The economic geology among the geosciences and deals with the natural accumulation of solid, liquid or gaseous raw materials ( resources ) within the earth's crust that can be used for economic and / or industrial purposes. Their task is to supply industrial society with these raw materials. For this reason, the term deposit customer is often as a synonym for economic geology (English: economic geology ) used. As a sub-area of applied geology , deposit science is also responsible for a wide range of services during the mining of raw materials.
Exploration and assessment of deposits
An essential area of responsibility of the deposit theory is the exploration ( prospecting ) of prospecting areas and the development ( exploration ) of the deposits found. This requires careful sampling in the field, e.g. B. by trenches, boreholes, test tunnels, etc. The aim of this work is to determine the exact position of the deposit, as well as the reserve calculation. In this context, deposit science is often referred to as mining geology .
Classification of deposits
Often deposits are simply classified according to the economically most important component, such as gold deposits or uranium deposits . On the other hand, the methods of deposit science go far beyond the immediate practical needs of raw materials management and overlap with the scientific disciplines of structural geology , geochemistry , geophysics , mineralogy , petrography , petrology and, in the case of biogenic deposits, paleontology . The aim of these investigations is a system of deposits that is not only economically but also scientifically justified.
According to the external shape (morphology)
A first distinguishing feature of deposits is their storage relationship to the host rock: the raw material body can (similar to an igneous intrusion ) break through the rock packages discordantly or follow their internal stratification concordantly . Second, a distinction is made between the different types of spatial expansion:
- Tubular deposits, such as ore-containing, volcanic chimneys , or the pipes of explosive eruptions that are filled with mineral-bearing debris ( breccias ), usually extend vertically, or almost vertically, from the depths to the surface.
- Where mineralizing solutions rise up fissures and meet particularly reactive rocks, elongated mineralizations sometimes form, which often run more or less horizontally: the so-called “mantos”.
- Plate-shaped raw material bodies that are very large in width and length, but only small in thickness ( thickness ) are very common and important . This includes not only tunnels , but also coal seams and various layered (stratiform) deposits.
- Isometric deposits consist of evenly distributed ore minerals (so-called impregnations). The accumulations of crude oil and natural gas distributed in pores or cavities in the host rock also belong here. However, if the mineralization is bound to small, irregular tunnels and networks of veins, or veins that cross many times, they are called sticks , floors , or nests .
- Irregular zones of mineral new formations, the so-called skarns and greisen , often form on the contact surfaces of igneous intrusions with their host rock .
After the formation (genesis)
The classification of deposits according to their genesis is much more important. However, this approach is afflicted with fundamental problems, because the ideas about the processes that lead to the enrichment of certain raw materials in the earth's crust are still in flux today and not always finally clarified. In addition, there is often more than one process involved in the formation of a deposit. A strict classification based on genesis would mean that one and the same deposit would belong to several different deposit types. For this reason, some mineral explorers prefer a nomenclature that simply refers to the best-known example of a particular type of deposit, such as Olympic Dam -type , Broken Hill -type , or Savage River-type etc. This method is no less arbitrary, but it is far more obscure.
On the other hand, there is a gap that can hardly be bridged between the mineral explorers' need for the simplest and clearest possible classification, and the simultaneous need for the most complete possible recording of all observed facts. A good example here provide some of the so-called layer-linked deposits (English: strata-bound deposits ). Like the stratified deposits mentioned above, they are tied to certain stratigraphic horizons: B. the uranium deposits of the Colorado Plateau in Wyoming on sandstones, and the lead - zinc deposits of the Mississippi Valley type on carbonate rocks such as reef limestone and carbonate mud banks. However, the mineralization itself does not have to be layered, but can also have vein-like or irregular shapes. Most of these mineralizations are believed to have formed ( epigenetically ) after the sediments were deposited . Nevertheless, there is evidence of simultaneous ( syngenetic ) formation in others , and in some deposits both are found. Investigations of the isotope ratios in the various deposits mostly point to the origin of the metals from deeper layers of the earth, but sometimes also to concentrations in narrowed sea basins. So it is not surprising that some mineral explorers would like to subsume all of these deposits under a single generic term, while others suggest a separate deposit type for practically every single known deposit.
According to the place of origin, a distinction is made between roughly hypogenic (inside the earth) and supergenic formation, which occurs on the earth's surface or near the surface, for example in weathering deposits.
