What is a greisen in geology ?

Greisen : definition

A greisen is a quartz and muscovite rock of hydrothermal origin present at the edge of granite massifs or pneumatolytic veins. It results from the replacement of granite feldspars by the quartz-muscovite (and sometimes lepidolite) association under the action of pneumatolytic fluids, rich in Li, F, B and in various metals (Sn, W, Nb, Ta, etc...). Greisens can constitute notable concentrations of cassiterite, fluorite, tourmalineberyl or topaz. The kaolin quarries at Echassières (Allier, France) are located in mica schist and greisen.