fault zone hydrogeology
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Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings, such as
clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
,
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
and carbonate rocks. Fluid movements, that can be quantified as permeability, can be facilitated or impeded due to the existence of a
fault zone In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
. This is because different mechanisms that deform rocks can alter porosity and permeability within a fault zone. Fluids involved in a fault system generally are
groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
(fresh and marine waters) and
hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
(Oil and Gas). *Please note that Permeability (k) and Hydraulic Conductivity (K) are used interchangeably in this article for simplified understanding__TOC__


Architecture

A fault zone can be generally subdivided into two major sections, including a Fault Core (FC) and a Damage Zone (DZ) (Figure 1). The fault core is surrounded by the damage zone. It has a measurable thickness which increases with fault throw and displacement, i.e. increasing deformations. The damage zone envelopes the fault core irregularly in a 3D manner which can be meters to few hundred meters wide (perpendicular to the fault zone). Within a large fault system, multiple fault cores and damage zones can be found. Younger fault cores and damage zones can overlap the older ones. Different processes can alter the permeability of the fault zone in the fault core and the damage zone will be discussed respectively in the next section. In general, the permeability of a damage zone is several orders of magnitude higher than that of a fault core as damage zones typically act as conduits (will be discussed in section 3). Within a damage zone, permeability decreases further away from a fault core.


Permeability classification

There are many classifications to group fault zones based on their permeability patterns. Some terms are interchangeable; while some have different subgroups. Most of expressions are listed in the following table for comparison. Dickerson's categorisation is commonly used and easier to understand in a broad range of studies. The classification of a fault zone can change spatially and temporally. The fault core and damage zone can behave differently to accommodate the deformations. Moreover, the fault zone can be dynamic through time. Thus, the permeability patterns can change for short-term and long-term effects. *K = Permeability/ Hydraulic Conductivity *fz = fault zone *hr = host rock = Undeformed rock surrounds the fault zones


Mechanisms (permeability)

Fault zone results from
brittle deformation In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
. Numerous mechanisms can vary the permeability of a fault zone. Some processes affect the permeability temporarily. These processes enhance the permeability for a certain period, and then reduce it later on: in this case, like seismic events, the permeability is not constant through time. Physical and chemical reactions are the major types of mechanisms. Different mechanisms can occur different in fault core and damage zone since the intensities of deformation they experience are different (Table 3). *+ = more likely to occur at


Enhancing fault zone permeability


Deformation bands

The formation of a dilation band, in unconsolidated material, is the early result of applying extensional forces. Disaggregation of mineral fabric occurs along with the band, yet no offset is accommodated by the movement of grains (Figure 3). Further deformation causes offsets of mineral grains by rotation and sliding. This is called a shear band. The pore network is rearranged by granular movements (also called particulate flow), hence moderately enhance permeability. However, continuing deformation leads to the
cataclasis A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as ''cataclasis''. Cataclasis involves the granulation, crushing, or milling of the ori ...
of mineral grains which will further reduce permeability later on (section 3.2.3) (Figure 4).


Brecciation

Breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
tion refers to the formation of angular, coarse-grained fragments embedded in a fine-grained
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
. As breccia (the rock experienced brecciation) is often non-cohesive, thus, permeability can be increased up to four or five orders of magnitude. However, the void space enlarged by brecciation will lead to further displacement along the fault zone by cementation, resulting in a strong permeability reduction (Figure 5).


Fracturing

Fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
s propagate along a fault zone in direction responding to the stress applied. The enhancement of permeability is controlled by the density, orientation, length distribution, aperture, and connectivity of fractures. Even fracture with aperture of a 100-250
μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
can still greatly influence fluid movement (Figure 6).


Reducing fault zone permeability


Sediment mixing

Sediments, typically from distinct formations, with different grain sizes, are mixed physically by deformation, resulting in a more poorly- sorted mixture. The pore space is filled by smaller grains, increasing tortuosity (mineral scale in this case) of fluid flow across the fault system.


Clay smears

Clay minerals are phyllo-silicate, in other words, with sheet-like structure. They are effective agents that block fluid flows across a fault zone. Clay smears, deformed layers of clay, that are developed along fault zone can act as a seal of hydrocarbon reservoir i.e. extremely low permeability that nearly prohibits all fluid flows (Figure 7).


