Economic geology

The geology is one of the earth sciences and deals with the natural accumulations of solid, liquid or gaseous materials (resources) within the Earth's crust that can be used for commercial and / or industrial purposes. Your task is to supply the industrial society of these raw materials. For this reason, the term geology often as a synonym for Economic Geology (English: economic geology ) related. As a branch of applied geology, the geology is also responsible for a wide range of services during the mining extraction of raw materials.

3.1 Magmatic formations 3.1.1 Early crystallization
3.1.2 Main and late crystallization
3.1.3 kimberlites and Lamproite

3.2.1 vein deposits
3.2.2 Impregnation and floor deposits
3.2.3 Volcanic - exhalative deposits

3.3.1 Marine sedimentation
3.3.2 evaporation
3.3.3 weathering
3.3.4 deposits of hydrocarbons

3.4.1 skarns
3.4.2 Greisen

Exploration for and evaluation of mineral deposits

An essential activity of the Economic Geology is the exploration ( prospecting ) of Höffigkeitsgebieten and development ( exploration ) of the discovered resources. This requires careful sampling in the field, such as by trenching, drilling, test tunnel, etc. The aim of this work is to determine the exact position of the deposit, such as the stock calculation. In this context, the geology is often referred to as mining geology.

Classification of deposits

Often deposits are divided simply according to the most economically important part of how gold deposit or uranium deposit. On the other hand, the methods of geology go far beyond the immediate practical needs of the commodity economy out and overlap with the scientific disciplines of structural geology, geochemistry, geophysics, mineralogy, petrography, petrology, and, in the case of biogenic deposits of paleontology. Objective of these studies is a classification of deposits that is not only economical, but also scientifically justified.

After the outer shape (morphology)

A first distinctive feature of deposits is their relation to the storage host rock: the raw material body can ( similar to a magmatic intrusion ) by hitting the rock packages discordant or concordant follow their internal stratification. Second, we distinguish between different types of spatial extension:

Tubular deposits, such as ore-bearing, volcanic vents, or the breakdown tubes (English: pipes ) explosive eruptions with mineral leading breccias ( breccias ) are filled, usually extend vertically, or almost vertically from the depths to the surface.

Very often, and the importance of plate-shaped raw material body, which have a great extension in length and width, but only a small thickness ( width). This includes not only transitions but also coal seams and various layered ( stratiform ) deposits.
Isometric deposits consist of evenly distributed ore minerals (called impregnations ). Also distributed in pores or voids of the host rock accumulations of oil and natural gas belong here. If the mineralization, however, are bound to small, irregular transitions and transition networks, or many intersecting veins, they are referred to as poles, floor, or nests.

After the creation (Genesis )

Much more important is the classification of deposits according to their genesis. This approach is, however, fraught with fundamental problems, because the ideas about the processes that lead to the accumulation of certain raw materials in the earth's crust, are still in flux, and not always finally resolved. In addition, often more than one process is involved in the formation of a deposit. In a strict classification of the genesis then one and the same deposit would belong to several different types of deposits. For this reason, some deposit watchers prefer a nomenclature simply refers to the best-known example of a certain type of deposit, such as Olympic Dam - type Broken Hill - type, or Savage River - type, etc. However, this method is no less arbitrary, but but far unanschaulicher.

On the other hand, there is an almost bridging gap between the need of the deposit watchers after a possible simple and clear classification, and the simultaneous need for the fullest possible coverage of all the observed facts. A good example here provide some of the so-called stratabound deposits (English: strata -bound deposits ). So, as the above-mentioned layered deposits are tied to specific stratigraphic horizons such as the uranium deposits of the Colorado Plateau in Wyoming sandstones, and the lead - zinc deposits of the Mississippi Valley - type of carbonate rocks, as reef limestones and Karbonatschlammbänke. However, the mineralization need not be stratified itself, may also have special vein-like or irregular shapes. In most of these mineralizations, it is assumed that they have formed after deposition of the sediments ( epigenetic ). Nevertheless, there are other indications of a simultaneous ( syngeneic ) education, and in some deposits can be found both. Studies of the isotopic ratios in the various deposits indicate usually indicates an origin of the metals from deep within the Earth, but sometimes to concentrations in confined sea basin. It is not surprising that some deposit watchers want to subsume all of these deposits under a single generic term, while others practically propose its own type of deposit for each known deposit.

