DEFINITION: A placer deposit is a concentration of a natural material that has accumulated in unconsolidated (loose) sediments of a stream bed, beach or residual deposit. Gold freed by weathering or other processes from lode deposits is likely to accumulate in placer deposits because of its weight (gravity causes it to migrate toward the lowest spot) and resistance to corrosion. In addition, its characteristically sun-yellow color makes it easily and quickly recognizable even in very small quantities.
Placer gold mining in the United States spans a period of nearly 200 years. Earliest mining took place in the Eastern States and particularly in the southern Appalachian region during the late 1700's and early 1800's, but the richer deposits were soon exhausted, and interest turned to the West. The earliest production of any note in the West was from the Old and New Placer Diggings near Golden, Santa Fe County, N. Mex. These deposits were worked as early as 1828.
A few other deposits were mined in the succeeding years until the discovery by James Marshall on January 24, 1848, of gold in the gravels of the American River at Coloma, Calif. This discovery was a major factor in the rapid settlement of the West and triggered the SECOND of the great gold rushes in the United States. The FIRST was, of course, Conrad Reed's find on Little Meadow Creek in 1799. Because of the lure and excitement of gold mining, prospectors spread throughout the West and in subsequent years many more rich placer gold deposits were found.
DEFINITION:Lode - or "vein" - gold occurs within the solid rock in which it was deposited. If the particles of lode gold are very small - as is often the case - they may not be visible to the naked eye. In such cases, detection will depend on the results of laboratory analyses.
Among the early methods developed for recovering gold were the gold pan, the rocker, the dip-box, the long tom and the sluice. The pan is generally too slow to be effective for anything more than prospecting. The rocker is a time-honored tool of the small-scale miner with limited means. The dip-box and long tom might be considered more like simplified sluicing methods than distinct methods. The long tom method was never very popular. The simpler methods all normally involve hand-mining operations (shoveling and/or picking of the gold-bearing materials).
The gold pan or miner's pan is a shallow sheet-iron vessel with sloping sides and flat bottom used to wash gold-bearing gravel or other material containing heavy minerals. The process of washing material in a pan, referred to as "panning," is the simplest, most commonly used and least expensive method for a prospector to separate gold from the silt, sand and gravel of stream deposits.
Prior to the 1950's, gold pans were made primarily of stiff sheet iron. Today, most pans are made of plastic. The original standard pan was 16 inches in diameter at the top and 2-1/2 inches deep. The rim flared outward at an angle of about 50 degrees from the vertical. Smaller pans were used for testing. A 10-12 inch size pan is usually best for ease of handling and the 12-inch is the most widely available. Frying pans or other cooking utensils may also be used for panning gold but are less effective.
Before any kind of container is used for panning, it should be cleaned thoroughly. A good washing with soap and water will usually do the trick in most plastic pans. Metal pans are another story. New pans generally are greasy and should be heated over a fire until this coating is burned away by the heat.
There are different techniques and subtle variations in the art of panning - experience teaches which is best. Those with wide experience and much practice can recover the most gold with the least effort. It is often said that good panning technique lies in the action of the wrists. After much practice the good panner should be able to save even the very fine gold that may be nearly but not quite free from the black sands.
The pan is usually filled level with the top, or slightly rounded, depending somewhat upon the nature of the material being washed and the personal preference of the panner. It is then submerged in water. Still water 6 inches to 1 foot deep is best. While under water the contents of the pan are kneaded with both hands until all clay is dispersed and the lumps of dirt are thoroughly broken. Stones and pebbles are picked out after the finer material (referred to as "fines") are washed off. The pan is then held flat and shaken under water to permit the gold (which will be much heavier than any other material in the pan) to settle to the bottom. The pan is then tilted and raised quickly - keeping it under water. Such action will result in a swirling motion in the water causing the lighter material at the top of the pan to washed off. This operation is repeated until all the lighter material is gone from the pan leaving only the gold and heavy minerals behind.
Nuggets and coarse pieces of gold (called "colors") can now be picked out readily with a tweezer or with the point of a knife. Cleaning the black sand from the finer gold is more difficult and requires careful attention to prevent the finer colors from washing out of the pan during the process. This part of the operation is usually done over in a tub or over another pan so that if any gold is lost it can be recovered by repanning.
The "concentrates" (the remaining material) should be dried, and the black sands (composed largely of magnetite) can then be removed by using a magnet or by spreading the concentrate on a flat surface and gently blowing across it. In the early days, if there was an excessive quantity of black sand, the gold was usually "amalgamated" by putting a portion of a teaspoonful of mercury in the pan. The mercury would absorb the gold (see the section below on "Recovering Your Gold" for a more detailed description of this process).
