Sedimentary exhalative deposits
Encyclopedia
Sedimentary exhalative deposits (SedEx deposits) are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir (usually the ocean
), resulting in the precipitation of stratiform
ore.
SedEx deposits are the most important source of lead
, zinc
and barite
, a major contributor of silver
, copper
, gold
, bismuth
and tungsten
.
SedEx deposits are distinctive in that it can be shown that the ore mineral
s were deposited in a marine second-order basin environment, related to discharge of metal
-bearing brine
s into the seawater. This is distinct from other Pb-Zn-Ag and other deposits which are more intimately associated with intrusive or metamorphic processes or which are trapped within a rock
matrix and are not exhalative.
of SedEx mineralisation is varied, depending on the type of ore which is deposited by sedimentary exhalative processes.
Alternately, SedEx deposits may be sourced from magmatic fluids from subseafloor magma chamber
s and hydrothermal fluids generated by the heat of a magma
chamber intruding into saturated sediments. This scenario is relevant to mid-ocean ridge
environments and island arc
volcanic chains where black smokers are formed by discharging hydrothermal fluids.
are precipitated onto the seafloor to form an orebody and mineralization halo which are congruent with the underlying stratigraphy and are generally fine grained, finely laminated and can be recognized as chemically deposited from solution.
Arkose
-hosted SedEx deposits are known in some cases, associated with arkosic strata adjacent to faults which feed heavy brines into the porous sand
s, filling the matrix with sulfides, or deposited within a predominantly arkosic layer as a distinct chemical sediment layer usually associated with a shale interbed or at the lowermost levels of a shale formation directly overlying arkosic sands (for example, copper deposits near Maun
, Botswana
).
Occasionally, mineralization is developed in faults and feeder conduits which fed the mineralizing system. For instance, the Sullivan orebody in south-eastern British Columbia
was developed within an interformational diatreme
, caused by overpressuring of a lower sedimentary unit and eruption of the fluids through another unit en route to the seafloor.
Within disturbed and tectonized sequences, SedEx mineralization behaves similarly to other massive sulfide deposits, being a low-competence low shear strength
layer within more rigid silicate sedimentary rocks. As such, boudin
age structures, dikes
of sulfides, vein sulfides and hydrothermally remobilized and enriched portions or peripheries of SedEx deposits are individually known from amongst the various examples worldwide.
However, other forms of SedEx mineralization are known;
Deep formational brines are defined as saline
to hypersaline waters which are produced from sediments during diagenesis
.
Metals such as lead and copper and zinc are found in a trace amount in all sediments. These metals are bound weakly to the hydrous clay minerals on the edges of the crystals and are held by weak bonds with hydroxyl groups. Zinc is found within carbonate
minerals bound within the carbonate crystal lattice at vertices and along crystal twin planes and crystal boundaries. These metals enter the sedimentary minerals due to adsorption
from the seawater which deposited them; few freshwater
sediments are considered to have as much metal carrying capacity as saline waters.
Salt
is also bound within the matrix of the sediments, generally in pore waters, trapped during deposition. In a typical mud
on the seafloor up to 90% of the sediment volume and mass is represented by hydrogen and oxygen either trapped in pore space as water or attached to phyllite minerals (clays) as hydroxyl bonds.
During diagenesis
, pore water is squeezed out of the sediments and, as burial continues and heat
increases, water is liberated from clay minerals as the peripheral hydroxyl bonds are broken. As the rock enters the submetamorphic field, generally Zeolite facies
metamorphism
, clay minerals begin to recrystallize into low-temperature metamorphic phyllite minerals such as chlorite
, prehnite
, pumpellyite
, glauconite
and so forth. This liberates not only water but incompatible elements attached to the mineral and trapped within crystal lattices.
Metals liberated from clay and carbonate minerals as they are changed from clays and low-pressure disordered carbonate forms enters the remaining pore fluid which by this time has become concentrated into what is known as a deep formation brine. The solution of metal, salts and water produced by diagenesis is produced at temperatures between 150 - 350°C
. Hydrothermal fluid compositions are estimated to have a salinity of up to 35% NaCl with metal concentrations of 5-15 ppm
Zn, Cu, Pb and up to 100ppm Ba and Fe. High metal concentrations are able to be carried in solution because of the high salinity. Generally these formational brines also carry considerable sulfur.
s, quartz
and carbonate
veining
and pervasive ankerite
-siderite
-chlorite
-sericite
alteration.
