6. CONCENTRATION OF VALUABLE MINERALS
In theory, the richest part of a placer should be found near bedrock and, because of this, many people think that gold placers invariably are richer on bedrock than elsewhere within the deposit. On this basis, they believe that if indications of value are found in the upper horizons, pay gravel will surely be found on bedrock and countless mining ventures have been launched on this premise. Needless to say, many failed. In practice, it is not uncommon to find deposits in which the pay is scattered through a gravel mass without a significant bedrock enrichment. Some deposits, in fact, have their concentrations at the surface rather than on bedrock. This type of occurrence is explained in the section titled "Flood Gold Deposits."
Another popular idea is that concentration of gold in a stream is analogous to concentration in a sluice box. But in nature the process is by no means this simple as will be shown in following paragraphs.
- a. Bedrock concentration: The development of bedrock concentrations in simple stream deposits may be generalized as follows: Consider a down-cutting stream in which the bed load of sand, gravel and boulders is progressively shifting downstream and subjected to general agitation during times of high water. During high-water periods some of the smaller and lighter particles will be picked up and advanced by the current, usually in a series of short jumps, while the heavier rocks and boulders roll and slide along the bottom. Each time the bed load moves or is generally agitated, the contained gold particles can work downward toward bedrock. Under favorable conditions this may be quite rapid but in many deposits, such bed movements occur only at long-spaced intervals-perhaps not within the lifetime of anyone observer. Nevertheless, it must be realized that without such movement and rearrangement of the entire bed load, a downward migration of the gold and, in particular, its eventual concentration on bedrock would not be possible. Under flood conditions fine gold may be swept up and carried away but once coarse gold has settled to the bedrock it is very difficult for the current to dislodge it. For this reason coarse gold is generally found near its source.
- b. Types of bedrock: The ultimate richness of a placer is dependent to a large extent on the physical characteristics of the bedrock. Slates and jointed rocks, particularly those dipping at steep angles are considered most effective in capturing and holding gold. Compact clay, clayey volcanic tuff and decomposed granite make effective bedrocks. A smooth, hard bedrock such as serpentine is generally considered to be a poor gold saver. In some important placer fields, the pay gravels rest on a false bedrock. In the Folsom, California, area for example, this is usually a layer of volcanic tuff found well above the true bedrock.
- Gold works its way into soft or decomposed bedrocks and settles into the cracks and crevices of hard formations. lt commonly migrates downward to an extent requiring the mining of several feet of bedrock to effect a complete gold recovery. In some cases, inability to dig a hard, rough bedrock has been the direct cause for failure of dredging ventures. In others, having to dig unexpected amounts of soft bedrock has seriously upset the initial cost estimates. Such possibilities should always be considered when evaluating a placer property.
- c. Pay Streaks: Bedrock enrichments known as "pay streaks" usually follow a sinuous course and sometimes have no discernible relation to the present stream channel. A pay streak may split or terminate suddenly and its lateral limits may be irregular or indefinite, its location and eccentricities being dictated by local controls. Bear in mind that conditions which caused the pay streak, particularly in the case of older deposits, may no longer be in evidence. Pay streaks in gravel-plain or similar widespread-type placers are usually less definite than those in stream placers.
7. PRESERVATION OF THE DEPOSIT
Placer deposits do not suddenly come into being but, instead, they are progressively accumulated and enriched by a succession of stream actions and interactions. Although the land forms and geologic controls responsible for the creation of a particular placer may persist for a long period of time (as measured by the affairs of man), the deposit itself is transitory. Unless preserved by some disruption of the normal erosion cycle, the very forces which create the placer will in time destroy it. Knowing how and why a particular deposit was preserved is important to the prospector or mine operator and, obviously, such knowledge is also important to the examining engineer-particularly when he is called upon to project the course of a buried channel or appraise its potential. The more common causes of preservation are:
- a. Abandonment: This is probably the most frequently encountered form of preservation. It is brought about where a stream, by change of course or rapid downward cutting, abandons its former bed and the placers therein. The abandoned placers are preserved in the sense that they are no longer subject to erosion by the parent stream. High bench and terrace gravels provide typical examples of abandoned placers.
- b. Regional uplift: Elevation of the ocean shoreline has in some places preserved beach placers by raising them above the active surf. In some localities, such as Nome, Alaska, elevated beach placers are now several miles inland.
- c. Burial: The lava-capped Tertiary channels found in the Sierra Nevada region of California offer some of the best examples of buried placers but there are means other than volcanic flows by which burial and preservation may be affected. Among these are:
Covering by landslide material.
Covering by flood-plain gravels following regional subsidence or a net change in climate.
Burial under alluvial fans or outwash detritus in desert regions.
- Needless to say, a knowledge of past geologic or physiographic events is an important tool in the study of placers.
