2. SAMPLING GUIDES
For practical reasons, placer sampling incorporates a series of steps which may be carried out one at a time or in some cases are more or less combined. These may be defined as:
- Reconnaissance of the lands.
- Sampling.
- Sample processing.
- Data processing.
- Evaluation of results.
The first step can be further divided into three parts which will be discussed here under the following headings:
- a. Pre-sampling reconnaissance.
- b. Choosing a sampling method.
- c. Number and size of samples.
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a. Reconnaissance: A preliminary inspection of the lands should precede any placer sampling work. If the property is small this may take less than an hour but if it is large or if it contains significant mineral exposures, several days of pre-sampling reconnaissance may be in order. The importance of this preliminary work should be self-evident for, obviously, before the engineer can make intelligent sampling plans he must have some idea of the size, shape and the general characteristics of the deposit being investigated. Unfortunately, this first step appears short-changed in many sampling programs.
If aerial photographs of the lands under study are available, copies should be obtained before starting the reconnaissance. Photographs of the type available from the U.S. Geological Surveyor Forest Service are often the most useful reconnaissance tool. This is particularly true when used in stereoscopic pairs. Aside from providing a quick, overall perspective of the lands, good aerial photographs reveal details of topography, drainage and other features not easily discernible on the ground.
A review of published material relating to the lands under study should be part of any reconnaissance because information thus obtained can be a great time saver, particularly where the regional or local geology has been worked out by qualified observers and reported in authoritative publications. On the other hand, too much reliance on published material can lead to erroneous prejudgements, particularly among younger engineers not in a position to weigh the statements and conclusions of others against their own experience.
The history of a mining district can play an important role in the selection and interpretation of placer samples and should be considered an element of the reconnaissance. Any old diggings on the lands to be sampled merit careful consideration, particularly those from which some production has been made. Where production records are available, they often serve as a useful guide in the selection and interpretation of new samples. Nearby mines should also be visited and examined where possible to do so.
Because the reconnaissance must be tailored to suit the job at hand, no fixed procedures can be set out here. The effort and time required for an adequate job will depend on the examining engineer's experience and perception. -
b. Choosing a sampling method: What sampling method should be used? This question comes up early in most placer investigations but there is no easy answer. Because each deposit has unique characteristics there is no single "best" method of sampling and no procedure can be applied universally. In some cases, decisions will have to be made on a sample-by-sample basis but a good pre-sampling reconnaissance will llsually indicate if the ground should be tested by means of pits, shafts, churn drilling, or other means. It is a paradox that the very things which are to be determined by sampling are often things which govern the type of sample to be taken in the first place. This is a "chicken-and-egg" type of situation in which we sometimes cannot tell which comes first. Something to keep in mind is the fact that today's placer sampling is expensive. Because of this, the method, or sampling program adopted should be no more elaborate than needed to provide the amount of detail and the end accuracy required for a particular determination. For example, the prospector, whose main concern is to expose enough of a "showing" to interest a second party or mining company in his prospect, would be foolish to expend his own time and money attempting to carry out an elaborate sampling program; that is, sampling which any cautious mining company would do over again for their own satisfaction and protection. Very often the first placer sampling is of a cursory nature sufficient only to indicate if further interest in the property is warranted or to serve as a guide in future sampling or exploratory work.
Some methods to be considered are:
Sampling existing exposures.
Use of hand-dug excavations.
Machine-dug shafts.
Backhoe pits or trenches.
Bulldozer trenches.
Churn drill holes.
Bulk samples.
Grab samples.
The order in which these methods are presented has no bearing on their relative importance or applicability in a given situation. Their applications are described in the section titled "Sampling Methods."
