Definitions and images to illustrate geological terms, links to images and website articles


Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia. Courtesy of Roland Gotthard.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.

A banded, 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.

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