To assess the risk of acid and metalliferous mine drainage (AMD), most new mining projects require an initial assessment of the locations and volumes of potentially acid forming (PAF) waste that is likely to be moved and dumped in waste stockpiles. This requires sampling of the waste rock zones and submitting the samples for a series of tests. Key to the outcome of the waste characterisation process is obtaining sufficient samples that are representative of the various rock types, both in number and spatial distribution. Various regulatory bodies throughout Australia and the rest of the world are currently struggling with how to define minimum amounts of sampling to achieve realistic estimates without imposing excessive costs and time constraints on a project.
This paper addresses two aspects of the process:
1. What constitutes a ‘sufficient’ number of samples
2. How to use those samples in determining volumes of likely PAF material
Geostatistical methods of characterising ore bodies with large numbers of samples are routinely used in the mining industry by geologists and geostatisticians to estimate and classify mineral resources. In theory, the same methods can be used to estimate PAF waste locations and volumes. In practice, the number and spatial distribution of dedicated PAF waste samples at feasibility stage is typically so small that it is almost impossible to estimate the spatial distribution of PAF material with any degree of confidence. Consequently, the common approach is to characterise a few samples from a lithological unit and then assume that they represent the unit as a whole. This approach, however, can be fraught with risk.
This paper describes some of the findings of SRK’s evaluation of several coal projects with respect to PAF waste evaluation and highlights some common misconceptions.
