Placer Practices

With lots of new people joining the quest for gold, thought I would share this helpful article. 

 

 

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

1. PANNING

• a. Preparation: After filling the pan approximately level full, carefully submerge it in quite water, preferably resting it on the bottom of a shallow pool or tub with the top of the pan just below the water surface. After the material has become thoroughly wet, work over the contents with both hands and break up any lumps. If clay is present, knead and stir the material until the clay is dissolved and floated away. It is important that all clay be eliminated before actual panning begins. Wash off and throw out all large rocks. In this first step the eye and hands substitute for a screen.

• b. Suspension and stratification: Commence this stage by grasping the pan with hands on opposite sides and while holding the submerged pan level, twist it back and forth (clockwise and counterclockwise) with sufficient vigor to keep the contents loose. This allows the heavier minerals to migrate to the bottom of the pan and is similar to the action in a jig in which heavy mineral grains are separated from lighter grains by their ability to sink through a semi-fluid bed. If this second step is properly executed, the smallest and heaviest grains will migrate toward the bottom and the larger and lightest to the top. This will allow many of the pebble-size rocks to be manually removed by raking them out of the pan with the fingers.

• c. Washing: The third step is one which, depending on the nature of the material being washed, may take on many variations. It is like Step 2 in that the entire contents of the pan are kept in motion, but as stratification of the bed develops, the lighter particles are allowed to escape over the rim of the pan. To do this, raise the pan partially above water and move the hands slightly back of center (allowing the pan to tip forward with the low side away from the panner) and change from the twisting motion of Step 2 to a flat circular motion. While keeping the pan partially submerged and its contents loose, gradually work the lighter-weight material over the low side of the pan. The rate of discharge is regulated by raising or lowering the pan rim and by using a side to side motion along with the flat circular motion. Alternate Steps 2 and 3 and wash until the bed begins to pack or until heavy minerals begin to crowd to the surface.

• d. Cleaning: The fourth step involves selectively washing away surface grains and, in effect, it can be compared to the action of wash water on a concentrating table. To prepare the now partially concentrated material for this step, the pan is given a short, quick side to side motion of sufficient vigor to thoroughly loosen the bed and further stratify the material. During this shaking phase the pan is tipped gradually forward until the surface of the mineral bed becomes flush with the lip. At this point the shaking is stopped and the mineral bed allowed to settle. Next, a thin layer of the lighter material is removed by carefully dipping and raising the pan with a forward-and-back motion which will wash off the surface grains a few at a time. The washing can be effectively controlled by use of a so,mewhat circular motion as well as the forward-and-back dipping motion. When the panner decides that further washing would cause a loss of values, the bed is re-stratified and more light material brought to the surface by repeating thc vigorous side-to-side motion. Repeat the washing and shaking operations until the heavy-mineral concentrate is clean or until it is reduced to a volume small enough to permit inspection or removal of the gold. During the finishing steps the panner can save time by raking any remaining pebbles out of the pan with his fingers and flicking out smaller particles with his thumb. These and other tricks come with practice.

• e. Inspection and estimating: At the end of the panning operation the original material will normally have been reduced to a small quantity of concentrate consisting mostly of black sand minerals. After putting a little clear water in the pan, the experienced panner will fan out the concentrate on the bottom of the pan and by "tailing" the gold he can inspect or count the colors. At this point he can estimate the tenor of the sample. There are perhaps as many ways of tailing the gold as there are panners but this is usually accomplished bymoving the pan in a way that causes the water to gently swirl around the trough formed at the intersection of the bottom and side of the pan. This swirl of water carries the lighter particles ahead of those which are heavier or finer and with careful manipulation, brings the gold colors into view at the tail of the slowly moving fan of concentrate.

• f. Removing the gold: Final separation of the gold from other heavy minerals can be made in a number of ways. Larger pieces can be picked out with tweezers or the point of the knife and small colors or specks can be picked up by pressing down on them with the end of a wooden match or a dry finger tip. Remove the gold by placing the finger tip over a vial of water and washing it off with a splash of water.
  
  A small globule of clean quicksilver (mercury), if rolled around in the pan, will pick up the gold providing it is untarnished and free of oil or grease. Tarnished gold can be brightened by rubbing it in the pan.
  
  Where there is a considerable amount of tlack sand, particularly if it is fine and densely packed, it may be easier to separate the gold by blowing. This is done by first drying the concentrate and then removing the magnetite with a common horseshoe magnet, and finally, by blowing the non-magnetic black sand residue away from the gold. To do this, place the non-magnetic residue in a dry gold pan or on a suitable sheet of stiff paper and holding it level, blow gently across its surface while tapping the pan or paper. This takes some practice but with care a clean separation can be made in a surprisingly short time.

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).