Certain accessory or uncommon constituents of magmas become enriched into bodies of sufficient size and richness to constitute valuable mineral deposits eg. Chromite and platinum. Magmatic ore deposits are characterized by their close relationship with intermediate or deep seated intrusive igneous rocks. They themselves are igneous rocks whose composition happens to be of particular value to man. They constitute either the whole igneous mass or a part of it, or may form offset bodies. They are magmatic products that crystallize from magmas.
They are also called Magmatic Segregations, Magmatic Injections, or Igneous Syngenetic Deposits.
Mode of Formation:
There are several modes of formation of magmatic deposits. They originate during different periods of magma crystallization – in some the ore minerals crystallize early, in others late, and in still others they remained as immiscible liquids until after crystallization of the host rock.
Classification of Magmatic Deposits:
I. Early Magmatic Deposits: Those which resulted from straight magmatic processes (orthotectic and orthomagmatic).
A. Dissemination
Deep seated crystallization will yield a granular rock in which the early formed crystals are disseminated. If such crystals are valuable and abundant, the whole rock or a part thereof becomes the orebody. The individual crystals may be phenocrysts eg.
B. Segregation
Concentration of early formed crystals in-situ. These are early concentrates of valuable constituents of the magma that have taken place as a result of gravitative crystallization differentiation, eg. Chromite. These orebodies are generally lenticular and small in size, commonly disconnected pod shaped lenses, stringers or eg.
II. Late Magmatic Deposits: Those which consist of minerals crystallizing from a magma towards the close of magmatic period. The ore minerals are later than the rock silicates and cut across them, embay them, and yield reaction rims around earlier minerals. They are always associated with mafic igneous rocks.
A. Residual Liquid Segregation
In certain mafic magmas, the residual liquid becomes enriched in iron, titanium and volatiles. This liquid settles to the bottom of the magma chamber, or crystallizes in the interstices of early formed crystals. Examples: Titaniferous magnetite layers of the Bushveld Igneous Complex, S. Africa.
B. Residual Liquid Injection
The iron-rich residual liquid accumulated in the above manner may be subjected to movement because of:
a. Gentle tilting (causing lateral movement).
b. Pressure and be squirted out to places of lesser pressure.
In both cases it may be injected into adjacent rocks and even in the earlier consolidated parent silicate mass. Examples: Titanomagnetite Deposits, Adirondack Region, New York; Allard Lake Deposits; Magnetite Deposits of Kiruna, Sweden.
C. Immiscible Liquid Segregation
Sulfide-rich magmas are immiscible in silicate rich magmas. This gives rise to separation even before crystallization. The accumulated sulfide may not necessarily be pure – in fact it quite often is an enrichment of sulfides in the lower parts of the magma. Deposits formed in this manner are pyrrhotite-chalcopyrite-pentlandite nickel-copper ores confined to rocks of the gabbro family. Examples: Ni-Cu Deposits of Insizwa, S. Africa; Nickeliferous Sulfide Deposits of Bushveld, S. Africa & Norway; Nickel Sulfide Deposits of Sudbury, Ontario.
D. Immiscible Liquid Injection
Examples: Vlackfontein Mine of S. Africa; Nickel Deposits of Norway.
Association of Rocks and Mineral Products:
Characteristics of different rock types:
Peridotite: A coarse grained mafic igneous rock composed of olivine with small amounts of pyroxene and amphibole.
Anorthosite: A plutonic rock composed mainly of Ca-rich plagioclase feldspars.
Gabbro: A black, coarse grained intrusive igneous rock, composed of calcic plagioclases and pyroxenes. The intrusive equivalent of basalt.
Syenite: A group of plutonic rocks containing alkali feldspars, a small amount of plagioclase, one or more mafic minerals, and quartz only as an accessory, if at all. The intrusive equivalent of trachyte.
Kimberlite: A peridotite that contains garnet and olivine and is found in volcanic pipes.
Pegmatite: An igneous rock with extremely large grains (> 1 cm in dia). It may be of any composition, but is most frequently granitic.
