Sampling Placer Deposits
Sampling is defined as taking a small portion of a whole mass that accurately represents the whole mass.
Sampling placer deposits is a little different from sampling a lode or vein deposit, because of the nature of a placer deposit. It is not a solid rock, but a sandy or gravelly material, and can be spread over a wide area. The basic principals of sampling still apply, though.
First, one must have a basic plan. The deposit must be identified as potentially containing a valuable mineral. This is done be field locating possible areas of interest and taking samples, of approximately the same size, from different locations. This material can then be panned or processed on a concentrating device, such as a Knelson or Falcon concentrator, a Deister or Wilfley Table, to determine if any "color" exists. If no visible gold is present, there probably is none in the material. So move on to another location. A word of advice, do not initially look for gold, look for the presence of heavy metals found in "black sands". If these are present, there is a possibility that gold may also be present. Also be aware, that if gold deposits are or historically have been found in the upper elevations of the drainage area, there may be some in the lower elevations as placer deposits.
If there are a few specs of gold visible in the material, it should be assayed at a reputable assay office, to determine if it is really gold. Once the shinny content is verified as gold, then further sampling may be justified. To begin with, mark the location of the samples on a contour map, such as a USGS topo map, and delineate the area to be sampled, realizing that it will be in the drainage area, which is located at the lower elevations on the map. Then set up a grid, with equal spacing between sample points. Initial sample points could be spaced further, as additional points can be added, if the sampling reveals favorable results.
There are many methods to sample placer, rotary drilling, using backhoes, various clam shell devices, etc. But, the easiest and most widely used method is to trench sample. This method can be utilized with simple hand tools, pick, shovel, and manpower. The trench should take a cross section of the area under consideration. For example if the placer area is 500 feet wide, the trench should be 500 feet wide. The width of the cut should be at least one foot, and several feet deep. A sample trench should generally not be deeper than 7 feet. If the deposit extends deeper than 7 feet, it should be drilled, in all probability. If drilling is used, then a grid should be constructed, again using equal spacing between drill holes.
The Nugget Effect is especially critical in placer sampling, since a large nugget will drastically skew the assays of one drill hole or trench sample. For example a small .007 g nugget will add $3 value to a 1 cubic foot sample. A 10 g nugget will add $193 to the same sample. For a 7 inch drill hole the effect is worse. A 0.007 g nugget will add $10 to a cubic yard projection from the sample, since the sample size is smaller and the nugget's value is multiplied by the projection. The 10 g nugget will increase a projection from a 7 inch drill hole by $580/cubic yard! With an over eager, overly "optimistic" sampler, it would be easy to go broke, with those types of errors. Sampling should be done using good common sense, and a basic scientific approach. Results should be unbiased, and representative, if these guides are followed.
Return to First Sampling Page
First, one must have a basic plan. The deposit must be identified as potentially containing a valuable mineral. This is done be field locating possible areas of interest and taking samples, of approximately the same size, from different locations. This material can then be panned or processed on a concentrating device, such as a Knelson or Falcon concentrator, a Deister or Wilfley Table, to determine if any "color" exists. If no visible gold is present, there probably is none in the material. So move on to another location. A word of advice, do not initially look for gold, look for the presence of heavy metals found in "black sands". If these are present, there is a possibility that gold may also be present. Also be aware, that if gold deposits are or historically have been found in the upper elevations of the drainage area, there may be some in the lower elevations as placer deposits.
If there are a few specs of gold visible in the material, it should be assayed at a reputable assay office, to determine if it is really gold. Once the shinny content is verified as gold, then further sampling may be justified. To begin with, mark the location of the samples on a contour map, such as a USGS topo map, and delineate the area to be sampled, realizing that it will be in the drainage area, which is located at the lower elevations on the map. Then set up a grid, with equal spacing between sample points. Initial sample points could be spaced further, as additional points can be added, if the sampling reveals favorable results.
There are many methods to sample placer, rotary drilling, using backhoes, various clam shell devices, etc. But, the easiest and most widely used method is to trench sample. This method can be utilized with simple hand tools, pick, shovel, and manpower. The trench should take a cross section of the area under consideration. For example if the placer area is 500 feet wide, the trench should be 500 feet wide. The width of the cut should be at least one foot, and several feet deep. A sample trench should generally not be deeper than 7 feet. If the deposit extends deeper than 7 feet, it should be drilled, in all probability. If drilling is used, then a grid should be constructed, again using equal spacing between drill holes.
The Nugget Effect is especially critical in placer sampling, since a large nugget will drastically skew the assays of one drill hole or trench sample. For example a small .007 g nugget will add $3 value to a 1 cubic foot sample. A 10 g nugget will add $193 to the same sample. For a 7 inch drill hole the effect is worse. A 0.007 g nugget will add $10 to a cubic yard projection from the sample, since the sample size is smaller and the nugget's value is multiplied by the projection. The 10 g nugget will increase a projection from a 7 inch drill hole by $580/cubic yard! With an over eager, overly "optimistic" sampler, it would be easy to go broke, with those types of errors. Sampling should be done using good common sense, and a basic scientific approach. Results should be unbiased, and representative, if these guides are followed.
Return to First Sampling Page