Greisens appear to be restricted to intrusions which are emplaced high in the
crust, generally at a depth between 0.5 and 5 km, as the hydrous fluid separation from granite to produce greisenation cannot occur deeper than about 5 kilometres. The roof or upper aureole is mostly sealed shut to prevent most
fluids escaping. This sealing is largely due to
hornfelsing and silicification of the overlying rocks, and fracturing of these rock typically forms greisen veins. They are generally associated mostly with potassic plutonic rocks;
Alkali feldspar granite, and are rare in less potassic rocks like
granodiorite or
diorite. Greisens are prospective for mineralisation because the last fluids of granite crystallization tend to concentrate incompatible metals such as
tin,
tungsten,
molybdenum and
beryllium, and in places other metals such as
tantalum,
gold,
silver, and
copper.
Tectonically, greisen granites are generally associated with generation of S-type suites of granites in thick arc and
back-arc fold belts where
subducted sedimentary and
felsic rock is melted. ==Distribution==