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Thursday 13 July 2017
Ore Deposits Related to Magmatic Activity
Ore Forming Minerals: Metallic & Non-Metallic Minerals
Certain kinds of mineral can be treated for metal extraction more easily than others; these are commonly referred to as ore minerals. Quite often, different minerals containing a particular metal occur together in a deposit, and are referred to as ore forming minerals. Ore minerals form as a result of special geologic processes and often occur in isolated, small, localized rock masses. Such local concentrations are called mineral deposits. Mineral deposits are what prospectors seek. The terms ore mineral and mineral deposit were originally applied only to minerals and deposits from which metals are recovered, but present usage includes a few nonmetallic minerals, such as barite and fluorite, that are found in the same kinds of deposit as metallic minerals.
No deposit consists entirely of a single ore mineral. There is always an abundance of worthless minerals, collectively called gangue. The more abundant an ore mineral, the more valuable the mineral deposit. For every mineral deposit there is a set of conditions, such as the level of concentration and the size of the deposit that must be reached if the deposit is to be worked at a profit. A mineral deposit that is sufficiently rich to be worked at a profit is called an ore deposit. The assemblage of ore minerals and gangue in a deposit is called the ore.
Metals used in industrial and technological applications can be divided into two classes on the basis of their abundance in the Earth's crust. The geochemically abundant metals, of which there are five (aluminum, iron, magnesium, manganese, and titanium), constitute more than 0.1 percent by weight of the Earth's crust, while the geochemically scarce metals, which embrace all other metals (including such familiar ones as copper, lead, zinc, gold, and silver), constitute less than 0.1 percent. In almost every rock, at least tiny amounts of all metals can be detected by sensitive chemical analysis. However, there are important differences in the way the abundant and scarce metals occur in common rocks. Geochemically abundant metals tend to be present as essential constituents in minerals. For example, basalt, a common igneous rock, consists largely of the minerals olivine and pyroxene (both magnesium-iron silicates), feldspar (calcium-aluminum silicate), and ilmenite (iron-titanium oxide). Careful chemical analysis of a basalt will reveal the presence of most of the geochemically scarce metals too, but no amount of searching will reveal minerals in which one or more of the scarce metals is an essential constituent.
Geochemically scarce metals rarely form minerals in common rocks. Instead, they are carried in the structures of common rock-forming minerals (most of them silicates) through the process of atomic substitution. This process involves the random replacement of an atom in a mineral by a foreign atom of similar ionic radius and valence, without changing the atomic packing of the host mineral. Atoms of copper, zinc, and nickel, for example, can substitute for iron and magnesium atoms in olivine and pyroxene. However, since substitution of foreign atoms produces strains in an atomic packing, there are limits to this process, as determined by temperature, pressure, and various chemical parameters. Indeed, the substitution limits for most scarce metals in common silicate minerals are low--in many cases only a few hundred substituting atoms for every million host atoms--but even these limits are rarely exceeded in common rocks.
One important consequence that derives from the way abundant and scarce metals occur in common rocks is that ore minerals of abundant metals can be found in many common rocks, while ore minerals of scarce metals can be found only where some special, restricted geologic process has formed localized enrichments that exceed the limits of atomic substitution.
Ore minerals
Two factors determine whether a given mineral is suitable to be an ore mineral. The first is the ease with which a mineral can be separated from the gangue and concentrated for smelting. Concentrating processes, which are based on the physical properties of the mineral, include magnetic separation, gravity separation, and flotation. The second factor is smelting--that is, releasing the metal from the other elements to which it is chemically bonded in the mineral. Smelting processes are discussed below, but of primary importance in this consideration of the suitability of an ore mineral is the amount of energy needed to break the chemical bonds and release the metal. In general, less energy is needed to smelt sulfide, oxide, or hydroxide minerals than is required to smelt a silicate mineral. For this reason, few silicate minerals are ore minerals. Because the great bulk of the Earth's crust (about 95 percent) is composed of silicate minerals, sulfide, oxide, and hydroxide ore minerals are at best only minor constituents of the Earth's crust--and in many cases they are very rare constituents.
The preferred ore minerals of both geochemically abundant and geochemically scarce metals are native metals, sulfides, oxides, hydroxides, or carbonates. In a few cases, silicate minerals have to be used as ore minerals because the metals either do not form more desirable minerals or form desirable minerals that rarely occur in large deposits.
