In
geologyGeology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed...
, a
vein is a finite volume within a
rockIn geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
, having a distinct shape, filled with
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
s of one or more
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
s, which were precipitated from an (
aqueousWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
)
fluidA fluid is a substance that continually deforms under an applied shear stress. All gases are fluids, but not all liquids are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
.
Veins are formed by fluids carrying mineral constituents into a rock mass as a consequence of some form of
hydraulic flowIn physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
within the rock. Usually this is the result of hydrothermal circulation.
Veins are classically thought of as being the result of growth of crystals on the walls of
planarIn mathematics, a plane is a flat surface. Planes can arise as subspaces of some higher dimensional space, as with the walls of a room, or they may enjoy an independent existence in their own right, as in the setting of Euclidean geometry....
fractureA fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures, or to crystals or crystalline materials, such as gemstones or metal. Sometimes, in crystalline materials, individual crystals...
s in rocks, with the crystal growth occurring
normalIn geometry, two lines or planes , are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective. Thus, referring to Figure 1, the line AB is the perpendicular to CD through the point B...
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
geologyGeology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed...
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
Open space filling is the hallmark of epithermal vein systems, such as a
stockworkIn geology, a stockwork is a complex system of structurally controlled or randomly oriented veins. Stockworks are common in many ore deposit types and especially notable in greisens. They are also referred to as stringer zones....
, in
greisenGreisen is a highly altered granitic rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn.Greisens appear as highly altered rocks, partly coarse, crystalline granite, partly vuggy with miarolitic cavities, disseminated halide minerals such as...
s or in certain
skarnSkarn is a metamorphic rock that is usually variably colored green or red, occasionally grey, black, brown or white.It usually forms by chemical metasomatism of rocks during metamorphism and in the contact zone of magmatic intrusions like granites with carbonate-rich rocks such as limestone or...
environments. For open space filling to take effect, the confining pressure is generally considered to be below 0.5
GPaThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
, or less than 3-5 kilometres. Veins formed in this way may exhibit a colloform,
agateAgate is a microcrystalline variety of quartz , chiefly chalcedony, characterised by its fineness of grain and brightness of color...
-like habit, of sequential selvedges of minerals which radiate out from
nucleation pointsNucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form via nucleation in liquids are gaseous bubbles, crystals, or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
on the vein walls and appear to fill up the available open space. Often evidence of fluid boiling is present.
Vugthumb|300px|right|In the lower left, a spherule, or sphere-shaped grain, can be seen penetrating the interior of a small cavity called a vug.Vugs are small to medium-sized cavities inside rock that may be formed through a variety of processes...
s, cavities and
geodeGeodes are geological rock formations which occur in sedimentary and certain volcanic rocks. Geodes are essentially rock cavities or vugs with internal crystal formations or concentric banding...
s are all examples of open-space filling phenomenon in hydrothermal systems.
Alternatively,
hydraulic fracturingHydraulic fracturing is a method used to create fractures that extend from a borehole into rock formations, which are typically maintained by a proppant, a material such as grains of sand or other material which prevent the fractures from closing...
may create a
brecciaBreccia is a rock composed of angular fragments of minerals or rocks in a matrix , that may be similar or different in composition to the fragments. The word is a loan from italian, and in that language indicates both loose gravel and stone made by cemented gravel...
which is filled with vein material. Such breccia vein systems may be quite extensive, and can form the shape of tabular
dippingStrike and dip refer to the orientation or attitude of a geologic feature. The strike of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map this is represented with a short straight line segment oriented parallel...
sheets,
diatremeA diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes...
s or laterally extensive mantles controlled by boundaries such as
thrust faultA thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger. Thrust faults are the result...
s, competent
sedimentSediment is any particulate matter that can be transported by fluid flow, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers...
ary
layersIn geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over...
, or cap rocks.
Crack-seal veins
When the confining pressure is too great, or when
brittleA material is brittle if it is liable to fracture when subjected to stress. That is, it has little tendency to deform before fracture...
