Sampling and Evaluation
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If the reader will pause and study Figure 2, he can better visualize the more pertinent placer sampling problems, as they are taken up in the following paragraphs.
1. GENERAL CONSIDERATIONS
a. Problems: Contrary to popular belief, representative placer samples are seldom easy to obtain and in almost all cases sample results need a large measure of interpretation. Some of the underlying reasons are:
b. Industry practice: How do the established placer mining companies evaluate their prospects? First, the prospecting is put in charge of an employee who has had wide experience with the type of deposit or mining project involved. Second, they recognize that anyone sample is rarely representative of the overall deposit and by itself may mean little. Third, after studying the combined prospecting data and perhaps taking special check samples, experience-based adjustments known as "correction factors" are applied to the initial sample data where needed and, finally, the calculated values are based on these "adjusted" data. Here, it should be emphasized that the successful placer companies place as much reliance on the experience and insight of their prospecting and management personnel as they do on the sample results. If there is a mathematical formula or a general rule which will replace experience-based judgement, the operating companies have not found it.
c. Minerals other than gold: Parenthetically, it should be noted that placers chiefly valuable for minerals such as monazite, rutile, cassiterite, ilmenite, etc., are generally easier to sample and evaluate than gold placers. There are several reasons. First, the valuable mineral makes up a larger part of the mass. Second, the mineral sought has a relatively low unit value which means that extraneous particles of such mineral (in a sample) may have little effect on the calculated value. Third, deposits containing such minerals are often made up of well sorted, small-size detrital materials such as beach sands and, in such cases, the mechanics of sampling may be simplified by permitting the use of augers or jet drills in lieu of churn drills. Speaking generally, the sampler has considerably more latitude when dealing with minerals of low unit-value than when dealing with gold and his mistakes are less likely to seriously affect the end results.
d. Other factors to be considered: Many things other than mineral content have a direct bearing on the commercial value of a placer deposit. The mineral value in itself may be of secondary concern where sampling or examination of the lands for example, show unfavorable bedrock conditions, excessive amounts of sticky clay, large boulders or other factors that would adversely affect a placer mining operation. A good sampling program will provide the information needed to evaluate adverse physical conditions, should they exist, but here again experience is needed to interpret the information and make correct decisions.
e. In brief: Placer samples yield limited information. Correct interpretation of this information depends upon the engineer's powers of deduction and experienced judgement, rather than on the rigid application of formulae.