Geostatistical support for categorization of metal ore resources in Poland

Abstract

Abstract The authors attempted to introduce some components of the Australasian JORC Code system to the categorization of Polish Cu-Ag and Zn-Pb ore resources. The proposed geostatistical method of resource categorization applies two criteria: continuity of deposit parameters described by semivariograms and permissible, relative standard error of resources estimation determined with the ordinary kriging procedure. Considering the first criterion, we propose the following values of autocorrelation coefficients, which define the ranges (distances) of the resources categories around the measurement sites (e.g., exploration wells): “measured” category (A + B in the Polish system) – the values of the autocorrelation coefficient from 1 to 2/3, “indicated” category (C1 in the Polish system) – the values of the autocorrelation coefficient from 2/3 to 1/3, “inferred” category (partly C2 in the Polish system) – the values of the autocorrelation coefficient from 1/3 to 1/20, “out-of-doors” category (partly D in the Polish system) – the values of autocorrelation coefficient from 1/20 to 0. The second criterion of resources categorization is based upon the relative, standard errors of resources estimations calculated for the parts of deposit defined with the first criterion. The following permissible values of errors determined as the errors of ordinary kriging have been proposed: “measured” category (A + B in the Polish system) – 10% error, “indicated” category (C1 in the Polish system) – 20% error, “inferred” category (partly C2 in the Polish system) – 30% error, “out-of-doors” category (partly D in the Polish system) – 50% error. It was found that the Polish metal ore deposits reveal low continuity of deposit parameters, as indicated by a high share of nugget variance in the overall variability of these parameters. Moreover, an inconsistency was observed between the semivariograms of deposit parameters based upon samplings of drill cores and underground mine workings, which results in extreme differences in the ranges of resources categories around the sampling sites. This, in turn, causes radical discrepancies in estimated resources. Thus, it was concluded that the sufficiently credible categorization of resources is only possible when a significant part of the deposit is explored with mine workings, in which the grid of sampling sites is much denser than that of exploration wells. It was proposed that the principal criterion of resources categorization should be the permissible error of estimations whereas the continuity of deposit parameters should only be a supplementary criterion.