Sampling, when applied to mining, means to take samples from veins or formations including mill pulp at various stages of the mining/milling operations for analysis in a laboratory.
In the exploration stage of a mine development, the main source of sampling should be core drilling. Properly done, many cores are packed in boxes showing the grid location and depth where the sample was obtained.
Tailings, dumps, tunnels and head ore, are usually sampled with pick and shovel, with powered equipment being used on large masses.
The residue from milling operations, gravity, cyanide leaching, or other recovery operations, leave tailings. Tailing are usually finely ground, while dumps contain the waste rock blasted from tunnels.
Tailings from old cyanide works usually show very little value. An example is near Oatman, Arizona where huge tailing piles encompass the town.
Old mine tunnels may have in front of them a large dump pile of blasted waste rock sloping down the hill or mountain below the tunnel. This material was considered to have no value, or a very low grade ore that was not worth the transport cost to the mill. Sometime the lack of water or the distance to a mill necessitated the leaving low grade ore on the dump. Nowadays these sites may be processed economically with modern methods. Tailing piles are normally near the site of a mill and vary from the blasted rock on dumps in that the processed ore is like fine sand.
Many dumps when sampled show that precious values still exist due to low grade or distance and difficulty getting the ore to a mill.
Sampling in adits, tunnels and drifts:
If one wonders through the darkness with a miners light knocking off the better looking specimens of ore along the ceiling, walls or hanging sides, the results may show very good results. In actual practice when a tunnel or drift is extended, a large percentage of the challenged mass will be waste rock unless one has a vein as large as the advance. The width and height of a tunnel and the size of the vein it follows determine the percentage of ore to be recovered to the amount of waste rock that will be only good for the mine dump.
The proper method of sampling mine heads is to chisel a V-groove approximately 1-1/2 inches deep across the face of the head in the shape of a giant X running from opposite corners to the floor of the tunnel. By spreading a tarp beneath, all of the falling sample can than be gathered and poured into sampling bags. An average of this sampling will give one an idea of what might be recovered in relationship to the waste that is generated. - This method is not preferred by mine promoters who seek out only the richest areas with no thought about the cost.
A methodical grid approach to core drilling will result in a more accurate snapshot of the area being sampled. It is common for promoters to look for the richest samples or areas they can find. The results are then used to convince the potential investor of thousand acres or hectares containing untold riches to be gained. Again, the real test is to drill many core holes to the depth that it is practical to mine. Since this is usually beyond the means of most small miners, a more practical approach is a backpack drill rig that can reach depths of six to 12 feet. Beyond that depth, especially in hard rock, the strength required may be beyond the average man as all equipment and water has to be packed into the remote areas most prospecting occurs in.
It is very important that sample bags have an accurate location and description just in case an assay should show promising results.
This type of sampling is more of a control method to determine if the milling processes are properly adjusted and producing the expect results.
Sampling should be done at every important stage of the process being used where precious metals can be lost due to inattention or misadjustment. In the case of a ball mill, this might be small samples being taken every 30 minutes to one hour, and sent to the assay laboratory at the end of each half or full shift. The flotation process especially is critical as to the densities and chemical strengths necessary to proper flotation separations.