Fluids eventually discharge onto the seafloor, forming areally extensive, stratiform deposits of chemical precipitates. Discharge zones can be breccia diatreme
s, or simple fumarole
conduits. Black smoker chimneys are also common, as are seepage mounds of chert
, jaspilite and sulfides.
In the majority of cases the overprint of metamorphism
and faulting, generally thrust fault
ing, deforms and disturbs the sediments and obscures sedimentary features, although this is generally patchy so that the original configuration will be seen within the deposit.
Most deposits fit the model of having been formed late in the basin history and in most cases feeder systems and metal zonation support exhalative models. However, in the case of diatreme
related deposits, such as the giant low-grade Abra deposit, the mineralization is intra-formational, lacks sedimentary textures (is epigenetic and replacement type) and is too low in the basin profile (ie; in the basal formation).
Following the discovery of hydrothermal vent
s, deposits similar to those of oceanic vents and fossilized vent life forms have been found in some SedEx deposits.
The ore genesis of the Sullivan ore body is summarized by the following process:
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
), resulting in the precipitation of stratiform
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
ore.
SedEx deposits are the most important source of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
and barite
Barite
Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
, a major contributor of silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
and tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
.
Classification
The palaeoenvironmental setting and palaeogeologic setting of these ore deposits sets them apart from other lead, zinc or tungsten deposits which generally do not share the same source or trap morphologies as SedEx deposits.SedEx deposits are distinctive in that it can be shown that the ore mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s were deposited in a marine second-order basin environment, related to discharge of metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
-bearing brine
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...
s into the seawater. This is distinct from other Pb-Zn-Ag and other deposits which are more intimately associated with intrusive or metamorphic processes or which are trapped within a rock
Rock (geology)
In geology, rock or stone 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...
matrix and are not exhalative.
Genetic model
The process of ore genesisOre genesis
The various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore genesis theories are very dependent on the mineral or commodity....
of SedEx mineralisation is varied, depending on the type of ore which is deposited by sedimentary exhalative processes.
- Source of metals is sedimentary strata which carry metal ionIonAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s trapped within clayClayClay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
and phyllosilicate minerals and electrochemically adsorbed to their surfaces. During diagenesisDiagenesisIn geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
, the sedimenary pile dehydrates in response to heat and pressure, liberating a highly saline formational brineBrineBrine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...
, which carries the metal ions within the solution.
Alternately, SedEx deposits may be sourced from magmatic fluids from subseafloor magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
s and hydrothermal fluids generated by the heat of a magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
chamber intruding into saturated sediments. This scenario is relevant to mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
environments and island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
volcanic chains where black smokers are formed by discharging hydrothermal fluids.
- Transport of these brines follows stratigraphic reservoir pathways toward faults, which isolate the buried stratigraphy into recognisable sedimentary basinSedimentary basinThe term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
s. The brines percolate up the basin bounding faults and are released into the overlying oceanic water.
- Trap sites are lower or depressed areas of the ocean topographyTopographyTopography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
where the heavy, hot brines flow and mix with cooler sea water, causing the dissolved metal and sulfurSulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
in the brine to precipitate from solution as a solid metal sulfideSulfideA sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
ore, deposited as layers of sulfide sediment.
Morphology
Upon mixing of the ore fluids with the seawater, dispersed across the seafloor, the ore constituents and gangueTailings
Tailings, also called mine dumps, slimes, tails, leach residue, or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction of an ore...
are precipitated onto the seafloor to form an orebody and mineralization halo which are congruent with the underlying stratigraphy and are generally fine grained, finely laminated and can be recognized as chemically deposited from solution.
Arkose
Arkose
Arkose is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose....
-hosted SedEx deposits are known in some cases, associated with arkosic strata adjacent to faults which feed heavy brines into the porous sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
s, filling the matrix with sulfides, or deposited within a predominantly arkosic layer as a distinct chemical sediment layer usually associated with a shale interbed or at the lowermost levels of a shale formation directly overlying arkosic sands (for example, copper deposits near Maun
Maun
Maun is the fifth largest town in Botswana. As of 2001, it had a population of 43,776. It is an eclectic mix of modern buildings and native huts. Maun is the "tourism capital" of Botswana and the administrative centre of Ngamiland district...
, Botswana
Botswana
Botswana, officially the Republic of Botswana , is a landlocked country located in Southern Africa. The citizens are referred to as "Batswana" . Formerly the British protectorate of Bechuanaland, Botswana adopted its new name after becoming independent within the Commonwealth on 30 September 1966...
).
Occasionally, mineralization is developed in faults and feeder conduits which fed the mineralizing system. For instance, the Sullivan orebody in south-eastern British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
was developed within an interformational 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, a filled relatively shallow crater known as a maar, or other volcanic pipes.- Word origin :...
