Geological Sampling. How to collect a representative sample for testing Metallurgy and Assays.
Sampling is the process of taking a small representative portion of a Iarger mass. By analyzing the sample to determine the concentration of metaI it contains, the potential value of a larger mass can be determined.
The first samples taken from a mineral showing are called grab samples. Prospectors and geological field crews grab samples from outcrops, road cuts, trenches or river beds. These rocks are selected specifically because they appear to contain a significant amount of metal, so they are not considered representative of the outcrop or road cut from which they come.
In the field, grab samples are gathered, their original location is recorded, each rock is labeled and the most promising ones are sent to a lab for metal analysis.
If worthwhile or significant amounts of metal are present in such grab samples, channel sampling may be warranted. In this sampling technique, bedrock where the sample was taken is exposed as fully as possible, typically by using a backhoe or some such piece of earth-moving equipment.
Next, the outcrop is hosed down with water and, if a zone of mineralization is revealed, representative surface samples are taken at regular intervals across the exposed zone. These samples are usually cut with a portable circular saw equipped with a diamond-studded blade, leaving a linear channel across the outcrop.
The surface channel is the most desirable type of sample. It is normally cut about 10 cm (4 inches) wide and 2 cm (3/4 inch) deep across the supposed ore zone. The chips of rock removed are carefully collected, marked and bagged for analysis.
Chip samples are sometimes taken by the geologist or engineer for a quick approximation of contained value. Random pieces are quickly knocked off the outcrop with a hammer and chisel, with an effort made to take representative amounts. Chip samples can not be relied upon fully, so they generally do not enter final mathematical calculations of possible reserves.
It is highly desirable, but not often practical, to space surface channels at regular intervals along the mineralized zone. This obviates one mathematical calculation in the interpretation process.
In certain circumstances, particularly when sampling kimberlite rock for diamonds, it is useful to collect a bulk sample, which may range from a few hundred kilograms to several tonnes in weight, It is important for a bulk sample to be representative of the zone since this material can be used later for definitive metallurgical test work and grades.