Certain minerals with very high melting points can be precipitated early on in molten rock melts ( magmas ), such as B. Chromite . If these minerals are specifically heavier than the residual melt, they sink to the bottom of the magma chamber , where they accumulate. Magma flows and other differentiation processes can then lead to the formation of streaks or stratification of the minerals within the magmatic intrusion, e.g. B. in the chrome deposits of the Bushveld Complex in South Africa.
In other cases, certain immiscible components may separate in the melt, just as water separates from oil (liquid mixture). In sulphide - silicate melts z. B. sulfide droplets. Copper and nickel in particular , as well as metals of the platinum group , are concentrated in these droplets . When the individual droplets combine with one another, extensive sulphide deposits can arise, such as B. in Greater Sudbury (Canada).
See also: orthomagmatic deposits .
Main and late crystallization
After the crystallization of the deep rocks ( plutonites ) and dike rocks in the subsurface, and the effluent rocks ( volcanites ) on the surface, residual melts usually remain in which certain elements have accumulated, which, for chemical and physical reasons, are difficult to convert into the most common Have rock-forming minerals incorporated. Phosphorus and aluminum- rich apatite and nepheline deposits as well as inorganically formed limes are considered products of late crystallization from this residual melt . These carbonatite - alkali rock complexes include important deposits of rare earth metals and inorganic phosphate deposits .
A characteristic of the remaining melts is that they increasingly contain more and more water. However, because of the enormous pressure of the rock, the strongly overheated water cannot boil. The precipitates in this supercritical phase are z. B. the deposits of rare earth metals in pegmatite corridors.
Kimberlite and lamproite
An extreme case of igneous formation is the violent eruptions that the volcanic breakthroughs, or pipes , created by kimberlites and lamproites . Fortunately, most of these catastrophic events belonged to a long distant past. However, deposits with a radiometric age of only 20 million years exist in Western Australia. The economically most important mineral in this type of deposit is diamond , which is an exotic component found in the volcanic breccias.
Since diamonds only form under high pressures and temperatures, they must come from regions of more than 120 kilometers overlaid by at least 60 kilometers of continental crust. On the earth's surface, the breakthrough tubes usually have a diameter of less than a square kilometer. They narrow further towards the depths and finally end at duct-like structures that are filled with unrecognized kimberlite. It is assumed that these "root zones" are linked to regional weakness zones with expansion cracks and rift formations that reach down into the upper mantle. After the explosive eruption of very gaseous magmas, there can occasionally be a silent intrusion of smaller igneous bodies in the pipes.
After all, after all rock-forming minerals have been eliminated from the residual melt, only hot, mineral-saturated solutions remain, the so-called hydrothermal brines, or fluids . Because of the extreme conditions and the extremely variable composition of these fluids, their properties are difficult to replicate in the laboratory. It is assumed, however, that they are able to dissolve a wide variety of substances, to transport them and to excrete them again elsewhere. They thus make a significant contribution to the formation of most of the vein deposits and ore stocks.
Corridor storage facilities
Well into the 20th century, vein deposits around the world provided many of the richest deposits of gold , silver , copper, tin , lead and other metals. Accordingly, they were important for the formation of theories in deposit science. With high ore grades, they are still of economic importance today. Because of their small size, they are often difficult to mine with today's mining technology.
In the higher parts of the earth's crust, the fluids mix with surface waters, which are set in circulation by the warmth of the igneous intrusions. In this way, geothermal systems are formed in open crevices and crevices that can reach to the surface of the earth. Another mechanism that could transport the mineralizing solutions to the surface is what is known as "seismic pumping". That is, the sudden opening and closing of tectonic faults in the course of an earthquake .
With decreasing pressure and temperature, hydrothermal mineralization is divided into katathermal , mesothermal , and finally epithermal (e.g. hot springs and deposits from vapors).
Impregnation and storey deposits
In rocks that have been heavily tectonically stressed and sheared, the faults and shear zones often expand into extensive rustling zones in which the fluids can create many small passages and veins. The adjacent rock is then mostly also strongly attacked by the mineralizing solutions and shows characteristic changes ( alteration ).
