gold placer deposits

[size=18]Placers can be found in virtually any area where gold occurs in hard rock (lode) deposits. The gold is released by weathering and stream or glacier action, carried by gravity and hydraulic action to some favorable point of deposition, and concentrated in the process. Usually the gold does not travel very far from the source, so knowledge of the location of the lode deposits is useful. Gold also can be associated with copper and may form placers in the vicinity of copper deposits, although this occurs less frequently.

Geological events such as uplift and subsidence may cause prolonged and repeated cycles of erosion and concentration, and where these processes have taken placer deposits may be enriched. Ancient river channels and certain river bench deposits are examples of gold-bearing gravels that have been subjected to a number of such events, followed by at least partial concealment by other deposits, including volcanic materials. Residual placer deposits formed in the immediate vicinity of source rocks are usually not the most productive, although exceptions occur where veins supplying the gold were unusually rich. Reworking of gold-bearing materials by stream action leads to the concentrations necessary for exploitation. In desert areas deposits may result from sudden flooding and outwash of intermittent streams.

As material gradually washes off the slopes and into streams, it becomes sorted or stratified, and gold concentrates in so-called pay streaks with other heavy minerals, among which magnetite (black, heavy, and magnetic) is almost invariably present. The gold may not be entirely liberated from the original rock but may still have the white-to-gray vein quartz or other rock material attached to or enclosing it. As gold moves downstream, it is gradually freed from the accompanying rock and flattened by the incessant pounding of gravel. Eventually it will become flakes and tiny particles as the flattened pieces break up.

Some gold is not readily distinguishable by the normal qualities of orange-yellow to light yellow metallic color and high malleability, where it occurs in a combined form with another element, such as tellurium. Upon weathering, such gold may be coated with a crust, such as iron oxide, and have a rusty appearance. This "rusty gold," which resists amalgamation with mercury, may be overlooked or lost by careless handling in placer operations.

As mentioned before, the richest placers are not necessarily those occurring close to the source.Much depends on how the placer materials were reworked by natural forces. Streambed placers are the most important kind of deposit for the small-scale operator, but the gravel terraces and benches above the streams and the ancient river channels (often concealed by later deposits) are potential sources of gold. Other types of placers include those in outwash areas of streams where they enter other streams or lakes, those at the foot of mountainous areas or in regions where streams enter into broader valleys, or those along the ocean front where beach deposits may form by the sorting action of waves and tidal currents. In desert areas, placers may be present along arroyos or gulches, or in outwash fans or cones below narrow canyons.

Because gold is relatively heavy, it tends to be found close to bedrock, unless intercepted by layers of clay or compacted silts, and it often works its way into cracks in the bedrock itself. Where the surface of the bedrock is highly irregular, the distribution of gold will be spotty, but a natural rifflelike surface favors accumulation. Gold will collect at the head or foot of a stream bar or on curves of streams where the current is slowed or where the stream gradient is reduced. Pockets behind boulders or other obstructions and even moss-covered sections of banks can be places of deposition. Best results usually come from materials taken just above bedrock. The black sands that accumulate with gold are an excellent indicator of where to look.

It should be kept in mind that each year a certain amount of gold is washed down and redeposited during the spring runoffs, so it can be productive to rework some deposits periodically. This applies chiefly to the near-surface materials such as those deposited on the stream bars or in sharp depressions in the channels. The upstream ends of stream bars are particularly good places for such deposits. Where high water has washed across the surface by the shortest route, as across the inside of a bend, enrichment often occurs.

A rifflelike surface here will enhance the possibility of gold concentration. In prospecting areas with a history of mining, try to find places where mechanized mining had to stop because of an inability to follow and mine erratic portions of rich pay streaks without great dilution from nonpaying material. Smaller scale selective mining may still be practical here if a miner is diligent[/size

Basic Equipment

Among the essential implements needed for prospecting are a pick; a long-handled, round-pointed shovel; and a gold pan, preferably a 30-40cm diameter pan which can usually be purchased at hardware stores in gold-mining areas. A small prospector's pick is also useful, and a magnet and a small amount of mercury should be carried to separate the gold from black sand after panning.Specialty stores and manufacturers can provide the more elaborate equipment, such as skindiving gear, ready-built sluices, and mechanical gold separation devices, if desired.

In some cases, a bucket or wheelbarrow may be needed to transport materials to the washing site, and in addition, a heavy .25-.75cm -mesh screen is handy to separate out coarse materials. A small screen cut to nest in the upper part of a gold pan can be useful for the same purpose in panning. A gold pan the same size as the one used for panning will make a most efficient nesting screen if a close pattern of holes is drilled in the bottom. Holes usually should be .24-.75cm in diameter, depending on the average size of the material being sampled. Distance between holes should be about the same as the diameter of the holes. In some areas these pans can be purchased readymade. For weighing gold, a small balance scale graduated in milligrams may be desirable. A compact, folding type of balance is available for this purpose.

A compass will be needed for establishing claim lines and for finding your way out of the woods if lost. Adequate maps should be carried. A hand magnifying lens is helpful in identifying minerals. Bags may be needed to carry out samples; plastic bags are the best because samples may be damp. A rocker may be transported to the site either assembled or in a knocked-down condition. If mining is planned, lumber and other materials to build a sluice may be carried to the site. (See construction details under respective headings.) More elaborate equipment such as pumps, pipes, hoses, and light plants might be taken in by pack animals if desired.

Personal gear includes a good pair of boots, sturdy clothing, weatherproof gear, sleeping bag, tent, and such other things as one might want for comfort and sanitation. A foam pad or air mattress adds comfort to sleeping. A length of rope is useful for many purposes around camp, from raising the food out of reach of animals to extracting a car from a mudhole. For hiking, all necessary equipment for the period away from camp should fit into a manageable backpack of some kind.

An ax, a flashlight, a knife, and matches are almost indispensable. (Fires in the National Forest should be made only in designated areas or after consulting the local forest ranger.) A water bucket is often required, and a good crosscut saw will be found useful. Guns and fishing equipment can be taken to supplement the food supply and to provide some additional recreation. Guns are seldom necessary for protection from animals. A canteen with a 2-quart or larger capacity is advisable in many areas, depending on dryness of the climate. You will need water-purification tablets where streams are contaminated, whether by grazing stock or for other reasons. A miner's lamp, which consumes calcium carbide, is sold at some hardware stores and can be used for a serviceable light, although most people when away from electricity prefer gasoline or propane lamps. A carbide lamp will also be useful for any underground work. The special miner's safety lamp is recommended wherever air may be bad. Stoves that burn gasoline or pressurized gas are in wide use in camping and even gas refrigerators may be taken along "to cool the beer." (For low-budget operations, a swift-running stream will serve this same purpose well.) For any length of time in the field, an oven for baking is a valuable amenity. A reflector oven for use next to a campfire can be made of light sheet metal and will give excellent results, also serving as a place to keep food warm