Greisen is a highly altered granite|granitic rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of skarn|endoskarn.

Greisens appear as highly metasomatism|altered rocks, partly coarse, crystalline granite, partly vugh|vuggy with miarolitic cavities, disseminated halide minerals such as fluorite, and occasionally metallic oxide and sulfide ore minerals, borate minerals (tourmaline) and accessory phases such as sphene, beryl, topaz, etcetera.

Greisens are formed by endoskarn alteration of granite during the cooling stages of emplacement. Greisen fluids are formed by granites as the last highly gas- and water-rich phases of complete crystalisation of granite melts. This fluid is forced into the interstitial spaces of the granite and pools at the upper margins, where boiling and alteration occur.

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