Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents
The first step in placer formation is release of valuable mineral from its parent rock. The various phenomena which combine to decompose and disintegrate rocks are embraced in the general term "weathering." The chief agents of rock weathering are:
- a. Ground water.
- b .Temperature change.
- c. Plant growth.
- d .Surface erosion.
a. Ground water: The great solvent of rocks is, of course, water; particularly ground water that has become charged with products of mineral and vegetable decay. Some rocks such as limestone may be completely dissolved but the majority are more commonly dccomposed through the processes of oxidation! carbonation, and hydration.
b. Temperature: Change of temperature combined with frost wedging plays a part in rock weathering, particularly in desert regions where changes may be rapid and extreme. The effects of frost wedging arc readily apparent where it loosens large blocks, but its lesser-known and more important function is thc wedging apart of minute rock scales and mineral grains, which in some cases is an effective agent of rock destruction.
c. Plant growth: Plants contribute to rock destruction and mineral release in two ways.
(1) Root growth forces open and extends surface and near-surface cracks and crevices.
(2) Decaying plants contribute rock solvents to the percolating ground waters. Decomposing vegetation is a very powerful agent of rock decay and, consequently, wet humid climates favor the development of deep mantles of decomposed rock and the liberation of placer-forming minerals.
d. Surface erosion: On the steeper slopes surface erosion becomes more effective and, in turn, chemical rock decay assumes a lesser role in the weathering process. The rapid surface erosion of relatively unweathered rocks in desert regions can result in the formation of placers, but as shown by Lindgren (1933, p. 219), such placers are not often rich or highly concentrated.
5. STREAM PROCESSES RELATED TO PLACERS
Because streams are the dominant agent in the formation of most placer deposits, the field engineer should understand the fundamental processes involved in the movement and deposition of sediments by flowing water. In particular. he should be able to relate stream processes to the mechanics of placer formation. Actually this is a complex subject but several of the more important concepts of stream processes related to placer formation are:
- a. Stream bed erosion occurs only when the flow has the ability to transport more material than is being supplied, and conversely, deposition will begin when the stream becomes overloaded or has its velocity sufficiently checked. Temporary or semi-permanent deposits are usually formed where velocity is locally reduced on the downstream side of rocks or other channel obstructions and along the inside banks of curves. Although at anyone point in a stream the balance may be precarious, alternating between erosion and deposition, the net effect will be one or the other. Where there is a net reduction the stream is said to be degrading, and where there is a net deposition it is said to be aggrading.
- b. It can be stated generally that streams do most of their transporting while in flood, and most of their depositing when the floods recede. In general, the coarse material transported by a stream moves intermittently and much of the coarsest material may be at rest for all but brief periods of time. Other things being equal, the length of time between moves and the distance moved is largely a function of particle size.
- c. Stream conditions wherein the entire bed load is shifted or is agitated sufficiently to allow a downward movement of gold particles may occur only during exlreme flood conditions or periods of regional climatic change, many years or centuries apart. Nevertheless, bed scour or movement of the whole contents of a stream bed, or of all except the large boulders, is essential to formation of the typical stream placer. Concentration of gold on the bedrock would not be possible without channel scour or a general agitation of the bed load.
- d. Today's climate and present stream conditions may have lillie or no resemblance to those which controlled formation of a given placer. Also, it should be borne in mind that old stream deposits such as those of Tertiary age may have no relation to today's topography or drainage pattern.
- e. In the case of desert placers, the usual rules of stream deposition and of placer concentration are often subordinate to other factors. These conditions are discussed under the heading of DESERT PLACERS.
Longwell, Knopf and Flint (1948) present in concise language, a broad picture of fluvial processes and the movement of alluvial material by streams. Although this reference does not discuss the formation of placers it can serve as a quick review of fundamental principles. Jenkins (1946), in an article titled "New Technique Applicable to the Study of Placers," discussed stream processes and their relation to placers in considerable detail. This well illustrated article is possibly the best single reference on the subject.
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