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Geologic controls on the composition of natural waters and mine waters draining diverse mineral-deposit types

Sulfide-bearing mineral deposits formed in reduced conditions out of contact with an oxygenated atmosphere. When sulfides in the deposits are exposed by natural erosion or by mining to atmospheric oxygen and water, weathering of the sulfides can produce natural or mining-related acid-rock drainage. The prediction of water quality that results from mining and mineral processing activities has therefore become a high priority in the permitting of mining activities worldwide, in order to prevent the formation of or mitigate the environmental effects of deleterious drainage waters. In addition, estimating the compositions of natural waters that drained mineral deposits prior to mining is crucial to establish appropriate baseline environmental standards at mine sites. There are a variety of techniques currently in use to predict the acidity or metal content of mine-drainage waters, most common of which are static and kinetic testing procedures. In static procedures such as acid-base accounting (White et al., 1997, 1999), the contents of acid-generating sulfide minerals from ores and wastes from a proposed mine are measured and balanced against the measured contents of acid-consuming minerals such as carbonates; based on this balance, the materials are determined to be acid generating or non-acid-generating. In kinetic tests such as column or humidity-cell tests (ASTM, 1996), samples of ores and wastes are allowed to react over a period of time under laboratory conditions with oxidized waters or moist air, and the pH and metal contents of the resulting leachates are then measured. Although both static and kinetic methods are widely used to help predict the compositions of mine waters, they have several potential limitations. Most important of these are (1) whether the samples used in the tests adequately represent the range of mineralogic characteristics commonly present in complex mineral deposits, (2) whether the time scale and laboratory conditions of kinetic tests adequately reproduce the time scales and physical, geochemical, and biological conditions actually present in the mine, mine dump, or tailings impoundment environment, and (3) whether kinetic test leachate compositions accurately reproduce actual drainage quality. Another approach to mine-drainage prediction that can be used to supplement the static and kinetic engineering tests is one in which the compositions of existing mine waters draining geologically similar deposit types in similar climates are measured empirically and then interpreted in a geologic and geochemical context. By evaluating the compositions of waters draining geologically comparable deposits in comparable climates, it is possible to place constraints on the potential ranges in composition of chapter 19