Greisen is formed by alteration of which rock type and is considered a class of endoskarn?

Greisen is formed by alteration of which rock type and is considered a class of endoskarn?

• A. Metamorphosed limestone
• B. Basaltic lava flow
• C. Sandstone cement
• D. Highly altered granitic rock or pegmatite formed by self-generated alteration of a granite and is a class of endoskarn

Answer: (D)

Greisen forms from intense hydrothermal alteration of granitic rocks, specifically granites and pegmatites, driven by fluids generated within the intrusion itself. This late-stage, silica-rich alteration produces a quartz- and mica-rich assemblage, with minerals like muscovite and often topaz or tourmaline, and it is commonly associated with tin-tungsten mineralization. Because the alteration occurs inside the granitic body, greisen is considered a form of endoskarn, the class of skarn that develops within the intrusion rather than in the surrounding country rock. The other rock types listed would not produce greisen textures or mineral suites: metamorphosed limestone would form marble, basaltic lava flow would lead to different alteration products, and sandstone cement represents diagenetic cement rather than greisen-type hydrothermal alteration.

Greisen forms from intense hydrothermal alteration of granitic rocks, specifically granites and pegmatites, driven by fluids generated within the intrusion itself. This late-stage, silica-rich alteration produces a quartz- and mica-rich assemblage, with minerals like muscovite and often topaz or tourmaline, and it is commonly associated with tin-tungsten mineralization. Because the alteration occurs inside the granitic body, greisen is considered a form of endoskarn, the class of skarn that develops within the intrusion rather than in the surrounding country rock. The other rock types listed would not produce greisen textures or mineral suites: metamorphosed limestone would form marble, basaltic lava flow would lead to different alteration products, and sandstone cement represents diagenetic cement rather than greisen-type hydrothermal alteration.