, caused by overpressuring of a lower sedimentary unit and eruption of the fluids through another unit en route to the seafloor.
Within disturbed and tectonized sequences, SedEx mineralization behaves similarly to other massive sulfide deposits, being a low-competence low shear strength
Shear strength
Shear strength in engineering is a term used to describe the strength of a material or component against the type of yield or structural failure where the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is...
layer within more rigid silicate sedimentary rocks. As such, boudin
Boudin
Boudin describes a number of different types of sausage used in French, Belgian, German, French Canadian, Creole and Cajun cuisine.-Types:*Boudin blanc: A white sausage made of pork without the blood. Pork liver and heart meat are typically included...
age structures, dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
of sulfides, vein sulfides and hydrothermally remobilized and enriched portions or peripheries of SedEx deposits are individually known from amongst the various examples worldwide.
Mineralization types
SedEx mineralization is best known in lead-zinc ore deposit classification schemes as the vast majority of the largest and most important deposits of this type are formed by sedimentary-exhalative processes.However, other forms of SedEx mineralization are known;
- The supergiant deposits of the ZambiaZambiaZambia , officially the Republic of Zambia, is a landlocked country in Southern Africa. The neighbouring countries are the Democratic Republic of the Congo to the north, Tanzania to the north-east, Malawi to the east, Mozambique, Zimbabwe, Botswana and Namibia to the south, and Angola to the west....
n Copperbelt are considered to be SedEx-style copper mineralization formed at arkose-shaleShaleShale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
interfaces within sedimentary sequences. Within the Botswanan extent of the Damaran Supergroup, the SedEx nature is confirmed by chemical sediment limestoneLimestoneLimestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s. - The vast majority of the world's barite deposits are considered to have been formed by SedEx mineralization processes
- The scheeliteScheeliteScheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten. Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws...
(tungsten) deposits of the Erzgebirge in the Czech RepublicCzech RepublicThe Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
are considered to be formed by SedEx processes - Some goldGoldGold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
associated with Carlin-type depositsCarlin–type gold depositCarlin–type gold deposits are sediment-hosted disseminated gold deposits. These deposits are characterized by invisible gold in pyrite and arsenopyrite...
of NevadaNevadaNevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
is interpreted to be stratiform chertChertChert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
or spillite formed by SedEx processes on the seafloor. This concept is controversial because most gold is clearly of later epigenetic origin.
Metal sources
The source of metals and mineralizing solutions for SedEx deposits is deep formational brines in contact with sedimentary rocks.Deep formational brines are defined as saline
Saline
Saline may refer to:* Salinity, the salt content of a solution** Saline water, water containing significant concentration of salts* Soil salinity, salt content of soil* Saline , a liquid with salt content to match the human body...
to hypersaline waters which are produced from sediments during diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
.
Metals such as lead and copper and zinc are found in a trace amount in all sediments. These metals are bound weakly to the hydrous clay minerals on the edges of the crystals and are held by weak bonds with hydroxyl groups. Zinc is found within carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
minerals bound within the carbonate crystal lattice at vertices and along crystal twin planes and crystal boundaries. These metals enter the sedimentary minerals due to adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
from the seawater which deposited them; few freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
sediments are considered to have as much metal carrying capacity as saline waters.
Salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
is also bound within the matrix of the sediments, generally in pore waters, trapped during deposition. In a typical mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
on the seafloor up to 90% of the sediment volume and mass is represented by hydrogen and oxygen either trapped in pore space as water or attached to phyllite minerals (clays) as hydroxyl bonds.
During diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
, pore water is squeezed out of the sediments and, as burial continues and heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
increases, water is liberated from clay minerals as the peripheral hydroxyl bonds are broken. As the rock enters the submetamorphic field, generally Zeolite facies
Zeolite facies
Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.The zeolite facies is generally considered to be transitional between diagenetic processes which turn sediments into sedimentary rocks, and prehnite-pumpellyite facies,...
metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
, clay minerals begin to recrystallize into low-temperature metamorphic phyllite minerals such as chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
, prehnite
Prehnite
Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most oftens forms as stalactitic or botryoidal aggregates, with only just the crests of small...
, pumpellyite
Pumpellyite
Pumpellyite is a group of closely related sorosilicate minerals:*pumpellyite-: Ca2MgAl2[2|SiO4|Si2O7]·*pumpellyite-: Ca2Fe2+Al2[2|SiO4|Si2O7]·*pumpellyite-: Ca22[2|SiO4|Si2O7]·H2O...
