Part III
Placer Prospecting: Part IV
Placer Prospecting: Part V
Placer Prospecting: Part VI

Return To:
All About Placer Gold Deposits



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