Abstract: |
When a mineral deposit is made of geological bodies such as breccias or lenses which concentrate high grades, and when the size of the production blocks is important compared to the average size of such bodies, estimating block grades by ordinary kriging may produce unrealistic spatial continuity. To improve the block model, a usual practice consists in (1) Estimating spatial proportions of facies (ore unit). (2) Estimating the grades, facies by facies. (3) Combining the results to obtain the block grades. We show that this practice assumes some links between the geological bodies, which shall be verified. We then address this general question: Given a set of samples where facies and grades are known, what is the best way to build a block model? We propose a methodological work flow which leads to a cokriging system where facies indicators are used together with their product with the grade (the ?partial metal grades?), a method that does not require the previous calculation of facies proportions, at the scale of the blocks. Three applications to porphyry copper deposits located in Chile are presented. We compare the results to kriging without any facies consideration, and to the usual approach. |