, glauconite
Glauconite
Glauconite is an iron potassium phyllosilicate mineral of characteristic green color with very low weathering resistance and very friable.It crystallizes with a monoclinic geometry...
and so forth. This liberates not only water but incompatible elements attached to the mineral and trapped within crystal lattices.
Metals liberated from clay and carbonate minerals as they are changed from clays and low-pressure disordered carbonate forms enters the remaining pore fluid which by this time has become concentrated into what is known as a deep formation brine. The solution of metal, salts and water produced by diagenesis is produced at temperatures between 150 - 350°C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
. Hydrothermal fluid compositions are estimated to have a salinity of up to 35% NaCl with metal concentrations of 5-15 ppm
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
Zn, Cu, Pb and up to 100ppm Ba and Fe. High metal concentrations are able to be carried in solution because of the high salinity. Generally these formational brines also carry considerable sulfur.
Deposition
The mineralizing fluids are conducted upwards within sedimentary units toward basin-bounding faults. The fluids move upwards due to thermal ascent and pressure of the underlying reservoir. Faults which host the hydrothermal flow can show evidence of this flow due to development of massive sulfide veins, hydrothermal brecciaBreccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
veining
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...
and pervasive ankerite
Ankerite
Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with formula: Ca2. In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varying amounts of iron and manganese.The crystallographic and...
-siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
-chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
-sericite
Sericite
Sericite is a fine grained mica, similar to muscovite, illite, or paragonite. Sericite is a common alteration mineral of orthoclase or plagioclase feldspars in areas that have been subjected to hydrothermal alteration typically associated with copper, tin, or other hydrothermal ore deposits...
alteration.
Fluids eventually discharge onto the seafloor, forming areally extensive, stratiform deposits of chemical precipitates. Discharge zones can be breccia 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, a filled relatively shallow crater known as a maar, or other volcanic pipes.- Word origin :...
s, or simple fumarole
Fumarole
A fumarole is an opening in a planet's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from...
conduits. Black smoker chimneys are also common, as are seepage mounds of chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
, jaspilite and sulfides.
Problems of classification
One of the major problems in classifying SedEx deposits is in identifying whether or not the ore was definitively exhaled into the ocean and whether the source was formational brines from sedimentary basins.In the majority of cases the overprint of metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
and faulting, generally thrust fault
Thrust fault
A 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...
ing, deforms and disturbs the sediments and obscures sedimentary features, although this is generally patchy so that the original configuration will be seen within the deposit.
Most deposits fit the model of having been formed late in the basin history and in most cases feeder systems and metal zonation support exhalative models. However, in the case of 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, a filled relatively shallow crater known as a maar, or other volcanic pipes.- Word origin :...
related deposits, such as the giant low-grade Abra deposit, the mineralization is intra-formational, lacks sedimentary textures (is epigenetic and replacement type) and is too low in the basin profile (ie; in the basal formation).
Following the discovery of hydrothermal vent
Hydrothermal vent
A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...
s, deposits similar to those of oceanic vents and fossilized vent life forms have been found in some SedEx deposits.
Sullivan Pb-Zn mine
The Sullivan Pb-Zn mine in British Columbia was worked for 105 years and produced 16,000,000 tonnes of lead and zinc, as well as 9,000 tonnes of silver. It was Canada's longest lived continuous mining operation and produced metals worth over $20 billion in terms of 2005 metal prices. Grading was in excess of 5% Pb and 6% Zn.The ore genesis of the Sullivan ore body is summarized by the following process:
- Sediments were deposited in an extensional second-order sedimentary basinSedimentary basinThe term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
during extension - Earlier, deeply buried sediments devolved fluids into a deep reservoir of sandy siltstoneSiltstoneSiltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
s and sandstoneSandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s - IntrusionIntrusionAn intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
of dolerite sillsSill (geology)In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
into the sedimentary basin raised the geothermal gradientGeothermal gradientGeothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is 25–30°C per km of depth in most of the world. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied...
locally - Raised temperatures prompted overpressuring of the lower sedimentary reservoir which breached overlying sediments, forming a breccia diatremeDiatremeA 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.- Word origin :...
- Mineralizing fluid flowed upwards through the concave feeder zone of the breccia diatreme, discharging onto the seafloor
- Ore fluids debouched onto the seafloor and pooled in a second-order sub-basin's depocentre, precipitating a stratiform massive sulfide layer from 3 to 8 m thick, with exhalative chertChertChert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
, manganeseManganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
and bariteBariteBaryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
.