Advances in estimations of number of undiscovered mineral deposits in three-part assessments

A central part of three-part mineral assessments is estimating number of undiscovered mineral deposits. Internal consistency and well-explored deposits in grade–tonnage models reduce chances of biased estimates in three-part quantitative assessments. Estimates are of specific types of deposit permitted by geology in delineated tracts. Regardless of estimation method, each deposit type needs a grade–tonnage model constructed with well-explored deposits and estimated numbers must be consistent with this model. Local well-explored deposits are typed and statistically tested to ensure the global grade and tonnage model is representative of them.

Three-part assessments used expert judgements since 1975 that relied on analogy-based guidelines to aid in estimation of number. The advantage of expert judgements is that they can use all available information and adapt to information supplied by other experts during the assessment. A targeting method has seen limited application due to the requirement that location indicators of each possible target need to be present. Another estimation method developed around 2008 is a generalized deposit density model based on multiple deposit types which uses deposit type's median tonnage, permissive tract area, and number of known deposits in well-explored control areas. It is remarkably robust and can be applied to any deposit type in a proper permissive tract.

Advanced deposit estimations use the generalized density method to guide expert estimates. Experts can then identify possibly flawed estimates and improve estimates based on local information such as local exploration results or observations by experts thereby further reducing chances of biased estimates.