Sampling is the collection of material to find its gold content. The amount of gold in a sample does not have to be as high as an economically exploitable deposit to be of interest to the explorer, because very small amounts of gold can be used to trace a path back to their source – a deposit.
The idea of sampling for gold exploration follows the trend in the general progression of work – from wide-spaced, cheap, uncertain, sampling, to closer-spaced, expensive but more confident sampling.
Where to Start and How to Sample
If we were able to test an entire landscape’s gold concentration at a reasonable cost, we would. This might be possible one day, but for now we must rely on taking samples of between 300g to a few kilograms that are representative of the area from which they came. In doing this we hope to determine whether that area should be looked at more closely for a gold deposit, or forgotten about.
The type of sampling will depend on the stage of exploration, which will in turn be a function of what is already known about an area. Early-stage exploration has little or no knowledge about the presence of gold in the area. In this situation the explorer needs to be able to start focusing their attention on smaller areas of land based on some evidence of potential gold mineralisation. But how to do this by sampling? The explorer can’t take rock samples every few metres over 100’s of square kilometres. The answer is often the use of stream sediment surveys. These are relatively wide spaced sampling programs (1 sample per several square kilometres) which use the sediment from streams as a representative sample of the wider catchment area up-stream. Any small trace of gold found in a stream sediment sample will focus an explorer’s attention on the area upstream of the sampling point.
At this stage of exploration patterns of gold values are more important than single isolated results. For example, it will be more encouraging to see multiple low level gold hits (50 to 400 parts per billion) in stream sediment over a continuos area than to have a single high gold in soil sample (>1000 parts per billion for example). The former indicates potential of a deposit with significant dimensions to shed consistent gold into stream sediments, whereas the latter could be a lucky grab from a small vein with no economic potential.
Once an area of interest is identified by gold in stream sediment samples, more densely spaced sampling such as soils will be employed. The position of the soil samples will be guided by the location of the positive stream sediment samples, but then specifically located according to the local geology and any other tools like geophysical surveys or ASTER imagery. Soil samples will always be in a grid or along line transects, for example samples spaced every 50m along a 1km long line, with each line being 100m apart.
Providing that gold continues to be found in the soil sampling, trenching and cuts will now focus on a scale of tens of meters rather than 100’s of metres. Trenching and cuts sample hard rock rather than the eroded loose sediments of the stream and soil samples. It is at this trenching stage that gold grades need to be at the level that can potentially be mined, ie. generally >0.5 parts per million (equal to 0.5 grams per tonne or 500 parts per billion)*.
At this stage sampling will have been done only at the surface. If the trenching results are good, the high cost of drilling will be justified and a drill program done. Drilling is essential for any gold deposit discovery as it confirms the existence of gold bearing under the ground. The sampling from drilling will usually be on 1m intervals down the hole. Drill holes will drilled at a density and number fit for the use in a gold resource estimate meeting JORC or NI43-101.
*Gold concentrations lower than this are mined economically, while other deposits with much higher grade may not be economic because their rock type makes it prohibitively expensive to extract the gold from it. This is important when thinking about gold deposit profitability.