Erzgenese

A common classification assigns the deposits of the three basic rock types: the igneous rocks, sediments and metamorphic rocks.

Magmatic formations

Early crystallization

( Magma ) can retire early on certain minerals with very high melting points, such as chromite in red-hot molten rock melts. When these minerals are specifically heavier than the residual melt, they sink to the bottom of the magma chamber, where they are concentrated. Magmenströmungen and other Differenziationsprozesse can then lead to streaking or layering of the minerals inside of igneous intrusion, such as in the chromium deposits of the Bushveld Complex in South Africa.

In other cases it may lead to separation of certain immiscible components in the melt, just as water from oil separates ( Liquident mixture ). In sulfide - silicate melts differ eg sulfide droplets from. In these droplets concentrate particularly copper and nickel, as well as metals of the platinum group. When combining the individual droplets with each other, may arise as extensive sulphide deposits, such as in Greater Sudbury ( Canada).

Main and late crystallization

After crystallization of the plutonic rocks ( plutonic ) and dike rocks in the subsurface, and the igneous ( volcanic ) at the surface, usually residual melts left behind, in which certain elements are enriched, which, from chemical and physical reasons, difficult frequently in the can install rock-forming minerals. As products of the late crystallization of these residual melts phosphorus and aluminum - rich apatite and nepheline deposits and limestones are formed inorganically. These carbonatite - alkaline rock complexes are important deposits of rare earth metals and inorganic phosphate deposits.

A characteristic of the now remaining residual melt is that they contain progressively more and more water. However, because of the enormous rock pressure, the strongly superheated water can not boil. The precipitates in the supercritical phase are eg the deposits of rare earth metals in pegmatite dykes.

Kimberlites and Lamproite

An extreme case of igneous formation represent the violent eruptions that produced the volcanic punch tubes or pipes, of kimberlites and Lamproiten. Fortunately, most of these belong to the catastrophic events of a long-ago past. However, there are also deposits in Western Australia with a radiometric age of only 20 million years. The most economically important mineral deposits in this kind of is the diamond, which is found as an exotic ingredient in the volcanic breccias.

Since diamonds form only under high pressures and temperatures, they must come from regions of over 120 kilometers, which are overlain by at least 60 km thick continental crust. On the surface, the dielectric tubes usually have a diameter of less than a square kilometer. At depth they constrict further and eventually end at vein-like structures, which are filled with unbreckziiertem kimberlite. From this " root zones ", it is believed that they are tied to regional weakness zones with strain cracks and grave formations that extend into the upper mantle down. After the explosive eruption very gassy magma may occasionally to silent intrusion of smaller igneous bodies in the pipes.

Hydrothermal phase

Finally, after the elimination of all rock-forming minerals from the residual melts only hot, mineral- saturated solutions remain, the so-called hydrothermal brines or fluids. Due to the extreme conditions and the highly variable composition of these fluids their properties in the laboratory are difficult to imitate. However, it is believed that they are able to solve a wide variety of materials, transport, and elsewhere excrete again. They contribute significantly to the formation of most vein deposits and Erzstöcke.

Vein deposits

Until well into the 20th century, traditional vein deposits around the world, many of the richest deposits of gold, silver, copper, tin, lead and other metals. Accordingly, they were important for the theory in geology. At high ore grades, they are still of economic importance. Because of their small size, but they are often only difficult to break down with today's mining technology.

In the higher parts of the earth's crust, the fluids mix with surface water, which are set by the heat of igneous intrusions in circulation. To form in open clefts and crevices Geothermal systems, which can range up to the surface. Another mechanism that may transport the mineralizing solution to the surface, is the so-called " seismic equipment", i.e., the shock-like opening and closing of faulting in the course of an earthquake.

With decreasing pressure and temperature, divided the hydrothermal mineralization in katathermal, mesothermal, and finally epithermal (eg, hot springs and deposits from vapors ).

