A Geostatistical Contribution To The Use Of Resource/Reserves Classification Systems

United Nations (UN) has been leading an international committee to define the guidelines in reporting resources and reserves. The system proposed by UN is very similar to the guidelines currently in use in Australia known as JORC guidelines (Joint Organisation Reserves Committee). The JORC guidelines show major differences in relation to the Brazilian system current in practice. The important aspects differing JORC from the current Brazilian guidelines for reporting reserves is the required sampling spacing. The JORC system requires the points of observation to be closer spaced than the Brazilian system for any class of resources. All classification systems share some common aspects in terms of defining the classes of resources based on distance separating samples and on the degree of confidence or accuracy associated with the results reported. Despite of being very clear in terms of stating sample distances, all the systems of classification do not provide clear definitions on how confidence limits should be calculated. This paper proposes a geostatistical framework to measure the uncertainty associated with the reported tonnages and grades. Results demonstrate that the error associated with an estimate can be approximated using multiple simulated deposits to define confidence limits. The methodology is illustrated in a large Brazilian coal deposit and proved to be appropriate to address the issue of quantifying the uncertainty necessary to be incorporated in the systems of reserves classification.