Gangue

In mining, gangue (/ɡæŋ/) is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. It is thus distinct from overburden, which is the waste rock or materials overlying an ore or mineral body that are displaced during mining without being processed.

The separation of mineral from gangue is known as mineral processing, mineral dressing, or ore dressing. It is a necessary and often significant, aspect of mining. It can be a complicated process, depending on the nature of the minerals involved. For example, galena, an ore of lead, is usually found in large pieces within its gangue, so it does not normally need extensive processing to remove it; but cassiterite, the chief ore of tin, is usually disseminated as very small crystals throughout its gangue, so when it is mined from hard rock, the ore-bearing rock first needs to be crushed very finely, and then has to be subjected to sophisticated processes to separate the ore.

For any particular ore deposit, and at any particular point in time, the concentration of the wanted mineral(s) in the gangue material will determine whether it is commercially viable to mine that deposit. The ease with which the ore can be separated also plays an important part. Early mining ventures, with their relatively unsophisticated methods, often could not achieve a high degree of separation, so significant quantities of minerals found their way into the waste mineral dumps of mines. As the value of a mineral increases, or when new and cheaper means of processing the gangue to extract the ore are introduced, it may become worthwhile to rework such old dumps to retrieve the minerals they still contain.

Minerals that were once thought of as gangue, and were dumped, may later find a commercial use. When this happens the old dumps are often reworked to extract the wanted mineral. For example, in copper mines in the 19th century the mineral arsenopyrite was dumped until arsenic became popular as an insecticide later in the century.

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Wikipedia