Often these floors or "floors" (in English: stockwork , not as a term for a floor or the like) develop at the interfaces between disturbances and certain layer boundaries. If the rocks are particularly reactive, such as carbonates, carbonaceous sediments, volcanic tuffs , or lavas that are particularly rich in bubbles , the finely distributed mineralization ( impregnation ) can extend far into these strata towards the sides. This includes: B. the high-volume deposits of the Carlin type in the USA (Nevada, Utah, Idaho, California), which are referred to as "invisible" gold deposits because of their very fine-grained mineralization.
It was not until the late 1960s that uranium impregnation deposits in Saskatchewan / Canada and in Australia became known, which are linked to the erosion surfaces ( discordances ) between the crystalline basement and overlying metamorphic sediments.
The so-called porphyry copper deposits represent an economically particularly important type of impregnation and multi-storey deposits . These are deposits with relatively low ore contents, but often huge volumes. Today, more than half of the world's copper production can be traced back to "copper porphyry", but also to the largest human-made holes in the earth's crust.
In contrast to the reservoirs listed above, copper porphyry typically forms in the upper parts of acidic and intermediate intrusions such as granite and diorite . While the minerals at the edge of these intrusions usually have the same grain size, larger crystals emerge inside in a uniform-grain matrix ( porphyry structure , hence the name). This structure is an indication of a relatively rapid cooling and crystallization of the intrusion. It is believed that the vapor pressure of the mineralizing solutions in the upper area of the magma chamber exceeded the rock pressure at some point , and that the surrounding rock was disrupted by retrograde boiling . The mineralizing solutions thus crystallized out on the spot before they could migrate to more distant passages or rocks.
Volcanic exhalative deposits
In the transition area from magmatic-hydrothermal to sedimentary processes, the volcanogenic massive sulphide deposits (VMS) of non-ferrous metals develop . They were recognized in the 1980s as syngeneic fumes (exhalations) from submarine volcanoes , the so-called black smokers . Where hydrothermal solutions rise to the surface, these deposits are underlain by epigenetic bedrock mineralization. Upon contact with sea water, the dissolved sulphides separate out, mostly as pyrite . These concordant, often banded ores interlock with basaltic lavas on mid-ocean ridges , but with more diverse volcanic rocks and the erosion debris from the mainland and the island arches ( greywacke ) near island arches .
There are also volcanic oxide deposits , such as the enormous iron deposit of Kiruna in Sweden, the world's largest underground mine. The predominant ore minerals here are magnetite , hematite and apatite. In addition to exhalative formation, the possibility of lava flows from oxidic magmas or liquid-magmatic segregation is also discussed here. The tungsten deposit of Mittersill (Felbertal, Austria) is probably also an exhalative formation.
At a greater distance from submarine volcanic centers, predominantly sedimentary sulphide deposits can form, such as Sullivan in British Columbia, the copper shale formed in large parts of Central Europe and the deposits of the Zambian copper belt . The historic deposit of Rammelsberg in the Harz Mountains is also assigned to this type today. Since it is assumed that the sulphides originate from volcanic exhalations, they are referred to as sedimentary exhalative deposits (SEDEX). Typically, however, these deposits are not found in deep-sea sediments, but in the deposits of shallow seas that have expanded across the continental crust ( transgression ).
The Proterozoic Banded Iron Formations (BIFs) represent a special case here . Because of certain similarities in the deposit conditions (interlocking with volcanic rocks and greywackes in the "Algoma type", as well as deposits on the shelf edge or in intracontinental basins in the "Superior type"), one suspects here also volcanic-exhalative or sedimentary-exhalative formations, but in waters under a practically oxygen-free atmosphere . Details of the formation, especially the origin of the enormous amounts of iron and the precise processes of ore precipitation, are still controversial. Some users prefer a largely sedimentary formation due to chemical and / or biochemical processes.
In principle, sedimentary rocks, and thus also sedimentary deposits, can be divided into two groups.
- First: sediments that were deposited autochthonously (at the place of their formation), such as most chemical, biochemical and biogenic deposits from the water, as well as some weathering deposits on the mainland. It is therefore a matter of taste whether the above-mentioned stratiform SEDEX deposits, or the BIFs, still belong to the hydrothermal deposits because of the hydrothermal origin of their metallic components, or to the sedimentary deposits because of their form of deposit.
- Second: sediments that were transported allochthonously (from elsewhere) to their place of deposition, such as clays , sands , conglomerates , or volcanic ejecta ( pyroclastics ).
