Placer deposits

Placer deposits are mechanical concentrations of debris weathered out of rocks. Commonly, economically important minerals have higher densities, so they are concentrated as the lighter-density minerals are winnowed out by the action of water or wind.

Background

Placer deposits are found throughout the world wherever the mechanisms of concentration, water and wind, have been active and the resulting concentrates have not been redispersed by later processes. The best known types of placers occur in river channels and in beach sediments.

The weathering and of rocks release particles of varying size, shape, and density. Soluble materials are dissolved and removed in or groundwater. Some minerals, such as feldspars, are hydrated and converted into minerals, which, being soft, small, and of low density, are relatively readily removed in suspension. Quartz (SiO₂), common in many kinds of rocks, generally weathers out as roughly equant grains that, because of their and insolubility, wash into streams and rivers, where they are moved by rolling, bouncing, and pushing along toward the oceans. The abundance of quartz and its resistance to mechanical and results in its being the most abundant placer mineral and the principal constituent of temperate and cold beaches throughout the world.

During the and erosional processes, other minor or trace minerals, which are resistant to breakdown, are also transported along with the quartz grains and pebbles in river channels to the ocean margins. If the mineral particles possess high densities, they may be selectively concentrated as the transporting agent (usually water) more readily removes the lighter-density minerals. Thus, gold nuggets, with densities of 15-19 grams per cubic centimeter, are commonly concentrated in residual materials as the quartz grains, with a of about 2.65 grams per cubic centimeter, are removed. The densities of several other valuable and resistant minerals are sufficiently higher than that of quartz to allow them to also be concentrated in placer deposits (examples include ilmenite, FeTiO₃, 4.8; rutile, TiO₂, 4.25; zircon, ZrSiO₄, 4.7; cassiterite, SnO₂, 7.0; and diamond, C, 3.5).

Economically Important Placer Minerals

Many types of minerals and materials can occur in placer deposits; among the most important are gold, titanium minerals, zircon, tin oxide, diamonds, platinum, and sand and gravel. Throughout history, gold has no doubt been the most important placer mineral. Gold is soft and malleable but is otherwise nearly inert in the weathering realm. Hence, once gold is weathered out of the lode deposits where it initially formed, the grains may survive transport in streams and rivers over long distances. It was the discovery of such gold grains that led to nearly all the world’s major gold rushes, including the California gold rush in 1849. Gold placers have formed throughout geologic time; the world’s largest gold in South Africa occur in placers formed 2 billion years ago.

The titanium minerals, ilmenite and rutile, occur in minor amounts as small grains in many types of and rocks. These minerals are highly resistant to weathering and hence are liberated intact from their host rocks. Although their densities are less than twice that of quartz, they are quite effectively concentrated by flowing water in rivers and by the agitation of waves along beaches as the lower density quartz grains are winnowed out. The zirconium zircon is a common accessory mineral in alkaline igneous rocks. It weathers out as the titanium minerals do and is generally found with those minerals in river and beach deposits.

The tin cassiterite, like the titanium minerals, is heavy, hard, and extremely resistant to weathering. Consequently, where there are cassiterite-bearing lode deposits, the cassiterite weathers out and may be concentrated into economic placers. The hardness of diamonds and their resistance to normal weathering agents have allowed them to wash down rivers that drain from the areas of exposed diamond pipes and to occur in placer river and beach deposits. Because the density of the diamonds is only about one-third greater than that of quartz, the diamonds are not as well selectively concentrated in these placers as are the heavier minerals noted above. Platinum, like gold, is chemically inert and has a high density, 15-19 grams per cubic centimeter. Platinum lode deposits are much more restricted geologically than gold deposits; hence, placer platinum deposits occur in only a few places in the world.

The sand and gravel deposits found in rivers and lakes and on beaches constitute the largest placer deposits in that they represent mechanically concentrated residual materials. Although they have a much lower per unit value than many other placer materials, the large volumes of sand and gravel mined from these deposits actually make them the most important placer deposits economically.

Types of Placer Deposits

Placer deposits have been classified into several different types on the basis of location of formation. The major types include residual placers, eluvial placers, stream or river placers, riverbank and flood placers, eolian placers, and beach placers. Residual placers are occurrences of minerals at or near their point of release from the original source rocks. There has been some degree of enrichment of the placer minerals as the result of the removal of other portions of the host rocks by weathering. Eluvial placers are transitional placers in which concentrations of placer minerals occur downslope from the source rocks but where the valued minerals have not yet washed into streams and rivers that would transport them for long distances.

Stream or river placers are the best known placers and are the types responsible for the most famous gold discoveries. The movement of the running water, especially where there is turbulence, is effective in sorting rock fragments and mineral grains according to size and density. Because of their higher densities, gold grains and several other placer minerals settle out. They are readily trapped in crevices and irregularities on the stream bed or among larger boulders, as the lower density materials are more easily washed away. This type of placer sometimes grades into deltaic beds where a river drains into a lake or the ocean.

Riverbank and flood placers are deposits adjacent to streams and rivers that have been left as the rivers meander, cut downward, or overflow their banks in flood conditions. During the natural development of rivers, they commonly shift laterally across their floodplains, eroding banks on one side while depositing materials on the other side. In some areas, changes in base levels result in rivers cutting downward through the sediments they had previously deposited in their floodplains. In both these circumstances, valuable placer deposits may be left in the riverbank sediments that are adjacent to the present rivers. Flood placers also occur in the sediments adjacent to rivers. They form during episodic flooding when water flow is sufficiently rapid and turbulent to transport gold or other valued placer mineral grains up and out of the channels onto the adjacent floodplains. As the water spreads laterally and its velocity decreases, the gold grains are left as placer deposits along the adjacent floodplains.

Eolian placers are wind-formed placers that occur locally in desert regions where high winds have removed lighter mineral grains, thereby enriching the heavy minerals in the residuum.

Beach placers are generally formed by the combined effects of river transport of weathered materials to coastal margins and the action of tides and storm waves along beaches. The ebb and flow of the waves and the generation of longshore currents, especially under storm conditions, can effectively winnow and sort beach materials such that certain areas are highly enriched in heavy minerals. Gold-bearing beach placers are known in many localities, but the most famous are probably those at Nome, Alaska, where the beaches were actively mined for many years. Diamond-bearing beach placers have been extensively mined along the west coast of central and southern Africa, where rivers draining the interior have transported diamonds into the beach sands. Beach placers containing ilmenite, rutile, and zircon are the world’s major sources of these minerals.

Mining of Placer Deposits

Gold panning is probably the best known method of exploiting placer deposits. A circular motion of water in a pan containing gold along with other sediments effectively separates the minerals on the basis of their densities. The same general principle is used in sluices, channel-like boxes with barriers to create turbulence in the water so that sorting can take place. On a large scale, modern placers are mined by the scooping up of the unconsolidated materials and the use of either spiral classifiers or heavy media to separate the heavy materials from the light materials. The differences in the densities of the minerals allow for effective separation.

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U.S. Geological Survey

Prospecting for Gold in the United States.