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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.

Salting

Salting of samples can occur intentionally or unintentionally. Intentional salting is the deliberate addition of valuable mineral to a sample. Unintentional or innocent salting is usually the result of careless or improper working procedures. No matter how salting occurs, the results can mean the failure of a project after many thousands or even millions of dollars have been spent bringing a property into production.

SAMPLE PROCESSING OR WASHING

Sample Washing

Once a sample has been taken from a placer deposit; it must be washed to separate the valuable mineral from the waste material. The valuable mineral thus separated is then weighed to determine the value of the ground being tested. Most washing devices use some type of riffled surface to retain the heavy minerals as the lighter waste material is washed away. In dry washers, a current of air floats away the lighter material while leaving the heavy minerals behind.
No production equipment, designed for the recovery of heavy minerals, actually recovers 100% of the mineral. Because of this, it is important to select a sample washing system that will indicate the commercially recoverable mineral content of a sample. Other essential features of a sample washing system are low initial cost, easy maintenance, easy transport and set-up, acceptability of a wide range of material size, efficient washing of the sample, efficient use of available water, ability to process large and small samples, good mineral recovery, ease and speed of cleanup, and reliability.

Sample Washing Equipment

Miner's Pan:
The old-fashioned gold pan is still the most widely used device for washing small placer samples. The pan is well suited for washing small samples but it is not well suited for handling samples over 13.6 kg (30 lb) or if samples are taken very frequently.

Sluice Box:
A sluice box is generally defined as an elongated wood or metal trough equipped with traverse riffles through which alluvial material is washed to recover the heavy minerals. Sluice boxes are sometimes, but erroneously, called "long toms."
The function of riffles in a sluice box is to retard the movement of heavy minerals allowing them to settle while the lighter material flows on through the sluice. Carpeting is often used under the riffles to increase the recovery of fine gold. In some operations, mercury is placed in the riffles to enhance gold recovery.

Rockers:
A rocker is a short, sluice like trough with curved traverse supports that permit it to be rocked from side to side. A flow of water, aided by the rocking motion, carries the material down the trough where the heavy minerals are trapped by riffles. Rockers have been used almost unanimously for washing samples by the dredging industry. A rocker requires less water than a sluice and is capable of recovering very fine gold.

Dry Washers:
Dry washers are used in and regions where water is scarce and a dry recovery plant is proposed for production. The dry washer uses a bellows to blow air up through a sluice box with a porous bottom, causing the lighter material to progress down the sluice while the heavy minerals are trapped behind the riffles. In order for a dry washer to work efficiently, the feed must be very dry. Damp, sticky material cannot be processed until it is dried.

Other Washing Equipment:
In addition to the previously discussed washing equipment that has been used since the early years of placer mining, there are two other types of washing machines that have come into use relatively recently.

The "Denver Gold Saver" and other comparable machines utilize a revolving trommel screen to separate the coarse and fine materials with the fines going to a shaking riffle for separation of the heavy minerals. The advantages of the "Denver Gold Saver" type of system are its ability to process relatively large samples and to recover fine gold.
A second machine that has been used for about the last 40 years is the mechanical pan. Once the coarse material in a sample is screened from the fines a mechanical panning device can collect the heavy minerals. As the name implies, the equipment is designed to duplicate hand panning.

DATA PROCESSING

Record Keeping

Accurate, systematic records must be kept for proper placer evaluation. As an example, data that must be recorded on a log sheet when churn drilling a placer gold deposit includes:
1) Name of property
2) Location
3) Date
4) Drill-hole line number
5) Hole number
6) Hole collar elevation
7) Time (the time is given for each bailing. A summary of time consumed in drilling, pulling, moving, repairs, etc. )
8) Depth of the drive shoe for each sample interval
9) Depth of pumping for each drive or sample interval
10) Total hole depth
11) Core rise in the pipe for each drive
12) Core left in the pipe after pumping, i.e., the plug thickness
13) The length of core removed
14) Volume bucket measurement
15) Classification of colours (count the number of No. 1, 2, and 3 colours)
16) Estimate the gold weight based on the colour count
17) Formation (note the visible physical characteristics of the formation being drilled)
18) Depth and nature of overburden
19) Labour used
20) Depth of the pay gravel
21) Depth to bedrock
22) Nature of bedrock
23) Thickness of pay zone
24) Diameter of the drive shoe
25) Theoretical volume of core removed
26) Volume of core removed
27) Weight of gold recovered in milligrams
28) Fineness of gold
29) Drive shoe constant used in calculations
30) Value in milligrams, grams or cents per cubic meter (cubic yard)
31) Price of gold used in calculations if cents per volume are used
32) Signature of the driller, panner, and helper

It is essential that detailed records are kept as they will be used to determine the value of the deposit as well as to determine its minability. Information included in the drill logs will also be used in selecting a mining method.