Impregnation and floor deposits

In rocks that were tectonically highly stressed and sheared, the faults and shear zones often extend for long Ruschelzonen in which the fluids can generate many small passages and veins. The host rock is then usually also strongly affected by the mineralizing solutions and has characteristic changes on ( Alteration ).

Often these sticks or " floors " to develop (after English: stockwork, not as a term for a floor or the like) at the interfaces between disorders and certain layer boundaries. If this is particularly reactive rocks such as carbonates, carbonaceous sediments, volcanic tuffs, or particularly bubble- rich lavas, the finely dispersed mineralization (impregnation ) can to the sides far into this layer packets expand. For example, here are the voluminous deposits of Carlin - type in the United States (Nevada, Utah, Idaho, California), which are referred to as " invisible" gold deposits because of their very fine-grained mineralization.

Only since the end of the 1960s, uranium Imprägnationslagerstätten in Saskatchewan / Canada and Australia were known, which are linked to the erosion surfaces ( unconformities ) between the crystalline basement and overlying metamorphic sediments.

A economically the most important type of Impregnation and floor deposits represent the so-called porphyry copper deposits dar. This is to deposits with proportionate low ore grades, but often huge volume. Today goes back over half of the world's copper production on " Kupferporphyries ", but even the largest holes ever produced by humans in the earth's crust.

In contrast to the floor deposits above is Kupferporphyries typically form in the upper parts of acidic and intermediate intrusions, such as granite and diorite. While the minerals on the outskirts of intrusions usually have the same grain sizes, come in the interior of larger crystals in an even-grained matrix states ( porphyritic structure, hence the name). This structure is regarded as an indication of a relatively rapid cooling and crystallization of the intrusion. It is assumed that the vapor pressure of the mineralizing solutions has eventually topped the rock pressure at the top of the magma chamber, and that the surrounding rock was shattered by retrograde boiling. The mineralizing solutions crystallized from so on the spot before they could migrate to more distant courses or rocks.

Besides that, there porphyry deposits of molybdenum, tin and tungsten.

Volcanic - exhalative deposits

In the transition zone of magmatic- hydrothermal sedimentary processes on the volcanogenic massive sulphide deposits develop (VMS ) of non-ferrous metals. They were recognized in the 1980s as syngenetic exhalations ( exhalations ) of undersea volcanoes, the so-called black smokers. Where hydrothermal solutions rise to the surface, these deposits are underlain by epigenetic Stockwerkvererzungen. Upon contact with the sea water, the dissolved sulfides are deposited, mostly as pyrite. These concordant, often banded ores interlock at mid-ocean ridge basaltic lavas with, near island arcs but with more diverse volcanic rocks and the detritus from the mainland and the island arcs ( greywacke ).

There also exist a volcanogenic oxide deposits, such as the enormous iron deposits of Kiruna in Sweden, the world's largest underground mine. The predominant ore minerals are here magnetite, hematite and apatite. In addition to an exhalative origin but also the possibility of lava flows of oxide magmas, or a liquid magmatic segregation is discussed here. In the tungsten deposit of Mittersill ( Felber, Austria ) it probably is also an exhalative formation.

Predominantly sedimentary sulphide deposits can form, as Sullivan in British Columbia, the educated in many parts of Central Europe and the copper shale deposits of the Zambian copper belt at a greater distance from submarine volcanic centers. The historic deposit of Rammelsberg in the Harz is now attributed to this type. Because it is expected that the sulphides from volcanic exhalations come, they are called sedimentary exhalative - deposits ( SEDEX ). Typically, these deposits are found but not in deep-sea sediments, but in the deposits of shallow seas, which have spread over continental crust ( transgression ).

A special case here, the Proterozoic banded iron formations ( BIFs ) dar. Because of certain similarities in the deposition conditions ( interaction with volcanic rocks and greywacke at the " Algoma - type", and deposition on the shelf edge or in intracontinental basins at the " superior type") is suspected here also volcanic - exhalative or sedimentary exhalative - training, but in waters under a substantially oxygen -free atmosphere. Details of the formation, particularly the source of enormous quantities of iron and the detailed processes of Erzausscheidung, but are still controversial. Some editors prefer a largely sedimentary formation due to chemical and / or biochemical processes.

