Veins
An Introduction to What Can Form Within Fractures
Figure 1: This photo shows tension gashes. Taken by Jim Talbot.
What Are Veins?
Definition of a Vein
Veins occur when cracks form within an existing rock. These cracks can be in the form of extensional fractures, which are fractures that extend perpendicularly to the two walls of the crack (Fossen, 2016). Within these fractures, fluid in or around the existing rock allows for the release of minerals. As the rock temperature cools these fluids precipitate and mineralize, forming veins. The minerals that can form as a result are numerous, but the most common is quartz and calcite. Although, other economically important minerals such as gold can also form as a result (Bons, 2000).
Figure 2: This is an example of en echelon veins running across the surface of a rock. Taken by Jim Talbot.
Vein Formation
Fluid
The first step in vein formation is the presence of a fluid. Fluids can transport dissolved materials that can later precipitate minerals. Fluids can either move materials through the rock or serve as a means for materials to pass through the rock without even flowing (Bons, 2000).
Pressure and Depth
The composition of fluid can depend on depth. Hydrothermal deposits occur when the fluid that allows precipitation to occur is hot. This is one of the most common types of deposits (Farooq)
Figure 3: This is a photo illustrating the mixture of hydrothermal fluids to help precipitate minerals such as gold, silver and copper. It accurately shows the source of the fluid as well as what raises its temperature, magma. Illustrated by earthsci.org.
There are three main types of hydrothermal fluids: epithermal, mesothermal, and hypothermal. The epithermal exists at the highest depths with the lowest pressure, while the hypothermal exist at the lowest depths with the highest pressures. The mesothermal is an intermediate area.
Figure 4: This image demonstrates the change in magma composition with depth.This includes several materials such as the minerals and fluids preset. We already see that the epithermal is labeled at around less than 1 km. The mesothermal makes up the remaining 2-5km of the diagram. Illustrated by earthsci.org.
Vein Morphology
Simply, morphology refers to how veins look because veins can appear in different shapes textures and sizes. Vein morphology can be used to group these veins which can further serve as a tool to identify formation conditions. There are many classifications of veins, but the sub-classifications of morphology that will be used in this section will include microscopic morphology and growth morphology. These terms were originally used by Doctor Paul D Bons, but their definitions have been modified and obtained from various sources.
Microscopic morphology
This section looks at the individual grains and can be described through texture, orientation, and shape.
Fibrous Veins
These minerals look longer and thinner than their blocky counterparts. They maintain a constant thickness and uniformity and do not usually undergo cracks in their formation (Fossen, 2016).
Figure 5: Fibrous Calcite Vein. Taken by Alex Strekeisen.
Blocky/SparryVeins
These vein materials usually have crystals that are of similar axial length, but vary in orientation. They are mosaic-like. These minerals are often random and don't exhibit a general uniform pattern but fill large areas within the fracture (Fossen, 2016).
Figure 6: Syntaxial Quartz Grains. Taken by Alex Strekeisen.
Elongated Blocky
These grains have a long axis that grows towards the direction of opening within veins and is perpendicular to its walls. They can sometimes be formed by the extension of cracks in the veins (Fossen, 2016)
Figure 7: Syntaxial Quartz Grain. Taken by Alex Strekeisen.
Growth Morphology
Many of the classifications used in the microscopic morphology can be used to further describe growth morphology. This section will describe the two classifications of vein growth.
Syntaxial
Definition
Syntaxial refers to the growth patterns of veins that result in older minerals being present near the wall of the vein and younger minerals near the center. These minerals are of the same mineral composition as the vein wall and precipitate inward from the vein wall towards the middle. Syntaxail veins extend minerals already present instead of forming new ones (Fossen, 2016)
Figure 8: Syntaxial Quartz Grain. Taken by Alex Strekeisen.
Syntaxial Median Plane
Where the precipitates from the vein wall meet is called the median line. The median line may be symmetrical or off center depending on the growth rate of the minerals from the wall. There may even be times when no median line is present because minerals only precipitated from one wall (Fossen, 2016)
Figure 9: Syntaxial Quartz Grain. Taken by Alex Strekeisen.
Syntaxial Formation
Syntaxial veins form either during a singular event or can form by several events. This is referred to as crack seal mechanism because it involves continuous fracturing and filling of those cracks, called healing, within the vein.The healing process, however, can be slower than the fracturing process leaving cracks unfilled (Fossen, 2016)
Figure 10: Syntaxial Quartz Grain. Taken by Alex Strekeisen.
Antitaxial Veins
Definiton
Veins are defined as antitaxial when minerals grow in such a way that the oldest minerals are in the middle and the youngest are on the vein wall. Antitaxial veins do not possess the same minerals as the vein walls (Fossen, 2016)
Figure 11: Antitaxial Quartz Vein. Taken by Alex Strekeisen.
Antitaxial Median Plane
While antitaxial median planes are in the middle just like syntaxial veins, they function differently. The median plane is not formed by the joining of two minerals ,in fact, the vein minerals grow from the median plane. It is not a line like in syntaxial veins, however it is usually symmetrical (Fossen, 2016)
Figure 12: Antitaxial Quartz Vein that exhibits a median plane. Taken by Alex Strekeisen.
Antitaxial Formation
Antitaxial veins grow from their median plane. The median plane is usually a vein that does not resemble the rest of the veins and may be filled with impurities from the wall rock. The minerals, however, precipitate from the median line to form thin fibers that are, again, usually uniform and have few impurities. Because the minerals grow from the middle they usually have a constant growth rate adding symmetry to the median plane. (Fossen, 2016).
Figure 13: Antitaxial Quartz Vein. Taken by Alex Strekeisen.
What do the Formation of Veins tell us?
Kinematic Indicators
En Echelon Tension Gashes
These are a type of vein formation that form to look like a stair step pattern. These show the direction of spreading within a rock. One can find this by placing a strain ellipse to find the direction of elongation. Tension gashes themselves are formed when there is resistance to a force by the axis of stress (Fossen, 2016)
Figure 14: En Echelon Gashes. Taken by Jim Talbot
Other Indicators
Precipitates: The mineral that precipitates, when antitaxial, can tell us about the fluid that flowed through the rock.
Morphology: Can indicate growth direcrection. Also median plane can indicate the rate of growth and potentially the environment that growth took place in (Fossen, 2016)
Figure 15: Tension Gashes on an overturned bed. Taken by Jim Talbot.
References
- Photos:
- Strekeisen, Alex. Veins. 2007. https://www.alexstrekeisen.it/english/meta/veins.php. Copyrighted. Granted Permission. (Used for figures 5-13)
- Talbot, Jim. Tension Crack Veins. http://myweb.facstaff.wwu.edu/talbot/cdgeol/Structure/Veins/Veins.html. Permission granted. (Used for Figures 1-2, 14-15)
- “Vein Deposits .” VEIN DEPOSITS, Earth Science Australia, http://earthsci.org/mineral/mindep/vein/vein.html. Granted Permission. (Used for figures 3-4)
- Academic
- Bons, P.D. 2000. The Formation of veins and their microstructures. In: Stress, Strain and Structure. A volume in honor of W D Means. Eds: M.W. Jessell and J.L. Urai. Volume 2, Journal of the Virtual Explorer.
- Farooq, S. “Hydrothermal and Skarn Deposits.” Department of Geology Aligarh Muslim University, http://www.geol-amu.org/notes/b3-3-4.htm.
- Fossen, Haakon. Structural Geology. Cambridge University Press, 2016.
- “Vein Deposits .” VEIN DEPOSITS, Earth Science Australia, http://earthsci.org/mineral/mindep/vein/vein.html.