Methodology for adequacy of sampling of mill tailings and mine waste rock

There are numerous suggestions in the literature on methods of obtaining representative samples of waste rock or mill tailings. Some suggested methods would result in thousands of samples and very high costs for a large facility. There is no general rule that can (or should) be followed, such as collection of a given number of samples per mass of material, per acre of impoundment, or per foot of drillcore. Each site is different, and the adequacy of sampling must be tailored to the site. A logical approach is to let the variability at the site dictate the number of samples that should be collected and analyzed, and then to quantify and evaluate the results by means of standard statistical methods. Each tailings impoundment or waste rock pile is unique, and the degree of heterogeneity must dictate how many samples will be required to yield an adequate and defensible characterization. In a characterization of a large tailings impoundment located in the western United States, 229 samples were found to be adequate to characterize approximately 1.5 billion tons of tailings. Statistical analysis for that site shows that hundreds of additional samples would yield only a very small increase in the level of confidence of the characterization, and that the estimated mean acid-base accounting (ABA) value from 229 samples was adequate for the intended purposes.