Where To Look For Placers
By: J. M West - Bureau of Mines publication, Information Circular 8517
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 (referred to as the "Tertiary channels" in California) 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.
Placer gold occurs in so many areas that it would be impractical to try to identify each of them here. One of the best recent publications covering individual districts and areas is U.S. Geological Survey Professional Paper 610, Principal Gold Producing Districts of the United States, published in 1968 (9) . Also, a series of reports is being written describing the individual placer gold deposits of various States or portions of States, to be published as Geological Survey Bulletins.
Specific locations and names of mines can often be found on the detailed maps prepared by the U.S. Forest Service, the U.S. Bureau of Land Management, the U.S. Army Corps of Engineers, or the U.S. Geological Survey. Various State agencies may also have appropriate maps on hand.
California
California has led all other States in placer mining and as would be expected has many gold-producing areas of interest, particularly including the deposits on the Feather, Mokelumne, American, Consumnes, Calaveras, and Yuba Rivers and their tributaries (2, 4-6, 9). These rivers reach into the famous Mother Lode area of the Sierra Nevada Mountains from which much of the gold is derived. Deposits are also found in the drainages of the Trinity and Klamath Rivers in northern California and at scattered points in the southern part of the State. Ancient Tertiary channels and gravels of the Sierra Nevada Range have been especially productive sources of gold, and maps have been published by the California Division of Mines and Geology showing approximate routes of these features. Two U.S. Forest Service maps that the prospector would find of particular value in considering the Sierra deposits are of (a) the Downieville, Camptonville, and Nevada City districts, Tahoe National Forest; and (b) the Foresthill and Big Bend districts, also in the Tahoe National Forest. Maps covering the Trinity and Klamath National Forests of northern California might also be of interest.
Alaska
Most of Alaska's gold production has come from placers, principally those in the Yukon River Basin, although deposits are known on nearly all major rivers or their tributaries (22, 33). Beach deposits in the Nome district have been notably productive, as have the river and terrace or bench placers in the drainages of the Copper and Kuskokwim Rivers. Climatic conditions play a great part in mining in Alaska, and the season for hydraulic operations of any kind is relatively short.
Northwest States (Montana, Idaho, Oregon, and Washington)
Montana's principal placer mining districts are in the southwestern part of the State (15). The Helena mining district and the many placers along the Missouri River in the vicinity of Helena and upstream are among the more important areas. The headwaters and tributaries of the Missouri in Madison County, particularly near Virginia City and Bannock, are noted for early placer production. Placer gold has also been produced on the headwaters of the Clark Fork of the Columbia River at a number of points.
The Boise basin, northeast of Boise, Idaho, is most noted for the dredging of placers (21). Other well-known placer areas lie along the Salmon River in Lemhi and Idaho Counties and on the Clearwater River and its tributaries, particularly in the vicinities of Elk City, Pierce, and Orofino. Placer gold is also found along the Snake River, but this is commonly fine-grained or "flour" gold that is difficult to recover.
Oregon's placers are located mainly in the southwestern part of the State, on tributaries of the Rogue River and on streams in the Klamath Mountains (16). Main gold-producing areas are the Greenback district in Josephine County and the Applegate district in Jackson County. Placer gold also occurs in many of the streams that drain the Blue and the Wallowa Mountains in northeast Oregon. The Sumpter area and the upper Powder River have had important production. Other areas include the Burnt River and its tributaries and the John Day River Valley.
Washington is not noted for placers, although gold has been found along a number of its streams, including some on the western slope of the Cascade Mountains. Generally, the few productive placers have been confined to the north-central part of the State.
Nevada
Nevada has not been a large placer gold producer, although lode gold deposits - potential sources - are widely distributed throughout the State (27). The problem has been chiefly one of too little water. In the past, dry washers were used extensively, as well as other methods that were very conservative of water. Producing areas were largely found in the western half of the State and included American Canyon and Spring Valley in the Humboldt Range, Pershing County, and the Manhattan and Round Mountain areas of Nye County. Placers were also worked below Virginia City and in northern Elko County near Charleston. Signs of limited placer diggings may be seen in many parts of the State.
Colorado
A few important Colorado placers of the residual type are found on slopes and hillsides in the immediate vicinity of gold veins. However, placers in Colorado are generally confined to narrow canyons below lode gold mining areas within the Rocky Mountains in a belt which extends northeast across the western part of the State. Almost every gold district has had some placer production. Many of the streams emerging from the Front Range, the headwaters of the South Platte River, and the Arkansas River and its tributaries as far upstream as California Gulch contain placer gold. Historically, placers were mined first and led to development of Colorado's rich lode deposits.
Other States
Among the other Western States, placer mining has been limited to only a few localized areas. In South Dakota, the Black Hills (particularly the Deadwood area) and French Creek, near Custer, have been productive sources. Arizona (32) and New Mexico (30) placers are in some instances related to copper deposits that carry gold.
In the Eastern States some of the streams draining the eastern slopes of the southern Appalachian Range have yielded gold (32 17). Saprolite deposits (rock decomposed at the original site) have been a source of placer gold in Georgia (13) and the Carolinas (29. Generally, the eastern placers are sparsely distributed and the gold is low in grade. Thus, few serious efforts have been made at mining them since the early 1800's. Nevertheless, many locations offer possibilities for small operations intended primarily for recreational purposes.