Cataclasis

Cataclasis A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as ''cataclasis''. Cataclasis involves the granulation, crushing, or milling of the ori ...
refers to pervasive brittle fracturing and
comminution Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part of ...
of grains. This mechanism becomes dominant at depth, greater than 1 km, and with larger grains. With increasing intensity of cataclasis, fault gouge, often with the presence of clay, is formed. The largest reduction occurs on the flows that are perpendicular to the band.


Enhancing and reducing fault zone permeability successively


Compaction and cementation

Compactions and cementation generally lead to permeability reduction by losing porosity. When a large region, which consist a fault zone, experience compaction and cementation, porosity loss in host rock (undeformed rock surrounding the fault zone) can be greater than that of fault zone rock. Hence, fluids are forced to flow through a fault zone.


Dissolution and precipitation

Solute carried by fluids can either enhance or reduce permeability by
dissolution Dissolution may refer to: Arts and entertainment Books * ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * ''Dissolution'' (Sansom novel), a 2003 historical novel by C. J. Sansom Music * Dissolution, in mu ...
or
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
(cementation). Which process takes place depends on
geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
conditions like rock composition, solute concentration, temperature, and so on. The changes in porosity dominantly control whether the fluid-rock interaction continues or slows down as a strong feedback reaction. For example, minerals like carbonates,
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
, and
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
s are dissolved by the fluid-rock interactions due to enhanced permeability. Further introduction of fluids can either continuously dissolve or otherwise re-precipitate minerals in the fault core, and thus alters the permeability. Therefore, whether the feedback is positive or negative heavily depends on the geochemical conditions.


Seismic event

Earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s can either increase or decrease permeability along fault zones, depending on the hydrogeological settings. Recorded
hot spring A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by circ ...
discharges show
seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. S ...
s dominantly enhance permeability, but reductions in discharge may also result occasionally. The timescale of the changes can be up to thousands of years. Taking the Chile earthquake in 2017 as an example, the discharge of streamflow temporally increased six times indicates a sixfold enhancement in permeability along the fault zone. Yet, seismic-induced effects are temporary that normally last for months, for Chile's case, it lasted for one and a half months which gradually decreased back to original discharge.


Mechanisms (porosity)

Porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
(φ) directly reflects the
specific storage In the field of hydrogeology, ''storage properties'' are physical properties that characterize the capacity of an aquifer to release groundwater. These properties are storativity (S), specific storage (Ss) and specific yield (Sy). According to '' ...
of rock. And brittle formation alters the pores by different mechanisms. If the pores are deformed and connected together, the permeability of rock enhances. On the other hand, if the deformed pores disconnect with each other, the permeability of rock in this case reduces.


Pore types


Enhancing porosity


Dissolution

The mineral grains can be dissolved when there is fluid flow. The spaces originally occupied by the minerals will be spare as voids, increasing the porosity of rock. The minerals that are usually dissolved are
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
,
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
and
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
. Grain
dissolution Dissolution may refer to: Arts and entertainment Books * ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * ''Dissolution'' (Sansom novel), a 2003 historical novel by C. J. Sansom Music * Dissolution, in mu ...
pores results from this process can enhance porosity.


Reducing porosity


Cataclasis, fracturing and brecciation

The mineral grains are broken up into smaller pieces by faulting event. Those smaller fragments will re-organise and further be compacted to form smaller pore spaces. These processes create intragranular fracture pores and transgranular fracture pores. Important to be aware is that reducing porosity does not equal to reduction in permeability. Fracturing, bracciation and initial stage of
cataclasis A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as ''cataclasis''. Cataclasis involves the granulation, crushing, or milling of the ori ...
can connect pore spaces by cracks and dilation bands, increasing permeability.


Precipitation

The mineral grains can be precipitated when there is fluid flow. The voids in the rocks can be occupied by precipitation of mineral grains. The minerals fill the voids and hence, reducing the porosity. Overgrowth, precipitation around an existing mineral grain, of quartz are common. And overgrown minerals infill pre-existing pores, reducing porosity.


Clay deposition

Clay minerals are phyllo-silicate, in other words, with sheet-like structure. They are effective agents that block fluid flows.
Kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...
which is altered from potassium feldspar with the presence of water is a common mineral that fills pore spaces.
Precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
and
infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...
only affect materials on shallow depth, hence, more clay materials infill pore spaces when they are closer to the surface. Yet, development of a fault zone introduces fluid to flow deeper. Thus, this facilitates clay deposition at depth, reducing porosity.


