Use of Tetrahedral Modelling for Variography and Grade Estimation of a Structurally Deformed Phosphate Deposit

The application of geological controls in orebody evaluation and modelling is critical. Stratigraphic orebodies require more accurate location in space and better seam boundary definition. This is achieved by either modelling boundary surfaces of the orebodies or their thickness at specific points in 3D space. Structurally deformed stratigraphic deposits pose a very special problem in variography and grade estimation – it is necessary to bring the locations of samples from the deformed space to the standard XYZ co-ordinate system. This is necessary in order to reconstruct the spatial distribution of grades at the time the deposit was formed and restore the relative positions of samples to their pre-deformed state. Earlier methods of unfolding deposits were based on various geometrical, mathematical or even manual techniques, while more recent methods were based on the use of an unfolded coordinate system for the transformation of every sample and estimation point. The alternative method presented in this paper, Tetrahedral Modelling, can be applied to deposits where mineralization is controlled by a pair of structural surfaces that can be modelled. In tetrahedral modelling the search ellipse is distorted to follow nominated structural surfaces leading to improved estimation accuracy. A case study is presented showing the benefits of this technique.