Quantifying mean grades and uncertainties from the ratio of service variables: accumulation to thickness. Applied to a phosphate deposit in Southern Mato Grosso, Brazil

Abstract Estimating mineral resources of a stratiform mineral deposit is not always a simple activity. Some difficulties can arise in mineral deposits where the thickness is relatively much smaller than the dimensions in the horizontal plane, or because these deposits have suffered events after their genesis, such as thrusts and folds. Usually, in these cases, we use a new variable named accumulation as a product between the average grade and thickness. Then, from these service variables, accumulation and thickness are estimated in a regular grid. Dividing accumulation by thickness, we have the mean grade. However, the mean grade is just an approach because the uncertainties of variables are not considered in this division. This calls for applying the Taylor series to computing, not only the mean grade, but also the associated uncertainty. In this article, we present the satisfactory results of this method applied to quantify the mean grades and uncertainties of the blocks to a stratiform phosphate deposit located in southern Mato Grosso, Brazil.