Lithological effects

Lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
has a dominant effect on controlling which mechanisms would take place along a fault zone, hence, changing the porosity and permeability. *↑ = mechanism that enhance permeability *↓ = mechanism that reduce permeability


Fault type effects

All faults can be classified into three types. They are
normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
, reverse fault (thrust fault) and
strike-slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
. These different faulting behaviours accommodate the displacement in distinct structural ways. The differences in faulting motions might favour or disfavour certain permeability altering mechanisms to occur. However, the main controlling factor of the permeability is the rock type. Since the characteristics of rock control how a fault zone can be developed and how fluids can move. For instance, sandstone generally has a higher
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
than that of shale. A deformed sandstone in three different faulting systems should have a higher
specific storage In the field of hydrogeology, ''storage properties'' are physical properties that characterize the capacity of an aquifer to release groundwater. These properties are storativity (S), specific storage (Ss) and specific yield (Sy). According to '' ...
, hence permeability, than that of shale. Similar example like the
strength Strength may refer to: Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human ca ...
(resistance to deform) also significantly depends on rock types instead of fault types. Thus, the geological features of rock involved in a fault zone is a more dominated factor. On the other hand, the type of fault might not be a dominant factor but the intensity of deformation is. The higher intensity of stresses applied to the rock, the more intense the rock will be deformed. The rock will experience a greater permeability changing event. Thus, the amount of stress applied matters. Equally important is that identifying the permeability category of the fault zones (barriers, barrier-conduits and conduits) is the main scope of study. In other words, how the fault zones behave when there are fluids pass through.


Studying approaches and methods


Surface and subsurface test

The studies of fault zones are recognised in the discipline of
Structural Geology Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informatio ...
as it involves how rocks were deformed; while investigations of fluid movements are grouped in the field of
Hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...
. There are mainly two types of methods used to examine the fault zone by structural geologists and hydrologists (Figure 7). In situ test includes obtaining data from boreholes, drill cores, and tunnel projects. Normally the existence of a fault zone is found as different hydraulic properties are measured across it as fault zones are rarely drilled through (except for tunnel projects) (Figure 8). The hydraulic properties of rocks are either obtained directly from outcrop samples or shallow probe holes/
trial pit A trial pit (or test pit) is an excavation of ground in order to st ...
s, then the predictions of fault structure are made for the rocks at depth (Figure 8).


Example of a subsurface test

An example of a large-scale
aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
test conducted by Hadley (2020), the author used 5 wells aligned perpendicular to the Sandwich Fault Zone in the US, and the drawdowns as well as the recovery rates of water levels in every well were observed. From the evidence that the recovery rates are slower for the wells closer to the fault zone, it is suggested that the fault zone acts as a barrier for northward groundwater movement, affecting freshwater supply in the north.


Example of a surface test

From an outcrop study of the Zuccale Fault in Italy by Musumeci (2015), the surface outcrop findings and cross-cutting relationship are used to determine the number and mechanism of deformational events happened in the region. Moreover, the presences of breccias and cataclasites, that formed under brittle deformation, suggest that there was an initial stage of permeability increase, promoting an influx of -rich
hydrous In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
fluids. The fluids triggered low-grade metamorphism and dissolution-and-precipitation (i.e. pressure-solution) in mineral scale that shaped a foliated fault core, hence, enhancing the sealing effect significantly.


Other methods


Geophysics

Underground fluids, particularly groundwater, create anomalies for superconducting gravity data which help study the fault zone at depth. The method combines gravitational data and groundwater conditions to determine not only the permeability of a fault zone but also whether the fault zone is active or not.


Geochemistry

The
geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
conditions of mineral fluids, water or gases, can be used to determine the existence of a fault zone by comparing the geochemistry of fluids' source, given that the conditions of
aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
s are known. The fluids can be categorized by the concentrations of common solutes like total dissolved solids (TDS), Mg- Ca- Na/ K phase, SO4-
HCO3 In inorganic chemistry, bicarbonate (International Union of Pure and Applied Chemistry, IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a Polyatomic ion, polyatomic anion wi ...
- Cl phase, and other dissolved trace elements.


