In geology, a vein is often defined as a long, regularly shaped occurrence of an ore (lode). However, more generally, a vein is a finite volume within a rock, which is filled with crystals of minerals precipitated from an (aqueous) fluid. The mineral-laden fluids, often of hydrothermal origin, circulated hydraulically before depositing the minerals by open-space filling or crack-seal growth.Open space filling occurs at relatively low pressure epithermal vein systems, such as stockworks, in greisens, or in some skarn environments. Crack-seal filling occurs at higher pressures, with reopening of the vein fracture by progressive deposition of minerals on the growth surface.
In a process called boudinage, veins may be pinched and distorted into sausage-shaped bodies called boudins. (image above left - click to enlarge - boudinaged quartz vein in dextral shear foliation, Fortnum Gold Mine, Western Australia.)
An accretion vein is formed by the repeated filling of channels, followed by their opening by pressure-related fracturing in the zone undergoing mineralization.
