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.
An asymmetrical vein
is a crustified vein of geologic material with unlike layers on each side.
, or ribbon vein
is composed of layers of different minerals lying parallel to the walls.
Barren vein matter, or a pinch in a vein, assumed to overlie an ore is called a cap rock
Labels: accretion vein, banded vein, boudin, boudinage, crack-seal filling, open-crack filling, ribbon vein, vein