-ductile
rheologicalIn geology, a rheid is a solid material that deforms by viscous flow. To be considered a rheid, deformation by flow should exceed elastic deformation by at least a factor of three.-Types of rheids:...
conditions predominate, vein formation occurs via crack-seal mechanisms.
Crack-seal veins are thought to form quite quickly during deformation by
precipitationPrecipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate...
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 millimetres or
micrometreA micrometre or micron is one millionth of a metre,or equivalently one thousandth of a millimetre....
s. Veins grow in thickness by reopening of the vein fracture and progressive
depositionIn chemistry, deposition is the settling of particles or sediment from a solution, suspension and mixture or vapor onto a pre-existing surface...
of minerals on the growth surface.
Tectonic implications
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.
In all cases except brecciation, therefore, a vein measures the plane of extension within the rock mass, give or take a sizeable bit of error. Measurement of enough veins will statistically form a plane of principal extension.
In ductilely deforming compressional regimes, this can in turn give information on the
stressIn continuum mechanics, the concept of stress, introduced by Cauchy around 1822, is a measure of the average amount of force exerted per unit area of a surface within a deformable body on which internal forces act...
es active at the time of vein formation. In extensionally deforming regimes, the veins occur roughly normal to the axis of extension.
Mineralisation and veining
Veins are of prime importance to mineral deposits, because they are the source of mineralisation either in or proximal to the veins. Typical examples include
goldGold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
lodeIn geology, a lode is a deposit of metalliferous ore that fills or is embedded in a fissure in a rock formation or a vein of ore that is deposited or embedded between layers of rock....
s, as well as
skarnSkarn is a metamorphic rock that is usually variably colored green or red, occasionally grey, black, brown or white.It usually forms by chemical metasomatism of rocks during metamorphism and in the contact zone of magmatic intrusions like granites with carbonate-rich rocks such as limestone or...
mineralisation. Hydrofracture breccias are classic targets for
oreAn ore is a type of rock that contains minerals such as gemstones or metals that can be extracted through mining and refined for use. Samples of ore in the form of exceptionally beautiful crystals, exotic layering visible when sectioned or polished or metallic presentations such as large nuggets or...
exploration as there is plenty of fluid flow and open space to deposit ore minerals.
Ores related to hydrothermal mineralisation which are associated with vein material may be composed of vein material and/or the rock in which the vein is hosted.
Gold-bearing veins
In many of the gold mines exploited during the
gold rushA gold rush is a period of feverish migration of workers into the area of a dramatic discovery of commercial quantities of gold.Gold rushes took place in the 19th century in Australia, Brazil, Canada, South Africa, and the United States....
es of the 19th century, vein material alone was typically sought as ore material. In most modern mines, ore material is primarily composed of the veins and some component of the
wall rockWall rock is the rock that constitutes the wall of an area undergoing geologic activity. Examples are the rock along the neck of a volcano, on the edge of a pluton that is being emplaced, along a fault plane, enclosing a mineral deposit, or where a vein or dike is being emplaced.In volcanoes, wall...
s which surrounds the veins.
The difference between 19th century and modern mining techniques and the type of ore sought is based on the grade of material being mined and the methods of mining which are used. Historically, hand-mining of gold ores permitted the miners to pick out the lode quartz or reef quartz, allowing the highest-grade portions of the lodes to be worked, without dilution from the unmineralised wall rocks.
Modern mining using larger machinery and equipment forces the miners to take low-grade waste rock in with the ore material, resulting in dilution of the grade.
However, modern mining and assaying allows the delineation of lower-grade bulk tonnage mineralisation, within which the gold is invisible to the naked eye. In these cases, veining is the subordinate host to mineralisation and may only be an indicator of the presence of
metasomatismMetasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
of the wall-rocks which contains the low-grade mineralisation.
For this reason, veins within hydrothermal gold deposits are no longer the exclusive target of mining, and in some cases gold mineralization is restricted entirely to the altered wall rocks within which entirely barren quartz veins are hosted.