The In-Situ Nugget Effect: A Transition between Geostatistics and the Theory of Sampling

Exploration and ore grade control programs often face great difficulties with minerals occurring as isolated nuggets, clusters of particles, or clusters of veinlets, leading to rapid changes over a very short distance. The random term of a variogram is made of many components, and the variance of the In-Situ Nugget Effect is only one of them. The original Theory of Sampling, created for sampling particulate materials, did not address the uncertainty created by the selection of a certain mass for an unbroken module of observation during diamond core, reverse circulation, or blasthole drilling. If such uncertainty is not well addressed, it can become a major source of sampling error. It is therefore critically important to include this new error in the Theory of Sampling, where it belongs, as it most certainly has an impact on the selection of a drilling diameter. In this chapter we explore the properties of this error, how it occurs, how to quantify it, and what we can do to minimize its negative effects. This error should not be confused with what geostatisticians refer to as the Nugget Effect in a variogram, which includes the In-Situ Nugget Effect, Fundamental Sampling Errors, Grouping and Segregation Errors, and the random component of Increment Delimitation Errors, Increment Extraction Errors, Increment Preparation Errors, Increment Weighting Errors, and Analytical Errors. It is of the utmost importance to clearly differentiate all these errors, so pragmatic solutions can be found to create a reliable database for geologists, miners, environmentalists, and geostatisticians.