Vein (geology)

In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation. In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation. Veins are classically thought of as being the result of growth of crystals on the walls of planar fractures in rocks, with the crystal growth occurring normal to the walls of the cavity, and the crystal protruding into open space. This certainly is the method for the formation of some veins. However, it is rare in geology for significant open space to remain open in large volumes of rock, especially several kilometers below the surface. Thus, there are two main mechanisms considered likely for the formation of veins: open-space filling and crack-seal growth. Open space filling is the hallmark of epithermal vein systems, such as a stockwork, in greisens or in certain skarn environments. For open space filling to take effect, the confining pressure is generally considered to be below 0.5 GPa, or less than 3–5 km (2–3 mi). Veins formed in this way may exhibit a colloform, agate-like habit, of sequential selvages of minerals which radiate out from nucleation points on the vein walls and appear to fill up the available open space. Often evidence of fluid boiling is present. Vugs, cavities and geodes are all examples of open-space filling phenomena in hydrothermal systems. Alternatively, hydraulic fracturing may create a breccia which is filled with vein material. Such breccia vein systems may be quite extensive, and can form the shape of tabular dipping sheets, diatremes or laterally extensive mantos controlled by boundaries such as thrust faults, competent sedimentary layers, or cap rocks. When the confining pressure is too great, or when brittle-ductile rheological conditions predominate, vein formation occurs via crack-seal mechanisms. Crack-seal veins are thought to form quite quickly during deformation by precipitation of minerals within incipient fractures. This happens swiftly by geologic standards, because pressures and deformation mean that large open spaces cannot be maintained; generally the space is in the order of millimeters or micrometers. Veins grow in thickness by reopening of the vein fracture and progressive deposition of minerals on the growth surface. Veins generally need either hydraulic pressure in excess of hydrostatic pressure (to form hydraulic fractures or hydrofracture breccias) or they need open spaces or fractures, which requires a plane of extension within the rock mass.