Geochemical and Mineralogic Characterization of Solids and Their Effects on Waters in Metal-mining Environments

The geologic characteristics (ore mineralogy, geochemistry, host rock lithology, etc.) that economic geologists use to classify mineral deposits are key to understanding their environmental signatures. Mineralogic characterization and chemical analysis are among the many scientific tools that can be applied to mining-environmental prediction, mitigation, and remediation (Plumlee and Logsdon, 1999a). Mineralogy is important because (1) the primary ore and non-ore (gangue) minerals are sources of metals and other elements released into solution upon weathering, (2) these weathering reactions can produce or consume acidity (H +), and (3) many secondary minerals that form during the weathering process can permanently or temporarily sequester metals, recycling them and generating or consuming acidity in reactions triggered by changes in moisture, temperature, or pH in the local environment. The pH of mine drainage varies depending on the balance between acid-producing and acid-consuming reactions that occur during weathering, the relative rates of these reactions, and the accessibility of minerals that contribute to these reactions (Smith and others, 1994). Thus, the chemical signature of waters and solids associated with a mineral deposit are intimately linked. Most mines are situated along streams and in many cases mine waste is deposited in or adjacent to streams (fig. 1), so surface runoff from and infiltration into mine waste is an important control of drainage water chemistry. Static tests (acid-base accounting) used to predict acid-rock drainage (ARD) and develop remediation options such as lime requirements are based on assumptions about mineralogy that may or may not be appropriate. These tests are at best a screening tool and provide no information about reaction rates. However, these tests are relatively rapid and inexpensive compared to long-term kinetic tests and are a reality of modern mining and environmental regulation. Knowledge of the mineralogy of the mine waste provides a guide to choosing the most appropriate methods for acid-base accounting because results for different methods vary depending on the minerals that represent the sources of acid and the minerals that provide sources of neutralization. Mineralogic characterization provides information on (1) sources and identity of potential metals and acidity, (2) sources of neutralization, (3) presence of minerals that may contribute to laboratory-based neutralization potential calculations, but behave differently under field conditions, (4) soluble and insoluble constituents of earth materials, and (5) textures and grain sizes that can affect reactivity of different minerals (Price and others, 1997; Jambor and Blowes, 1998). Many active mine operations are starting …