Sedimentary formations

In principle, sedimentary rocks, and therefore also of sedimentary deposits, divided into two groups.

First, sediments that were deposited autochthonous ( at source ), like most chemical, biochemical and biogenic deposits from the water, as well as some weathering deposits on the mainland. It is thus a matter of taste whether one wants to calculate the above-mentioned stratiform SEDEX deposits, or BIFs, because of the hydrothermal origin of their metallic components, nor to the hydrothermal deposits, or because of their deposit form to the sedimentary deposits.
Second, sediments that were transported allochthonous ( from elsewhere ) to their site of deposition, such as clays, sands, conglomerates, and volcanic ejecta ( tephra ).

Marine sedimentation

In the Lorraine minette iron ores are marine sediments, namely, small, shell beads ( ooids ) of quartz, lime and hematite, which constantly moves in the surf of a tropical sea and were unrolled until they finally got into deeper water and were deposited there. Such deposits were in the times of the Industrial Revolution not only in Lorraine, but also in the English Midlands to Manchester, and in Salzgitter of some importance, since they are among the most common Phanerozoic iron deposits. Today they can be, because of low grades and their silicate component, rarely break down economically.

Even with many mass commodities, which are for the construction industry and the industry of importance, such as lime, dolomite, phosphate and sulfate, are biochemical precipitates in the marine environment.

Evaporation

Salt deposits form in constricted ocean basins or lakes, when the supply of fresh water over a long period of time is less than the evaporation ( evaporation). Here, the dissolved salts in the water will not only precipitated, but also the minerals in the weathering solutions from the mainland, in a lawful order, according to their solubility. First fall of the most heavily soluble salts, such as carbonates and sulfates (gypsum), then rock salt, and eventually even the slightly soluble potassium and magnesium salts. Especially the potassium salts often represent the economically most interesting part of the salt deposits, because of their importance for fertilizer production.

Another important example is the saltpetre and borax deposits in closed Eindamfungswannen in desert areas. Here, the origin of nitrates for the sodium nitrate, and boron is moot for the borates. Usually, a volcanic origin is suspected.

Weathering

The mechanical weathering (erosion ) of rocks on the earth's surface by water and wind separates the individual minerals from each other, and leads to the accumulation of heavy minerals such as gold, cassiterite ( cassiterite ), rutile ( an important titanium ore), etc., in so called placer deposits. It has always belonged to the most important deposits soaps at all. Because they can be very easily discovered and mined but, they are today, exploited with a few exceptions almost all over the world.

The most important soap deposit in the world, the quartz conglomerates of Witwatersrand in South Africa are, in several respects represents an exception: first, they are as fossil solidified soap before. Second, they are exceptionally old ( Proterozoic ). Third, they have, in addition to free gold, and detrital pyrite ( an iron sulfide ) and the uranium mineral uraninite on. Today, these deposits would be as good as impossible to decompose these minerals in waters that are in equilibrium with an oxygen-rich atmosphere, fast.

By chemical weathering ( eg, under a tropical climate, in a landscape as flat as possible ) can cause the formation of Residuallagerstätten. This is to enrichment of sparingly soluble minerals in the oxidation and cementation of the soil, such as bauxite and laterite, but also to the "iron hat " that forms on iron sulphide or carbonate deposits, or residual soaps (eg nuggets over the outcrop of gold-bearing courses ).

In the dry ( arid ) climate accumulate in depressions and terrain depressions large amounts of debris. Metals may be contained there can be leached by saline groundwaters to be then precipitated in zones with a high content of organic substances ( eg crop residues) as sulphides. The name of the Katanga province in the southeast of the Democratic Republic of the Congo, means " copper ", and goes back to this type of Red -bed deposits.

In moist ( humid ) air can fail dissolved iron compounds, especially in boggy soils. Such Rasenerz was in many parts of northern Europe until well into the Middle Ages, the only source of wrought iron.

