Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents
2. GENERAL NOTES ON PANNING WITH SUGGESTIONS FOR IMPROVING TECHNIQUE
- a. New pan - preparation for use: The film of grease or other rust preventative found on a new pan must be removed before use. This is best done by passing the pan over a gas stove burner or other suitable flame until the metal turns blue. Although this process is sometimes called "burning", care should be taken to avoid excessive heat. Blueing a pan not only frees it of grease but equally important, the resulting dark color. makes fine specks of gold much easier to see in the pan. The "burning" process should be repeated as often as necessary to keep the pan free of body oil films which accumulate on a pan in normal use.
- b. Pan factor: Gold pans are made in a variety of sizes but the size generally referred to as "standard" has a diameter of 16 inches at the top, 10 inches at the bottom and a depth of 2 1/2 inches. A typical pan will hold 336 cubic inches or 0.0072 cubic yard. The number of pans representing a cubic yard of material in place (bank-measure) is called the pan factor. Pan factors vary according to the size and shape of the pan, the swell of the ground when excavated, and the amount of heaping when filling the pan. In practice, factors for a 16-inch pan range from 150 to 200 but an approximate figure of 180 is often used. This is based on a struck pan (i.e., level full) and an assumed 20 to 25 percent gravel swell.
- c. Recommended pan size: The average panner should not use a 16-inch pan but instead should use the so-called "half-size" pan which has a top diameter of 12 inches, a bottom diameter of 7 1/2 inches and a depth of 2 inches. The half-size pan level-full weighs approximately 9 pounds compared to about 20 pounds for the standard 16-inch pan. Halving the sample weight by use of the smaller pan not only reduces fatigue, particularly when much panning is to be done, but the fact that it is much easier to carry in the field and to use in a small stream or tub is an important consideration. The pan factor for a 12 x 7 1/2 x 2-inch pan is about 400, assuming a 20 to 25 percent gravel swell. Experience has shown that two half-size pans can usually be washed in less time than one full-size pan.
- d. Use of perforated pan: Panning, at best, is a tedious, back-breaking job and anything done to speed the operation or improve working conditions will be repaid many times over in the form of more reliable results. The beginner and experienced panner alike can profit by use of a sieve (sometimes referred to as a "grizzly" pan) made by drilling 1/4-inch holes in the bottom of a pan of the same size and shape as the one used for panning. To use the sieve, place it inside of the regular pan and then fill with gravel and submerge in water in the usual way. When the material is thoroughly wetted, lift the sieve slightly and twist it back and forth (under water) until all minus 1/4-inch material has passed into the regular pan. The plus 1/4-inch-material is discarded and the fines which dropped into the regular pan are washed in the usual way. Aside from speeding the overall panning operation, the use of a sieve enables the engineer to conveniently inspect the plus 1/4-inch rocks and to estimate the proportion of coarse material.
- e. Use of safety pan: Allowing the pan tailings to fall into a second pan generally referred to as a "safety" pan will guard against losing the sample by accident and will greatly expedite repanning where this is called for.
- f. Panning large samples: When a large multi-pan sample is to be washed, rather than complete each successive pan, it is best to reduce them only to a rough concentrate. The rough concentrates are accumulated and are eventually combined for finishing in the usual manner.
- g. Stage panning: Where a large amount of heavy black sand is encountered, a stage-panning procedure can be used to advantage. This is done by panning and repanning to successive high-grade concentrates without attempting to make a complete saving of black sand or values at anyone stage. As the proportion of heavy minerals decreases with each successive repan, it becomes progressively easier to make a high-grade concentrate with a low-grade tailing. Usually two or three repannings will make an acceptably clean tailing.
- h. Supplemental data: When panning a sample the experienced engineer will note a variety of things among which are: Its amenability to washing, particularly where clay or cementing materials are present; the proportion of coarse to fine material; any evidence of unusual muddy water problems; the composition and angularity of rocks; the relative ease of concentration; the quantity and composition of black sand; indications of valuable or potentially valuable accessory minerals; the size, shape and other physical characteristics of the gold including "rust", tarnish or other factors which would affect its amalgamation. Any of the foregoing could he important factors in a placer mining operation.
- i. Use as a geologic tool: Although the miners' pan is normally associated with gold deposits, it can be profitably employed when investigating a variety of heavy minerals such as monazite, scheelite, magnetite, ilmenite, cassiterite, chromite, cinnabar, etc. In general, it should be borne in mind that with few exceptions the pan can be employed in the study of either lode or detrital-type deposits containing finely divided minerals of relatively high specific gravity. The use of a miners' pan as a geologic tool has been studied and reported in detail by Mertie (1954) and by Theobald (1957).
3. FIRE ASSAY OF PLACER SAMPLES - MISLEADING RESULTS
Fire assaying, in essence, is a minature smelting process which recovers and reports the total gold content of the assay sample, including gold combined with other elements or locked in the ore particles. Because of this, a fire assay may report values that cannot be recovered by placer methods and it cannot be too strongly stressed that when dealing with gold placers, the sample values should not be determined by fire assay. Furthermore, no credence should be placed in placer valuations or reports that are based on the results of fire assays. Although this should be common knowledge among mineral examiners, a surprising number seem unaware that fire assaying although accurate per se yields misleading results when applied to placers.
There are other reasons: First, consider the small quantity of material used in a fire assay. The usual amount of sample taken for a crucible charge is either 29.166 grams (one assay-ton) or half of this amount. Next, consider that a particle of placer gold only 1/32-inch in diameter may weigh about 1/4 milligram. Now if the bank-run material from which it came averaged 10c per cubic yard,(Based on gold at $35 per ounce.) the 1/4-milligram gold particle would be contained in about 7 1/2 pounds of sand and gravel. But suppose this same 1/ 4-milligram gold particle found its way into a 29-gram fire assay charge. The resulting assay-value would be 1/4 ounce per ton or about $13 per cubic yard (Based on gold at $35 per ounce. ). It is seen that a single small particle of gold, by placer standards, will cause an intolerable error when injected into a standard fire assay charge.
Experience tells us that with few exceptions no amount of mixing or careful division can produce a fire assay charge representative of bank-run placer material. The practice of first panning a sample to reduce its bulk, and then fire assaying the resultant black sand concentrate does not entirely resolve this problem.
Even if we assume that a representative crucible charge could be obtained, the fire assay will detect all gold including that which is locked up in rock particles or is too finely divided to be recovered by placer methods. This in itself precludes the use of fire assays for evaluating placer ground.
In brief, experience has shown that fire assay results applied to placers usually results in a substantial over-valuation of the ground. This fact has been set out by Janin (1918, p.38), Vanderburg (1936, p. 33), Gardner and Allsman (1938, p.61) and others.
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