ABSTRACT
Regulatory bodies around the world have relied on similar definitions for mineral resource categories, and subdivide mineral resources in order of increasing geological confidence: Inferred, Indicated and Measured. Major investment and development decisions are based on a correct assignment of a resource to a proper category. The progression to mineral reserves reporting requires at least the categorization of Indicated and/or Measured resources. As such, the assignment of a mineral resource to an Indicated category should be recognized as a necessary step to advance towards the feasibility of a project.
Allocation of mineral resource categories often relies on some geostatistical tools, of which drill hole spacing is perhaps most common. This paper looks at procedures to determine the optimal drill hole spacing required for allocation of a portion of resources to an Indicated category in metal deposits. These procedures are differentiated into two main groups based on their simplicity and stage of application: (1) general assessment of adequate drill hole spacing; and (2) local assessment of uncertainty on estimated grade within large panels representing monthly or quarterly production. At an early exploration stage general assessment of optimal drill hole spacing for an assignment to an Indicated category is quite adequate. On the other hand, at a pre-feasibility stage simulation studies represent the best tool for resource categorization. Practical examples from a case study the authors have worked on will be presented.
