La ganga es el material que se descarta al extraer la mena de un yacimiento de mineral, por carecer de valor económico o ser demasiado costoso su aprovechamiento. Es posible que un mineral que se considere ganga en un yacimiento sea de interés en otro, o que la mejora en las técnicas extractivas o los usos industriales haga rentable el procesamiento de materiales anteriormente considerados ganga. La rentabilidad de una mina depende de su riqueza en el metal esperado así como de la forma en que se presente. Si la mena y la ganga se encuentran mezclados, y los procesos físicos y químicos de separación y preparación son demasiado costosos, la mena puede no ser rentable. Tampoco lo es la mena cuyo volumen es exiguo, o aquella cuya situación impide una explotación accesible.

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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 and 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 tailings (or dumps) of mines. As the value of a mineral increases, or when new and cheaper means of processing the ore are introduced it has often become worthwhile to rework such old dumps to retrieve the minerals they still contain.

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