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Review of Placer Theory and Geology

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Posted January 1, 2011 in Gold Mining

Publication Info:
Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents

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.

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:


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