Placer Deposits

A placer deposit is a concentration of a natural material that has accumulated in unconsolidated sediments of a stream bed, beach, or residual deposit. Gold derived by weathering or other process from lode deposits is likely to accumulate in placer deposits because of its weight and resistance to corrosion. In addition, its characteristically sun-yellow color makes it easily and quickly recognizable even in very small quantities. 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 and most commonly used and least expensive method for a prospector to separate gold from the silt, sand, and gravel of the stream deposits. It is a tedious, back-breaking job and only with practice does one become proficient in the operation.

Many placer districts in California have been mined on a large scale as recently as the mid-1950’s. Streams draining the rich Mother Lode region–the Feather, Mokelumne, American, Cosumnes, Calaveras, and Yuba Rivers–and the Trinity River in northern California have concentrated considerable quantities of gold in gravels. In addition, placers associated with gravels that are stream remnants from an older erosion cycle occur in the same general area.

Much of the gold produced in Alaska was mined from placers. These deposits are widespread, occurring along many of the major rivers and their tributaries. Some ocean beach sands also have been productive. The principal placer-mining region has been the Yukon River basin which crosses central Alaska. Dredging operations in the Fairbanks district have been the most productive in the State. Beach deposits in the Nome district in the south-central part of the Seward Peninsula rank second among productive placer deposits of Alaska. Other highly productive placers have been found in the drainage basin of the Copper River and of the Kuskokwim River.

In Montana, the principal placer-mining districts are in the southwestern part of the State. The most productive placer deposit in the State was at Alder Gulch near Virginia City in Madison County. Other important placer localities are on the Missouri River in the Helena mining district. The famous Last Chance Gulch is the site of the city of Helena. There are many districts farther south on the headwaters and tributaries of the Missouri River, especially in Madison County which ranks third in total gold production in the State. Gold has been produced at many places on the headwaters of the Clark Fork of the Columbia River, particularly in the vicinity of Butte. Placer production from the Butte district, however, has been over-shadowed by the total output of byproduct gold recovered from the mining of lode deposits of copper, lead, and zinc.

Idaho was once a leading placer-mining State. One of the chief dredging areas is in the Boise Basin, a few miles northeast of Boise, in the west-central part of the State. Other placer deposits are located along the Salmon River and on the Clearwater River and its tributaries, particularly at Elk City, Pierce, and Orofino. Extremely fine-grained (or “flour”) gold occurs in sand deposits along the Snake River in southern Idaho. Placers in Colorado have been mined in the Fairplay district in Park County, and in the Breckenridge district in Summit County. In both areas large dredges were used during the peak activity in the 1930’s.

The most important mining regions of Oregon are in the northeastern part of the State where both lode and placer gold have been found. Placer gold occurs in many streams that drain the Blue and Wallowa Mountains. One of the most productive placer districts in this area is in the vicinity of Sumpter, on the upper Powder River. The Burnt River and its tributaries have yielded gold. Farther to the west, placer mining (particularly dredging) has been carried on for many years in the John Day River valley.

In southwestern Oregon, tributaries of the Rogue River and neighboring streams in the Klamath Mountains have been sources of placer gold. Among the main producing districts in this region are the Greenback district in Josephine County and the Applegate district in Jackson County.

Minor amounts of placer gold have been produced in South Dakota (the Black Hills region, particularly in the Deadwood area, and on French Creek, near Custer) and in Washington (on the Columbia and Snake Rivers and their tributaries).

In addition to these localities, placer gold occurs along many of the intermittent and ephemeral streams of arid regions in Nevada, Arizona, New Mexico, and southern California. In many of these places a large reserve of low-grade placer gold may exist, but the lack of a permanent water supply for conventional placer mining operations requires the use of expensive dry or semidry concentrating methods to recover the gold.

In the eastern States, limited amounts of gold have been washed from some streams draining the eastern slope of the southern Appalachian region in Maryland, Virginia, North Carolina, South Carolina, Georgia, and Alabama. Many saprolite (disintegrated somewhat decomposed rock that lies in its original place) deposits in this general region also have been mined by placer methods. Small quantities of gold have been mined by placer methods in some New England States. Additional placer deposits may be discovered in the East, but prospecting will require substantial expenditures of time and money. The deposits probably will be low grade, difficult to recognize, and costly to explore and sample. Moreover, most of the land in the East is privately owned, and prospecting can be done only with the prior permission and agreement of the land owner.

Lode Gold

Lode gold occurs within the solid rock in which it was deposited. Areas likely to contain valuable lode deposits of gold have been explored so thoroughly that the inexperienced prospector without ample capital has little chance of discovering a new lode worth developing. Most future discoveries of workable lode gold ore probably will result from continued investigations in areas known to be productive in the past. The districts in which such new discoveries of gold may be possible are too numerous to be listed in detail in this pamphlet. Some of the famous districts are: in California, the Alleghany, Sierra City, Grass Valley, and Nevada City districts, and the Mother Lode belt; in Colorado, the Cripple Creek, Telluride, Silverton, and Ouray districts; in Nevada, the Goldfield, Tonopah, and Comstock districts; in South Dakota, the Lead district in the Black Hills; and in Alaska, the Juneau and Fairbanks districts. Deposits in these districts generally are gold-quartz lodes.

Prospecting for lode deposits of gold is not the relatively simple task it once was because most outcrops or exposures of mineralized rock have been examined and sampled. Today’s prospector must examine not only these exposures, but also broken rock on mine dumps and exposures of mineralized rock in accessible mine workings. Gold, if present, may not be visible in the rock, and detection will depend on the results of laboratory analyses. Usually, samples of 3 to 5 pounds of representative mineralized rock will be sent to a commercial analytical laboratory or assay office for assay. Obviously, knowledge about the geological nature of gold deposits and particularly of the rocks and deposits in the area of interest will aid the prospector.

Article by: Harold Kirkemo