When an orebody is mined it is inevitable that some of the adjacent unmineralized or weakly mineralized material is mined as well and sent to the extraction plant. The grade of this material will be much lower than the mineralized rock and the specific gravity may also be much different. It is important therefore to account for this material in the calculation of reserves (from resources) and the production schedule. The lower grade material may impact the metallurgical recovery of the minerals of interest by the introduction of deleterious substances.

It is important to analyze and study the mineralogy and grade of the adjacent rocks, therefore. Dilution is introduced into the mined material in a number of ways depending upon the mining method chosen. In open pit mines, the large excavation equipment will not be able to discriminate between the waste and ore materials resulting in waste materials being sent to the plant. In underground mines, waste material may slough off the walls of the stopes and become mixed with the ore material. This is particularly the case for indirect mining methods in large stopes.

The prediction of dilution is more art than science. Tests of the friability of the waste rock, the presence of open fractures and joint sets and differences in specific gravity are all important elements in the prediction of dilution. It is also important to look for similar types of operating mines and determine their experience with dilution using these results as a prediction of dilution in the new mine.

It can be appreciated that the calculation of a mining reserve must include some element of dilution as it is inevitable. Most of the reserve definitions accepted by modern securities regulators include dilution and an estimate for reserves which does not include dilution will not be accepted for publication.