Locating Minerals with Soil Samples
If there are no rock outcrops in the vicinity of an area of interest then soil samples are often taken. The theory behind soil samples is that the underlying mineralization (if it exists) will enrich the overlying soil with anomalous amounts of the metals of interest. “Anomalous value” is usually measured in parts per million and so soil samples are most often taken on a grid. That is a sample of the soil is excavated from a small hole every 25 or 50 meters along lines which are perpendicular to each other. Let’s assume that the grid consists of lines spaced every 25 meters apart with holes dug every 25 meters along the line. If there are 10 lines in both directions then there will be 100 samples taken.
The samples are all assayed for the element of interest or, more often, a lot of elements so that iso-grade contours can be drawn resulting from the sampling program. If the element of interest is gold then the contour plot will identify areas of higher gold values. The assay values may not be very high in absolute terms, but if there is a differentiation in the values then this may indicate where to focus the next phase of exploration.
Soil sampling then, is an indirect way of understanding the potential for any underlying mineralization. An individual soil sample is of dubious value but a gridwork of soil samples can be very useful in directing exploration activities.
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