A word should be said here about other minerals that you may see in your gold pan. Pyrite ("fool's gold," an iron sulfide) and mica are often mistaken for gold by the novice. Pyrite, which is usually a brassy yellow to white color, will shatter when struck with a hammer and becomes a black powder when finely ground. Mica, which may have a bright, bronzy appearance, is distinguished by its light weight and flat, platy cleavage. Both minerals are common in gold areas.
Other minerals that will collect with the gold and black sands because of high specific gravity include ilmenite (iron-titanium oxide), hematite (nonmagnetic iron oxide), marcasite (an iron sulfide), rutile (titanium oxide), scheelite (calcium tungstate), wolframite (iron, manganese tungstate), tourmaline (boron and aluminum silicate), zircon (zirconium silicate), chromite (iron and chromium oxides), and cinnabar (mercury sulfide). If present in sufficient quantity, these latter minerals may have some economic significance, although efforts to recover them as by-products are seldom worthwhile.
At least twice as much gravel can be worked per day with a rocker as with a pan. The rocker or cradle, as it is sometimes called (since it resembles one), must be worked carefully to prevent loss of fine gold. With the rocker, the job of washing is less strenuous but the method used in extracting the gold is virtually the same as panning.
The rocker, like the pan, is used extensively in small-scale placer work, in sampling, and for washing sluice concentrates and material cleaned by hand from bedrock in other placer operations. One to three cubic yards can be dug and washed in a rocker per man-shift, depending upon the distance the gravel or water has to be carried, the character of the gravel and the size of the rocker. Rockers are usually homemade and display a variety of designs. A favorite design consists essentially of a combination washing box and screen, a canvas or carpet apron under the screen, a short sluice with two or more riffles, and rockers under the sluice. The bottom of the washing box consists of sheet metal with holes about 1/2 inch in diameter punched in it, or a l/2-inch-mesh screen can be used. The bottom of the rocker should be made of a single wide, smooth board. The materials for building a rocker cost only a few dollars. If you are interested in seeing the plans for a rocker, click here then use the "Back" button on your browser to return to this page.
After being dampened, the gravel is placed in the box, one or two shovelfuls at a time. Water is then poured on the gravel while the rocker is "rocked" back and forth. A small stream from a pipe or hose may be used or water can be poured from a bucket. The gravel is washed clean in the box, and the oversize material is inspected for nuggets, then dumped out. The undersize material goes over the apron, where most of the gold is caught. Care should be taken that not too much water is poured on at one time, as some of the gold may be flushed out. The riffles stop any gold that gets over the apron. In regular mining work, the rocker is cleaned up (material caught behind the riffles is removed) after every 2 to 3 hours or perhaps more often when rich ground is worked and gold begins to show on the apron or in the riffles. In cleaning up after a run, water is poured through while the washer is gently rocked, and the top surface sand and dirt are washed away.
Then the apron is dumped into a pan. The material back of the riffles in the sluice is taken up by a flat scoop, placed at the head of the sluice, and washed down gently once or twice with clear water. The gold remains behind on the boards, from which it is scraped up and put into the pan with the concentrate from the apron. The few colors left in the sluice will be caught with the next run. The concentrate is cleaned in the pan.
Skillful manipulation of the rocker and a careful cleanup permit recovery of nearly all the gold. Violent rocking should be avoided, so that gold will not splash out of the apron or over the riffles. The sand behind the riffles should be stirred occasionally, if it shows a tendency to pack hard, to prevent loss of gold. If the gravel is very clayey it may be necessary to soak it for some hours in a tub of water before rocking it
Where water is scarce, two small reservoirs are constructed, one in front and the other to the rear of the rocker. The reservoir at the front serves as a settling basin. The overflow drains back to the one at the rear, and the water is used over again.
The gold recovered from the panning or rocking operation ends up in one of two forms - a nearly pure concentrate or an amalgam with mercury.
Placer gold in its natural form is almost always alloyed with a certain amount of silver, which decreases its fineness. The silver, much lower in value or price per ounce, lowers the value of the gold by a corresponding amount. Fineness is based on a scale of 0 to 1,000. As an example, gold 750 fine would be three-fourths gold and probably close to one-fourth silver. As a result, until the gold is refined, it will be worth less than the market price for pure gold. About the only exception to this is when the miner finds a nugget that may have special value in its natural form.
Gold that has been absorbed by mercury must be freed again. To accomplish this, the miner would heat the amalgam to drive off the mercury which partially vaporizes at ordinary room temperatures and will vaporize completely at about 675 degrees F leaving a gold "sponge." Great care had to be taken when using mercury to avoid inhalation of the fumes which are highly toxic and can cause a variety of ailments or even death. Since the mercury could be driven off at relatively low temperatures, small quantities of amalgam could be processed on a heated iron surface, such as a shovel face, out-of-doors where the vapors will be quickly dispersed.
The information used here is primarily edited from "How to Mine and Prospect for Placer Gold," J.M. West, US Bureau of Mines, 1971. To read the entire article, click here.