Deposits of hydrocarbons

In the border area of sedimentary deposits to metamorphic are the deposits of coal and hydrocarbons, since they must be sunk to their formation under the pressure of powerful sediment layers in depth. The formation of coal deposits is due to an increased local plant growth and low embedding conditions of plant residues, eg under the detritus of the uplifting mountains. By increasing the pressure and temperature under exclusion of oxygen, the organic substances through the process of coalification, peat, through lignite and hard coal, to anthracite.

Despite their paramount importance for the world economy, the processes of formation of oil and natural gas deposits are certainly not yet fully understood. Usually one starts with the formation of decomposition products of digested sludge ( sapropel ) under exclusion of air from. The problem lies in the nature of the power source, which converts the original, substantially oxidized organic material in a substantially oxygen-free hydrocarbons. Direct sources of heat can be excluded, since one can still find compounds in petroleum, which are unstable at temperatures above 200 ° C. A participation of natural radioactivity could not be confirmed. Instead, a reduction is discussed by anaerobic bacteria, or by mineral and chemical catalysts. Also over the periods and burial depths, which are necessary for petroleum formation, there are major differences among deposit Kund learning. In any case, the resulting liquids require the existence of an adjacent porous reservoir rock, such as sandstone or coral to which they can migrate. It is assumed that only the reactions in the pore waters cause the formation of petroleum hydrocarbons.

In the remaining petroleum source rocks (Bitumen) apparently form the short-chain hydrocarbons of natural gas. The gas then travels also in the reservoir rock. However, there are still other theories about the origin of natural gas. Even an inorganic form of carbon monoxide, hydrogen, sulfur dioxide and other gases from the upper mantle is still under discussion.

Metamorphic formations

Often, these are easy to magmatic or sedimentary deposits that were transported during a mountain building ( orogeny ) in the depth, where they came under great pressures and temperatures. Such metamorphosed deposits show characteristic changes in mineral content, such as recrystallization and mineral formations. Through a regional metamorphism, certain metals, such as zinc, lead or manganese, mobilized in the rocks. When they meet hydrothermal solutions, which are also driven by the metamorphosis, it may, in appropriate structures, such as faults or shear zones, coming to an ore concentration. Even with many quartz or quartz - calcite - level (eg in the metabasites in the Yellowknife gold field in the Northwest Territories Canada ) suspected you that the silica needed for the formation of the crystal (along with the metallic components ) from the surrounding rock is moved in the opening passages. This process is referred to as Lateralsekretion.

The emplacement of a magma body, it often comes to the contact metamorphism, that is, to mineral reactions and mass transfer between intrusion and host rock. Typical kontaktmetamorphe, or pyrometasomatic deposits are skarn and greisen.

Skarns

Originally it was in skarn not the name for a type of deposit, but a Swedish miner term for silicate gangue ( waste rock ). Even today, the vast majority of Skarnbildungen not mineable. Usually it involves almost pure limestone or dolomite that has been converted by the supply of large amounts of silicon, aluminum, magnesium and iron in often very irregular body of rock ( metasomatism or displacement ). Skarns can be confused with Kalksilikatfelsen, but they are not caused by repression but by a conversion from clayey- sandy ( impure ) limestone or siliceous dolomites, the same chemical composition as had the final products ( isochemische conversion ). As so often in the deposit customer is the origin of the metallic components in mineable skarns controversial. Some agents suspect the source of the intruding magma itself Others believe in an origin from circulating hydrothermal solutions.

Some skarns are important iron deposits, as in Magnitogorsk and Sarbai (Kazakhstan ), in Marmoraton (Ontario / Canada), or in the Cornwall Mine (Pennsylvania), the oldest continuously operated mine in the United States. There also exist a copper and Wolframskarnlagerstätten.

Greisen

Similar to Kupferporphyries old man deposits usually form in the roof area of granite intrusives and are sometimes accompanied by a Stockwerkvererzung. However, it is at the Greisen not impregnations, but irregular, but massive rock body. They usually consist of evenly - grained ( granoblastischen ) aggregates of quartz and muscovite with minor topaz, tourmaline and fluorite. As a rule, they are degraded because of their tin content, as the deposits in the Erzgebirge, occasionally on tungsten.

