The Australasian Code for Reporting of Mineral Resources and Ore Reserves, December 2004 (the JORC Code) deals with reporting exploration target size and type, which are expressed as ranges in order to properly convey the high level of uncertainty that exists early in the project evaluation process. To inform investors of the significant potential mineralisation in BHP Billiton’s Pilbara tenements and to enable strategic planning, a rigorous modelling approach has been devised for such early stage projects. An innovative approach to modelling scenarios that capture the uncertainty range has been developed. The approach integrates drill holes, mapping, geophysical surveys and the geologist’s interpretation. The modelling workflow consists of two parts: geology modelling, and grade modelling. An implicit 3D modelling approach is utilised to rapidly generate multiple regional models of stratigraphy. Mineralisation envelopes and weathering horizons, providing further controls on the target range, are also modelled using fit-for-purpose tools and data structures. Grades are estimated with a recoverable resource estimation approach. The estimation accounts for uncertainty of local grade distributions and provides multi-variate grade models that honour correlations between variables. The final product delivers a rigorous approach to the estimation of tonnage and grade ranges for reporting the exploration target that captures the uncertainty of the estimate.
