Assessment of the resources of a Gold Deposit by Transitive Kriging
Résumé
Transitive methods were developed to estimate the global in situ resources of an orebody without requiring a random function model. They are very useful when the geometry of the orebody is poorly known, as is demonstrated in the present study. The orebody is a carbonates replacement deposit, whose gold mineralization is strongly linked with igneous intrusives. It comprises two units: the Jasperoid unit in the central part, with the highest gold grades, and the Envelope unit at the periphery. New geostatistical tools show that the transitions from waste to the Envelope and from the Envelope to Jasperoid are sharp in terms of grade. Conversely, in each unit the variogram of gold grade shows a good continuity. In the absence of geological guidelines, modeling the geometry of the various units is the main concern. Transitive methods were put into practice to model the mineralized zone (Envelope + Jasperoid) and the Jasperoid unit alone. Transitive kriging provides an estimate of the indicator of these units in a grid of blocks. Gold grade was kriged separately for each unit over the same grid as the indicators. The resources of each block are derived from the indicator and grade estimates. The global resources are obtained by summing up the resources of the blocks. In practice, blocks with a too large transitive kriging variance or a too low proportion estimate are discarded, and the resources are parameterized by the corresponding thresholds, giving access to the uncertainty of these resources.