Deposit formation in the course of the earth

The earth, and especially the earth's crust has undergone many profound changes in the course of its 4.5 billion year history. For this reason, there are various types of deposits, which are limited to a certain period of Earth's history. In the preceding or following periods the conditions for their formation were no longer met.

Archean

The Archean comprises approximately the period from before 3.8 to 2.5 billion years ago. The tectonic situation this early epoch is characterized by two basic elements: the highly metamorphosed " migmatite - gneiss - granulite areas " that represent the first solid cores of forming lithosphere and the surrounding mobile " greenstone belt ". While in the former areas are just a few layered chromite deposits of importance, there are many significant deposits found in the greenstone belts, eg:

Ortho Magmatic deposits of nickel and copper sulphides, which are attached to basic and ultrabasic lavas ( Kalgoorlie belt in southwestern Australia, Abitibi Belt in Canada, in the Baltic Shield, and Zimbabwe ). Since these metallogenetic provinces are clearly limited, the metals and their source rocks probably derived from local anomalies in the upper mantle.
In the marginal zones of the greenstone belt, close to the adjacent granite intrusions, there are many gold-bearing vein deposits. In fact, the search was for gold used to be the main reason for the study and mapping of the greenstone belt.
Volcanogenic massive sulphide deposits of copper and zinc, especially in the Abitibi orogen.

Proterozoic

In the early and middle Proterozoic ( about 2.5 to 1.6 billion years ago) developed the first stable, albeit small, lithospheric plates. This completes the basic requirement for crustal movements in terms of plate tectonics was created. Now it came for the first time to the formation of sedimentary basins, deposits of platform sediments and the formation of geosynclines to the continental margins. Characteristic of this period are sedimentary and sedimentary - exhalative deposits that could only be formed under reducing conditions, in the absence of oxygen in the atmosphere:

The unique gold - uranium conglomerates of the Witwatersrand in South Africa.
The first banded iron formations date back to the Archean, but her most widely found them in time from 2.6 to 1.8 billion years ago. It is believed that eisenfällende bacteria had an important role in their deposition in intracontinental basin and the shelf Gebietn of young continents.
Various sedimentary, or sedimentary exhalative deposits, of manganese, lead and zinc (eg Mount Isa in Australia).
Leading diamond kimberlites and Lamproite occur for the first time. Previously, the scale thickness was not sufficient to produce the enormous pressures, which are necessary for the formation of diamond.
The presence of lithospheric plates also enabled the formation of regional fault systems, where could ascend huge vein-like body and magmatic complexes. The formation of the large layered chromite deposits in southern Africa is probably due to chromium anomalies in the upper mantle. The intrusions of many anorthosite plutons with ilmenite mineralization in Norway and Canada represent a magmatic event that has repeated ever again.
The virtual absence of ortho magmatic sulphide deposits in later periods is attributed to a depletion of the upper mantle of sulfur in the course of plate tectonic processes.

For the middle and late Proterozoic, many researchers suspect the mere existence of a supercontinent. This period is characterized by an unusually high concentration of copper in sedimentary rocks, such as the " Red Bed " deposits of Katanga. In Africa also formed three distinct belts of tin deposits, another in Brazil. The formation of BIFs went back further and further, which is recycled to the formation of an oxygen- rich atmosphere through plant photosynthesis.

Phanerozoic

Towards the end of the Proterozoic plate tectonic situation had set in about how it still exists today. Due to the displacement of the continents did the subduction of oceanic crust and the formation of fold mountains. Here it is particularly in the continental folds belts and the offshore island arcs to form the " copper porphyries ," which are among the largest metal enrichment of the Phanerozoic ( 0.57 billion years to the present). An example is Chuquicamata in Chile, the largest open pit in the world. Salt deposits worldwide show a striking accumulation in certain geological epochs, as in the period from the Permian to the Triassic, or tertiary. That they preferred to follow the great mountain building phase, if enough sub-basins exist, the relief differences, however, are no longer so great that the sinks are simply filled with detritus from the mountains. Coal deposits, however, go back to times with increased production of biomass, such as the eponymous Carboniferous age.

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