Gangue is the substance that must be extracted along with desired minerals or ores due to their close association. These materials are typically removed along with the target minerals or ores, and must then be separated out. Gangue may then be dumped as tailings. In some cases, there may be additional uses for the materials present in tailings. When situations like these occur, the tailings may be further processed to remove the newly valuable materials.
During the mining process, the materials that are unearthed are typically classified into two groups. The economic fraction is the desirable material that has a value, while the uneconomic fraction, otherwise known as gangue, is everything else. When determining whether a mining operation will be economically viable, the costs of separating the economic fraction from useless gangue are typically considered. Sometimes, a desirable material will be so integrated with useless gangue that it would be too costly to extract.
Mining technology has developed over the years, and more sophisticated methods have been been created to separate the economic and uneconomic fractions. Early mining operations could be inefficient, and often left relatively large amounts of useful minerals in the mine tailings. This was often due to the poor ability of older techniques to effectively separate useful minerals from gangue, or a result of the minerals being suspended in particularly difficult configurations. When new techniques are created, old tailings dumps are sometimes revisited for their stores of useful minerals and ores.
Tailings may also be reexamined if new uses for old materials are discovered. Arsenopyrite was once considered a useless gangue mineral and dumped in the tailings as part of the uneconomic fraction. Later developments led to arsenic becoming a popular insecticide, at which time the arsenopyrite contained in mine tailings could be considered an economic fraction. When developments like this occur, tailings are often reprocessed to extract the newly desired minerals and ores.
In certain mining operations, gangue may contain hazardous materials. This may be the result of mining hazardous materials, some of which invariably end up in the tailings due to processing that is not perfectly efficient. Hazardous or poisonous chemicals, such as cyanide, may also be used in the processing of minerals like gold and silver, in which case certain amounts often end up in the tailings. When this happens, the mining operation will often include an environmentally safe dump site to keep poisonous or hazardous materials present in the tailings from leeching out into the ground water.