Reserve Estimation and Placer Valuation:
Methods that can be used for reserve estimation and placer valuation are
The sampling methods outlined previously vary from simple grab samples on existing exposures to sophisticated drilling methods. The disposition and density of sampling must be adapted to each placer. The rule is: "A minimum of samples for a maximum of information." In this section, the various methods are discussed briefly and the applicability of each is indicated:
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.
These may be pits, trenches, or shafts and are suited to dry, shallow ground. This method of sampling is not used much today due to the high cost of labour but it can be effectively used in remote parts of the world where trained labour is not available, or general cost of labour is relatively low. The method provides a good bulk sample and is often used to verify drilling results by sinking a shaft over a drill hole. Bedrock values and characteristics can also be accurately determined when the excavations are sunk to bedrock.
Backhoes are a very versatile piece of equipment for sampling relatively shallow, up to about 6-m. (20-ft) deep deposits. Backhoes are mobile, fast, can dig fairly hard ground, and are inexpensive compared to hand-dug excavations. Once a trench is opened up, channel samples are taken by hand or by using the backhoe or a bulk sample can be made with all of the material from the excavation. For backhoe sampling programs, the ground must be fairly dry and stable. Care must be taken, when using a backhoe or any mechanical equipment, to keep all fuel and lubricants away from the sample material as sample contamination can result, causing the fine gold to float and thereby reducing the value of the sample.
Bulldozers are best suited to work in ground where trenches are to be dug 3 m ( 10 ft) deep or less and are in dry, stable ground. The greatest advantage of prospecting placers with a large bulldozer trench is that the trenches permit good visual inspection of the ground. Other advantages and disadvantages are similar to those associated with backhoe trenches.
Other Machine-Dug Excavations
Machine-dug excavations are shafts or pits that are dug using powered equipment such as large augers, bucket drills, or clamshell-type excavators. Digging shafts with the aid of powered equipment requires experienced operators and suitable machine access. Dry, stable ground is also needed for successful sampling operations when digging shafts and pits.
Augers for sampling placers can vary from small hand-held posthole size machines up to very large truck-mounted machines. Augers are relatively inexpensive to operate and can provide large volume samples, The disadvantages of using augers are their inability to penetrate ground with boulders, gold sorting may occur with spiral-type augers, and their inability to perform in water-saturated ground.
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 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:
Some of the special problems associated with placer sampling
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 value 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.
Rafal Swiecki, geological engineer email contact