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Gold Prospecting Methods - Sampling for Gold
Sampling:One of the most difficult tasks associated with placer mining is the sampling of the deposit. More placer projects have failed due to inaccurate assessment of the reserves than to any other reason. Within the realm of placers, those containing valuable minerals with a high unit value (diamonds) are more difficult to sample than those with larger bulk, lower unit value minerals (tin). Some items to consider when sampling a placer deposit are:
- A relatively large size sample is needed for accurate valuation of the ground being tested. Placers are composed of many sizes of gravel that make a representative sample difficult to obtain.
- When sampling placers for high unit value minerals such as gold, any error in mineral content of the sample will be magnified in the calculation of reserves.
- Values usually are erratically distributed within the gravel mass. Therefore, some placers with a more uniform value distribution may be adequately assessed with a minimum number of samples, while a deposit with a high erratic distribution of values may not be adequately sampled regardless of how many samples are taken.
- The investigation of a placer deposit should be made by or be under the direction of a person experienced in the art of placer sampling.
- During a sampling program, items that must be observed and noted in addition to the sample size and valuable mineral content should include boulder size and number, clay content, bedrock conditions, water, frozen ground, false bedrock, and any other physical characteristics that would affect mining of the deposit.
The steps to be followed in approaching a placer sampling program are outlined below:
- Reconnaissance: (1) check status of land ownership, (2) physical characteristics of area, and (3) research mining history of the area.
- Field inspection: (1) photogeomorphology, (2) surface grab sampling over all exposures of gravel, (3) few seismic cross section, (4) geobotanical study, and (5) survey for old workings.
- Choosing a Sampling Method: The main methods to consider are ( 1) existing exposures, (2) hand-dug pits or shafts, (3) backhoe trenches, (4) bulldozer trenches, (5) other machine-dug pits or shafts, (6) churn drill holes, (7) other drilling methods, or (8) bulk samples.
- Existing Exposures: If existing exposures are available, they can be tested for potentially valuable minerals by taking a grab sample and panning the sample. Advantages of taking samples from surface exposures are the low cost and the speed at which the samples can be taken. The disadvantages are that you can only sample what is on the surface and no quantitative information can be produced.
