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h. Grab samples: One thing to avoid or to use with caution when investigating placers is the so-called grab sample. This method of sampling relies on a theory that if enough small "grabs" or portions of material are impartially taken and then combined into a single sample, the combined material will be representative of the deposit or of the exposure, as the case may be. But in practice the average person fails to make allowance for the large rock or boulders normally found in bank-run material and, as a result, their grab samples contain a deceptive proportion of fines. Such oversight can cause serious error in the estimated value, particularly when based on a few pans of material. Every mineral examiner will at one time or another run into a situation where systematic sampling is not practicable or is not called for. For example, a property may have no discernible mineralization or mineral·bearing structure, or the mineral examiner may for other reasons be sure that no significant values exist. Here, a few check samples can serve as "written proof" and as such carry far more weight than a bald statement of fact. Grab samples, if fairly taken, serve this purpose well. Grab samples can sometimes be used to check the presence or the distribution of gold in a gravel exposure and thus used as a guide for subsequent sampling and, judiciously used, they provide an excellent check against salting.
Other uses for grab samples could be cited but the novice will be better served by stressing the fact that in nonexpert hands they are almost sure to be misleading. With few exceptions, grab sampling cannot be considered a valid method for testing placers.
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i. Drift mine sampling: Underground mining procedures are collectively known as "drift mining" where applied to placer gravels. Placers buried under deep masses of low-value gravel or capped by lava flows have been extensively worked in this manner.
Drift mining presupposes the concentration of values in a well defined stratum or channel. Pay streaks in these channels are typically confined within narrow limits, both laterally and vertically, and are commonly related to troughs or other depressions in the bedrock. But it is not unusual to find several generations of "pay leads", as the miners call them, within a given deposit where several periods of erosion and deposition have superimposed successive channels over one another. Such complications are usually quite difficult to decipher and speaking generally, drift-mine deposits seldom lend themselves to the usual investigative procedures.
These deposits are usually explored by adits or shafts from which drifts are driven into the richer portion of the gravels. Crosscuts are run from the main drifts to establish the outer pay limits and at the same time to block the ground for mining. In the usual drift mining venture, exploration, sampling, development and mining are of necessity integral parts of an overall package.
This is particularly true where the values are confined to sinuous, deeply buried pay streaks. In deep ground the cost of close-interval drilling can be expected to far outweigh its benefits. Drilling can, however, be of great value in guiding an underground exploration program when dealing with this type of placer. A relatively few well-placed drill holes may indicate the course of the pay channel, depth and character of the gravel, grade of bedrock, etc. This type of information is essential to a successful development program where drain tunnels and haulage levels are to be kept below the lowest point of the pay channel. An application of this type of churn drilling at the Vallecito Western drift mine has been described by Steffa (1932, pp. 4-6).
When evaluating drift mines, prospect data are usually meager and the mineral examiner should take into account the fact that drift mining, by its very nature, is typically a high-risk venture. He should also recognize that sampling, in the usual-sense, may not be practicable.
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j. Hydraulic mine sampling: Determining the mineral value of large deposits of the type worked by hydraulic mining is perhaps the most difficult task faced by today's mineral examiner. A typical large hydraulic deposit presents most of the sampling problems encountered in drift mines plus a few of its own. Anyone familiar with the immense masses of Tertiary gravels that have been partially worked in California's Sierra Nevada region need not be told that detailed sampling of such deposits ahead of mining would be too costly for practical application. The California hydraulic miner was no stranger to mining risks and, for the most part, he concurred with Whitney (1880, p. 369), who in assessing hydraulic mine sampling said: " ... the amount of gold in the gravel cannot be accurately determined by assay or any other proper method. All that can be positively known is the amount obtained in the clean-up." Whitney went on to say: "The miner, by panning a sufficient number of samples, judiciously selected, can form a pretty good idea whether the gravel is likely to pay for working; but this is not by any means the same as ascertaining the exact amount of gold which it contains. "
As indicated above many of the early hydraulic operators relied more on qualified estimates of value than on a formal sampling program. Such estimates were based largely on the production records or estimated gold yield of nearby or similar properties but, in most cases, the question of actual gold value was (and remains) secondary to such factors as the area of minable ground, its character, width and course of the deposit, the depth and slope of hedrock, facilities for washing, dump room, water supply, cost of water, etc.
It is obvious that some factors cannot he readily appraised without some underground exploration. Such work, where provided, is usually in the form of a few shafts and some drifting or small-scale operations. Churn drilling may he employed to a limited extent to investigate the course of the channel or bedrock contours. Some examples follow:
The initial sampling and exploratory work at the Malakoff mine (one of California's largest hydraulic operations) consisted essentially of four shafts and 2,000 feet of drifting along the channel, which, according to Bowie (1885, p. 88), cost a total of $63,956.20. It should be noted that at the time (circa 1880), the expenditure of this great a sum for prospecting and sampling was considered somewhat remarkable.
According to Eassie (1944, p. 93), the initial prospecting and sampling for a medium-size hydraulic property (150-foot hanks) in British Columbia was done hydraulically, with exception of a small amount of churn drilling undertaken to determine the course of the channels. In all, six exploratory pits were opened hut only one supported a continuing operation.
Existing faces in old hydraulic pits can, in some cases, be sampled by conventional means. At the McGeachin mine in Placer County, California, old hydraulic hanks roughly 50 feet in height were sampled by means of vertical hand-dug cuts 2 feet wide by 1 foot deep. The sampling crew worked from long ladders propped against the gravel faces.
A small hydraulic mine on Fox Creek, Alaska, was prospected and simultaneously worked in the following manner: The deposit, consisting of low bench gravels flanking a small creek, was superficially tested by panning and its 15 to 20-foot depth was determined by use of 3-inch, hand-driven drive pipes. Several hand-dug pits and shafts were put down in the more promising areas and those returning the best prospects were expanded into ground sluice cuts. Pay areas exposed by ground sluicing were, in turn, expanded into pits and worked out by hydraulicking. Once a pit was started, the hydraulic mining proceeded from clean-up to clean-up without further sampling. When the ground stopped paying, the pit was expanded in another direction or abandoned. In other words, this was a hand-to-mouth operation (as are many small placer mining ventures) and it illustrates the type and extent of sampling or testing generally relied on for working small hydraulic mines.
In most cases the initial prospect work has been engulfed or otherwise eradicated by subsequent mining and, today in most hydraulic areas, there remains little to see but abandoned pits and their sloughed faces. What does this mean to the mineral examiner? It means that in the absence of prospecting shafts or other openings in the unworked gravels, it is necessary to use what past records are available and a large measure of common sense in making an evaluation.