Metalic Minerals: are the source of metals including iron (steel), aluminum, copper, zinc, manganese, lead, chrome, platinum and others. Demand for new metals derived from ore deposits is decreasing, largely due to recycling.
Non-metallic Minerals: include stone, sand and gravel, limestone and cement rock, salt, clays, phosphate rock (fertilizer), sulfur from salt domes or volcanoes, diamond from kimberlites, F, Cl, Br, I from seawater or evaporates. Demand for non-metals is increasing.
Reserves and Resources: Reserves are the known amount of a mineral in the ground that is exploitable with current technology and under current economic conditions. Resource includes the reserves plus estimated undiscovered deposits.
Processes of Ore Formation
Current theories of the genesis of ore deposit can be divided into internal (endogene) and external (exogene) or surface processes. It must be understood that more than one mechanism may be responsible for the formation of an ore body. Example - stockwork porphyry copper deposit at depth (epigenetic) with a syngenetic massive sulfide deposit at the surface. The Table at the end of the document summarizes the principal theories of ore genesis,
Depending upon whether an ore deposit formed at the time of and together with the enclosing rock, or was introduced into it by subsequent processes, they are classed as:
Syngenetic - A deposit formed at the same time as the rocks in which it occurs. Ex. Banded Iron Formation
Epigenetic - A deposit introduced into the host rocks at some time after they were deposited. Ex. Mississippi Valley-type Deposits
Magmatic Deposits: Those deposits, not including pegmatites that have formed by direct crystallization from a magma. Two types:
· Fractional crystallization - Any process whereby early formed crystals can not re-equilibrate with the melt. Includes 1) gravitative settling; 2) flowage differentiation; 3) filter pressing and 4) dilation. Number 1 is the most important and results from the settling of early formed crystals to the bottom of the magma chamber. Rocks formed in this manner are termed cumulates and are often characterized by rhythmic layering. In ore deposits the alternating layers are often magnetite and/or chromite between layers of silicate. Ex. Bushveld igneous complex.
· Immiscible liquid - Typical example is oil and water. In ore deposits we deal with silicate and sulfide magmas. As a magma cools, sulfides coalesce as droplets and due to higher density settle out. Most common sulfides are iron sulfides, but nickel, copper and platinum also occur. Ex. Sudbury, Canada. The settling out of the heavier sulfides results in the peculiar net-textured ores often found in many of these deposits.
Pegmatitic Deposits: Pegmatites are very coarse grained igneous rocks. Commonly form dike-like masses a few meters to occasionally 1-2 km in length. Economic ore deposits are associated with granitic pegmatites since felsic magmas carry more water. Residual elements such as Li, Be Nb, Ta, Sn and U that are not readily accommodated in crystallizing silicate phases end up in the volatile fraction. When this fraction is injected into the country rock a pegmatite is formed. Temperatures of deposition vary from 250-750°C. Pegmatites are divided into simple and complex. Simple pegmatites consist of plagioclase, quartz and mica and are not zoned. Complex have a more varied mineralogy and are strongly zoned. Crystals in pegmatites can be large, exceeding several meters. Three hypotheses to explain their formation:
a. fractional crystallization
b. deposition along open channels from fluids of changing composition
c. crystallization of a simple pegmatite and partial to complete hydrothermal replacement
Hydrothermal Deposits: Hot aqueous solutions are responsible for the formation of many ore deposits. Fluid inclusion research indicates most ore forming fluids range in temperature from 50°C to 650°C. Analysis of the fluid in inclusions has shown that water is the most important phase and salinities are often much greater than those of seawater. The chemistry of ore fluids and the mechanism of deposition of ore minerals remains a subject of hot debate. Arguments boil down to a) source and nature of the solutions b) means of transport of the metals and c) mechanism of deposition.