Existing biases

The selection of an appropriate studying approach(es) is essential as there are biases existed when determining the fault zone permeability structure. In crystalline rocks, the subsurface-focused investigations favor the discoveries of a conduit fault zone pattern; while the surface methods favor a combined barrier-conduit fault zone structure. The same biases, to a lesser extend, exist in
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
as well. The biases can be related to the differences in studying scale. For structural geologists, it is very difficult to conduct outcrop study over a vast region; likewise, for hydrologists, it is expensive and ineffective to shorten borehole intervals for testing.


Economic geology

It is economically worth studying the complex system, especially for arid/ semi-arid regions, where freshwater resources are limited, and potential areas with hydrocarbon storages. Further research on the fault zone, the result of deformation, gave insights on the interactions between earthquakes and hydrothermal fluids along fault zone. Moreover, hydrothermal fluids associated with the fault zone also provide information on how
ore deposits Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April 2 ...
accumulated.


Artificial hydrocarbon reservoirs

Carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in land ...
is a modern method dealing with atmospheric carbon. One of the methods is pumping atmospheric carbon to specific depleted oil and gas reservoirs at depth. However, the presence of a fault zone act as either a seal or a conduit, affecting the efficiency of hydrocarbon formation. Micro-fractures that cut along the sealing unit and the reservoir rock can greatly affect the hydrocarbon migration. The deformation band blocks the lateral (horizontal) flow of and the sealing unit keeps the from vertical migration (Gif 1). The propagation of a micro-fracture that cuts through a sealing unit, instead of having a deformation band within the sealing unit, facilitates upward migration (Gif 2). This allows fluid migrations from one reservoir to another. In this case, the deformation band still does not facilitate lateral (horizontal) fluid flow. This might lead to the loss of injected atmospheric carbon, lowering the efficiency of carbon sequestration. A fault zone that displaces sealing units and reservoir rocks can act as a conduit for hydrocarbon migration. The fault zone itself has higher storage capacity (specific capacity) than that of the reservoir rocks, therefore, before the migration to other units, the fault zone has to be fully filled (Gif 3). This can slower and concentrate the fluid migration. The fault zone facilitates vertical downwards movement of due to its
buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
and
piezometric head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
differences, i.e.
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
/
hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
is greater at a higher elevation, which helps store at depth.


Seismic-induced ore deposits

Regions that are or were seismic active and with the presence of fault zones might indicate there are
ore deposits Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April 2 ...
. A case study in Nevada, US by Howald (2015) studied how seismic-induced fluids accumulate
mineral deposits In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
, namely sinter and
gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
, along spaces provided by a fault zone. Two separate seismic events were identified and dated by oxygen isotopic concentrations, followed by episodes of the upward hydrothermal fluid migrations through permeable normal fault zone.
Mineralization Mineralization may refer to: * Mineralization (biology), when an inorganic substance precipitates in an organic matrix ** Biomineralization, a form of mineralization ** Mineralization of bone, an example of mineralization ** Mineralized tissues are ...
started to take place when these hot silica-rich hydrothermal fluids met the cool meteoric water
infiltrated Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can reduce or eliminate the need for subtractive processes in manufacturing, lowering material losses and r ...
along the fault zone until the convective flow system was shut down. In order to deposit minerals, seismic events that bring hydrothermal fluids are not the only dominant factor, the permeability of the fault zone also has to be sufficient for permitting fluid flows. Another example taken from Sheldon (2005) also shows that development of fault zone, in this case by strike-slip faulting, facilitates mineralization. Sudden dilation happened along with strike-slip events increases the
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
and permeability along the fault zone. Larger displacement will lead to greater increase in porosity. If the faulting event cut through a sealing unit which seals a confined aquifer of over-pressured fluids, the fluids can rise through the fault zone. Then mineralization will take place along the fault zone by pressure solution, reducing the porosity of the fault zone. The fluid flow channel along the fault zone will be shut down when the pores are almost occupied by newly precipitated ore minerals. Multiple seismic events have to be occurred to form these economic ore deposit with vein structure.


See also

*
Carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in land ...
*
Fault (geology) In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
* Fault breccia * Fault gouge *
Hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on th ...
*
Hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
*
Hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...
*
Hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquif ...
*
Petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
* Permeability *
Structural geology Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informatio ...


References

{{Reflist Hydrogeology Faults (geology) Porous media Soil mechanics