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Confidence in classifying a resource is affected by the nugget effect. This is a term used to describe how well sampling results can be reproduced by repeated sampling at the same location. It incorporates both the natural inherent variability of the deposit plus variability due to sample size, sample preparation and analysis. The more homogeneous the mineralisation, the lower the nugget effect. Finely disseminated mineralisation will tend to give easily reproducible results but heterogeneous mineralisation will be sensitive to the method of sampling and could give variable results from a single location.
How to measure the Nugget Effect
The nugget effect can be measured by examining the results of repeated sampling from the same or nearby locations. It can also be measured using the semivariogram. The intersection of the semivariogram on the Y axis is an estimate of the nugget effect, that is the level of variability at zero separation between samples. For gold, this is usually of the order of 30% to 50% of the total variability. Other, more regularly distributed commodities, such as iron ore, manganese and zinc have lower nugget effects. Some coarse gold deposits and alluvial deposits may display a random distribution with a nugget effect tending towards 100%.
These are the most difficult deposits to evaluate because of the lack of spatial correlation.
Importance of the Nugget Effect in resource estimation and classification
Recognition of the level of the nugget effect is vital to resource estimation. The higher the nugget effect, the lower the likelihood of being able to achieve a high degree of selectivity during mining. This affects classification in that a high nugget effect resource quoted at a low cut-off may have a higher degree of resource confidence than if it were reported at a higher cut-off.