Metamorphic/Metasomatic Deposits: Pyrometasomatic deposits (skarns) developed at the contact of plutons and host rock. Generally, host rock is a carbonate and new minerals formed are the calc-silicates diopside, andradite and wollastonite. Temperatures involved are thought to be 300-500°C, but pressure is probably quite low. Three stage process:
1. Recrystallization
2. Introduction of Si, Al, Fe, Mg
3. Hydration and introduction of elements associated with volatile fraction
Other metamorphic processes are relatively unimportant, but hydration/dehydration during regional metamorphism may concentrate metals at the metamorphic front. Sodic metasomatism of K-spar is thought to have been important in the concentration of gold at Kalgoorlie. Conversion of feldspar from K-spar (1.33A) to Na plag (.97A) resulted in the expulsion of gold (1.37A) which could no longer be accommodated in the feldspar lattice. Skip over Mechanical-chemical sedimentary processes since they are covered in the other course.
Volcanic Exhalative Deposits: Some ore deposits often show spatial relationships to volcanic rocks. They are conformable with the host and frequently banded suggesting sedimentary processes. Principal constituent is pyrite with lesser chalcopyrite, aphalerite, galena, barite and Ag-Au. These were thought until the late 60’s to be epigenetic, but it is now realized they are syngenetic. They show a progression of types with three distinct end members:
1. Cyprus type - Associated with mafic volcanics and ophiolite sequences. Found in spreading centers and back arc basins. Consist predominantly of pyrite with lesser chalcopyrite. Typified by the Cyprus pyrite-cu ores.
2. Besshi type - Associated with basaltic to dacitic volcanism. Thought to form during the initial stages of island arc formation. Many Besshi type deposits occur in Precambrian rocks and these may have been generated in entirely different tectonic settings. Pyrite dominant, but chalcopyrite and sphalerite very common. Typified by many of the volcanogenic deposits of Canada.
3. Kuroko type - Associated with dacitic to rhyolitic volcanics. Form during the waning stages of island arc volcanism. Pyrite occurs, but is not dominant Usually galena or sphalerite are predominate with lesser chalcopyrite and tetrahedrite. Also significant silver in this type. Typified by the Kuroko deposits.
Although it is agreed ores are associated with volcanism the source of the ore bearing solutions continues to be debated. Many feel ore fluids are of magmatic origin, but others feel they are merely convecting seawater.
THEORIES OF ORE DEPOSIT GENESIS
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Origin Due to Internal Processes
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Magmatic Segregation
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Separation of ore minerals by fractional crystallization during magmatic differentiation.
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Pt—Cr deposits
Bushveld, S.A. Titanium deposit Tahawas, N.Y. |
Liquid immiscibility. Settling out from magmas of sulfide, sulfide-oxide or oxide melts which accumulate beneath the silicates or are injected into country rocks or extruded on the surface.
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Cu-Ni ores of Sudbury, Canada and the nickel extrusives of Kambalda, West Australia.
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Pegmatitic Deposition
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Crystallization as disseminated grains or segregations in pegmatites.
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Li-bearing pegmatites of Kings Mtn. N.C.
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Hydrothermal
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Deposition from hot aqueous solutions of various sources.
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Porphyry Cu-Mo deposits of the
W. Cordillera. |
Lateral Secretion
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Diffusion of ore and gangue forming materials
from the country rocks into faults and other structures. |
Gold deposits of Yellowknife, B.C. and the Mother Lode, CA.
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Metamorphic Processes
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Pyrometasomatic (skarn) deposits formed by replacement of wall rocks adjacent to an intrusive.
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W deposits at Bishop, CA. Fe deposits
Iron Mtn UT. |
Initial or further concentration of ore elements by metamorphic processes.
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Homestake Au Mine, Lead, South Dakota.
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Origin Due to Surface Processes
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Mechanical Accumulation
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Concentration of heavy minerals into placer
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Placer Au deposits of Alaska and California.
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Sedimentary Precipitation
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Precipitation of certain elements in sedimentary environments.
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Banded Iron Fm.
of the Canadian Shield. |
Residual Processes
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Leaching of soluble elements leaving concentrations of insoluble elements.
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Nickel laterites
of New Caledonia and Arkansas bauxite. |
Secondary or Supergene Enrichment
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Leaching of certain elements from the upper part of a mineral deposit and their reprecipitation at depth to produce higher concentrations.
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The upper portion of many porphyry copper
deposits. |
Volcanic Exhalative Process
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Exhalations of sulfide-rich magmas at the surface, usually under marine conditions.
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Mt. Isa, Aust., Sullivan and Kidd Creek,Canada, Kuroko,Japan.
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