Gold Prospecting Methods
- Placer Sampling

Bucket or Clamshell Type Excavators: These machines are usually quite large in size and allow taking a fairly large bulk sample. The advantages of using this equipment are its ability to allow visual inspection of bedrock, use of caissons to hold the hole open, and the capability of obtaining a fairly accurate sample volume. The disadvantages are the need for good access for the large equipment and a fairly slow digging speed.

Churn Drills: Churn drilling is used in deep or wet ground where sampling by pits, trenches, or shafts is not feasible. The churn drill utilizes a heavy casing with a drive shoe at the bottom, a chisel-shaped bit, and a vacuum type sand pump for removing the sample from the hole. There are three main types of churn drills that are differentiated by their size:

The hand operated "Banka", the light "Hillman" or "Airplane, "the heavy "Bucyrus-Erie" or "Keystone" drills. The advantages of using churn drills for sampling placer deposits are: (1) the sample is very reliable; (2) equipment is fairly portable; (3) few mechanical problems are encountered; and (4) technical data and interpretative information are available. The disadvantages of using churn drills are (1) very slow penetration rate, (2) large boulders create many problems,

 Special Problems Associated with Placer Sampling: These are (1) large rocks and b oulders, (2) erratic high values, (3) uncased holes, (4) small diameter holes, and (5) salting.

Large Rocks and Boulders: In sampling placers, there is a tendency to bypass areas containing many boulders or to sample the easily collected finer material around the boulders. In many cases, this is essentially salting the sample since the valuable material in a placer deposit usually occurs in the finer material. The most direct solution to sampling areas containing boulders would be to take samples large enough to contain a representative portion of the boulders to give accurate value estimates. Since it is not physically possible to take large samples to include the boulders in most sampling situations, how does one consider the effect of boulders without including them in the sample? The most common solution is to visually estimate the volume and insert a correction factor into the end sample volume calculations.

Erratic High Values: The methods used for estimating the values of placer ground rely on the assumption that the value found in a particular sample extends halfway to the next sample. While evaluation of ground having a generally low or fairly uniform average of values can be done using the standard reserve estimation methods, erratic high value samples in a deposit cause problems. Methods for adjusting erratic high values, so an over evaluation of the ground does not occur, include: (1) resampling erratic areas, (2) using the lower value determined, (3) determining what the highest reasonable value should be, and then keeping all sample values within that limit.

Uncased Holes: The use of uncased drill holes should be discouraged in placer sampling, especially in sampling ground with a high unit value mineral. If uncased holes are used, there is a tendency to unintentionally salt the sample and get overvalued sample results. This problem arises due to the fact that without casing, an excess of material can get into a sample without the evaluator knowing where, within the hole, the material came from. An exception to always casing drill holes is when drilling frozen ground. If the ground being sampled is well frozen, casing is usually not used.

Small Diameter Holes: Due to the large "nugget effect" associated with sampling ground containing high unit value minerals, the use of small diameter drill holes is not recommended for sampling deposits for gold, platinum, etc. When sampling deposits that have fine-grained material and contain low unit value minerals, small [50 mm (2-in.) diameter] holes may be used.