Characterization from Exploration Core

Diamond drilling, with geological core logging, is the most commonly used method for orebody delineation. Information obtained from drill intersections is extrapolated hole-to-hole using geological assumptions to provide estimates of lithological boundaries, alteration, weathering, hydrogeology, orebody size, shape, grades, continuity, tonnage, and some geotechnical characteristics.

The advantages are the depth to which the information can be obtained, and a relatively routine data analysis and interpretation. Holes near the center of the mineralization provide critical information for stope design, while holes near the periphery are critical to the design of mine infrastructure such as shafts, access declines, and crusher chambers.

Another advantage of geological logging is that characterization encompasses every drilled hole through a geological deposit. If some relevant geotechnical parameters are collected within this program, an extensive and representative database within and across the immediate boundaries of an orebody can be established. Parameters such as discontinuity linear frequency and rock mass classification data can be used to determine spatial variations in rock quality across an orebody. A perceived disadvantage is that a large number of individuals may perform the geological and geotechnical logging, introducing the chance of bias arising from different practices and interpretations. In addition, some of the drilling data may be collected from small-sized unoriented core that is not ideally suited for geotechnical logging.

The approach suggested here is to carry out geotechnical logging on a number of selected holes within each exploration ring as part of the orebody delineation drilling program. The approach does not require oriented core to carry out the geotechnical logging, with the level of detail required during geotechnical investigations usually depending upon the stages of a particular project (mine prefeasibility, feasibility, etc.). Estimates of the likely stable stope sizes and shapes, dimensions of regional pillars, the best locations for underground infrastructure, and reinforcement schemes can be provided by such investigations. Figure 4.4 shows a cross section of a typical exploration ring where horizontal, steeply inclined, and steeply declined holes were logged systematically across the orebodies.

Experience has shown that the choice of data format is important to facilitate the subsequent stages of the stope design process. In some cases, the computerized geological and geotechnical data are meshed as a three-dimensional model. In some mines, such a geological/geotechnical model is not available, and the information is presented on paper plans/sections from which it can be digitized for printing purposes only. It is important that the initial geological model built is not only for grade control purposes but also intended for use in predicting the likely engineering performance of the excavations. The following sections describe a procedure that can be followed to carry out a rock mass characterization program from routine underground orebody delineation drilling.

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