Vein (geology)

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Vein (geology)

In geology

Geology

Rock (geology)

In 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....
, having a distinct shape, filled with crystal

Crystal

A 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....
s of one or more mineral

Mineral

A mineral is a naturally occurring solid formed through Geology processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties....
s, which were precipitated from an (aqueous

Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
) fluid

Fluid

A fluid is defined as a substance that continually deforms under an applied shear stress. All liquids and all gases are fluids. Fluids are a subset of the Phase and include liquids, gas, Plasma physics and, to some extent, plasticity ....
. Veins are formed by fluids carrying mineral constituents into a rock mass as a consequence of some form of hydraulic flow

Fluid dynamics

In physics, fluid dynamics is the sub-discipline of fluid mechanics dealing with fluid flow — the natural science of fluids in motion....
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 planar

Plane (mathematics)

In mathematics, a plane is a curvature 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....
fracture

Fracture

A 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....
s in rocks, with the crystal growth occurring normal

Perpendicular

In geometry, two line or plane , are considered perpendicular to each other if they form congruence adjacent angles angles . The term may be used as a noun or adjective....
to the walls of the cavity, and the crystal protruding into open space.

This certainly is the method for the formation of some veins.

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Encyclopedia

In geology

Geology

Rock (geology)

Crystal

Mineral

Water

Fluid

Fluid dynamics

Plane (mathematics)

Fracture

Perpendicular

Geology

Geology is the science and study of the solid and liquid matter that constitute the Earth. The field of geology encompasses the study of the composition, structural geology, physical properties, dynamics, and History of the Earth 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 stockwork

Stockwork

In geology, a stockwork is a complex system of Structural geology or randomly oriented Vein . Stockworks are common in many ore deposit types and especially notable in greisens. They are also referred to as stringer zones....
, in greisen

Greisen

Greisen is a highly altered granite rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of skarn.Greisens appear as highly metasomatism rocks, partly coarse, crystalline granite, partly vugh with miarolitic cavities, disseminated halide minerals such as fluorite, and occasionally metallic oxide and sulfi...
s or in certain skarn

Skarn

Skarn 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 rock s such as limestone or dolostone....
environments. For open space filling to take effect, the confining pressure is generally considered to be below 0.5 GPa

Pascal (unit)

The pascal is the SI derived unit of pressure, stress , Young's modulus and tensile strength. It is a measure of force per unit area i.e. equivalent to one newton per square meter or one joule per cubic meter....
, or less than 3-5 kilometres. Veins formed in this way may exhibit a colloform, agate

Agate

Agate is a microcrystalline variety of quartz , chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks....
-like habit, of sequential selvedges of minerals which radiate out from nucleation points

Nucleation

Nucleation is the onset of a crystal in a small region. The phase transition can be the formation of a bubble or of a crystal from a liquid. Creation of liquid droplets in saturated vapor or the creation of gaseous bubbles in a saturated liquid 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. Vug

Vug

Vugs are small to medium-sized cavities inside Rock that may be formed through a variety of processes. Most commonly cracks and fissures opened by tectonic activity are partially filled by quartz, calcite, and other secondary minerals....
s, cavities and geode

Geode

Geodes are Geology Rock formations which occur in Sedimentary rocks 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 fracturing

Hydraulic fracturing

Hydraulic fracturing is a method used to create Fracture 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 breccia

Breccia

Breccia is a rock composed of angular fragments of several minerals or rocks in a Matrix , that is a Cementation material, that may be similar or different in composition to the fragments....
which is filled with vein material. Such breccia vein systems may be quite extensive, and can form the shape of tabular dipping

Strike and dip

Strike 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....
sheets, diatreme

Diatreme

A diatreme is a Breccia filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone or a filled relatively shallow crater known as a maar or other volcanic pipes....
s or laterally extensive mantles controlled by boundaries such as thrust fault

Thrust fault

A thrust fault is a type of Geologic fault, or break in the Earth's crust with resulting movement of each side against the other, in which a lower stratigraphic position is pushed up and over another....
s, competent sediment

Sediment

Sediment is any particulate matter that can be sediment transport by fluid dynamics, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers....
ary layers

Stratum

In geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers....
, or cap rocks.

Crack-seal veins

When the confining pressure is too great, or when brittle

Brittle

A material is brittle if it is liable to fracture when subjected to stress . That is, it has little tendency to deform before fracture. This fracture absorbs relatively little energy, even in materials of high Strength of materials, and usually makes a snapping sound....
-ductile rheological

Rheid

In geology, a rheid is a solid material that deforms by viscosity flow. To be considered a rheid, deformation by flow should exceed Elasticity deformation by at least a factor of three....
conditions predominate, vein formation occurs via crack-seal mechanisms.

Crack-seal veins are thought to form quite quickly during deformation

Deformation

In materials science, deformation is a change in the shape or size of an object due to an applied force . This can be a result of tensile strength forces, compressive strength forces, Simple shear, bending or torsion ....
by precipitation

Precipitation (chemistry)

Precipitation 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 pressure

Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
s and deformation mean that large open spaces cannot be maintained; generally the space is in the order of millimetre

Millimetre

The millimetre is a Units of measurement of length in the metric system, equal to one thousandth of a metre, which is the current International System of Units SI base unit of length....
s or micrometre

Micrometre

A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
s. Veins grow in thickness by reopening of the vein fracture and progressive deposition

Deposition

Deposition or Depose may refer to:* Deposition , taking testimony outside of court* Deposition , molecules settling out of a solution* Thin-film deposition, any technique for depositing a thin film of material onto a substrate or onto previously deposited layers...
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

Extension

Extension may refer to:* A List of cheerleading stunts* The building of community capacity by outsiders, for instance agricultural extension* Extension , relating to the pulling apart of the Earth's crust and lithosphere...
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 compression

Compression

Compression may refer to:In physical science:*Physical compression, the result of the subjection of a material to compressive stress**Compression member, a class of structural elements, of which a column is the most common specific example...
al regimes, this can in turn give information on the stress

Stress (physics)

In continuum mechanics, stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces....
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 gold

Gold

Gold is a chemical element with the symbol Au and atomic number 79. It is a highly sought-after precious metal, having been used as money, as a store of value, in jewelry, in sculpture, and for ornamentation since the beginning of recorded history....
lode

Lode

In geology, a lode is a deposit of wikt: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 skarn

Skarn

Skarn 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 rock s such as limestone or dolostone....
mineralisation. Hydrofracture breccias are classic targets for ore

Ore

An ore is a type of Rock that contains minerals such as gemstones and 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 crystalline formations of metals suc...
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 rush

Gold rush

A gold rush is a period of feverish migration of workers into the area of a dramatic discovery of commercial quantities of gold.Eight gold rushes took place throughout the 19th century in Argentina, Australia, Brazil, Canada, Chile, New Zealand, 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 rock

Wall rock

Wall 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....
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 metasomatism

Metasomatism

Metasomatism 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 Metamorphism source....
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.

Vein (geology)

Open space filling

Crack-seal veins

Tectonic implications

Mineralisation and veining

Gold-bearing veins

See also