The Lorraine Minette iron ores are marine sediments , namely small, shell-like spheres ( ooids ) of quartz, lime and hematite, which were constantly moved and rolled in the surf of a tropical sea until they finally reached deeper water deposited there. Such deposits were of some importance during the Industrial Revolution not only in Lorraine, but also in the English Midlands around Manchester and in Salzgitter , as they are among the most common Phanerozoic iron deposits. Today, due to the low ore content and their silicate component, they can only rarely be mined economically.
Salt deposits form in constricted sea basin or in lakes, if the supply of fresh water is less away for a long time, as the evaporation ( evaporation ). Here, not only the salts dissolved in the water are precipitated, but also the minerals in the weathering solutions from the mainland, in a regular order according to their solubility. First, the most difficultly soluble salts precipitate, such as carbonates and sulfates ( gypsum ), then rock salt , and finally even the easily soluble potassium and magnesium salts. Potash salts in particular often represent the economically most interesting part of the salt deposits because of their importance for the production of artificial fertilizers .
Another important example are the saltpetre and borax deposits in enclosed evaporation tanks in desert areas. The origin of the nitrate for the sodium nitrate and the boron for the borates is disputed. Mostly a volcanic origin is assumed.
The mechanical weathering ( erosion ) of rocks on the earth's surface by water and wind separates the individual minerals from one another and leads to the accumulation of heavy minerals such as gold, tin stone ( cassiterite ), rutile (an important titanium ore), etc., in so-called Soap deposits . Soaps have always been one of the most important deposits. However, since they can be discovered and dismantled very easily, they are today, with a few exceptions, exploited almost all over the world.
The most important soap deposit in the world, the quartz conglomerates from the Witwatersrand in South Africa, are exceptional in several respects: First, they are available as fossil, solidified soaps. Second, they are extraordinarily old (Proterozoic). Thirdly, in addition to free gold, they also contain detritic pyrite (an iron sulfide) and the uranium mineral uraninite . Today, such deposits would be almost impossible because these minerals would decompose quickly in water that is in equilibrium with an oxygen-rich atmosphere.
Chemical weathering (e.g. under a tropical climate, in as flat a landscape as possible) can lead to the formation of residual deposits. These are accumulations of poorly soluble minerals in the oxidation and cementation zone of the soil , such as bauxite and laterite , but also the " iron hat " that forms over sulphidic or carbonate iron deposits, or residual soaps (e.g. Nuggets over the outcrop of gold-bearing tunnels).
In the dry ( arid ) climate, large amounts of rock debris collect in depressions and depressions. Any metals contained there can be leached out by salty groundwater and then precipitated as sulfides in zones with a high proportion of organic substances (e.g. plant remains). The name of the province of Katanga in the southeast of the Democratic Republic of the Congo means "copper ore" and goes back to this type of red bed deposits .
In a damp ( humid ) climate, dissolved iron compounds can precipitate, especially in boggy soils. Such turf ore was the only source of wrought iron in many parts of Northern Europe well into the Middle Ages.
Deposits of hydrocarbons
In the border area between sedimentary and metamorphic deposits are the deposits of coal and hydrocarbons , as they have to be sunk into the depths under the pressure of thick layers of sediment in order to form. The formation of coal deposits is based on increased local plant growth and on favorable embedding conditions for the plant remains, e.g. B. under the rubble of a rising mountain range. By increasing the pressure and the temperature in the absence of oxygen, the organic substances go through the process of coalification , from peat to brown and hard coal to anthracite .
Despite their paramount importance for the global economy , the processes involved in the formation of oil and natural gas deposits are by no means fully understood. Usually one assumes the formation of decomposition products of digested sludge ( sapropel ) in the absence of air. The problem, however, lies in the nature of the energy source, which converts the original, largely oxidized, organic substances into largely oxygen-free hydrocarbons. Direct exposure to heat can be ruled out because there are still compounds in petroleum that become unstable at temperatures above 200 ° C. The involvement of natural radioactivity has not yet been confirmed. Instead, a reduction through anaerobic bacteria or through mineral and chemical catalysts is being discussed . There are also great differences of opinion among mineral explorers about the time periods and depths of immersion required for oil formation. In any case, the resulting liquids require the existence of a neighboring, porous storage rock, such as sandstone or coral limestone, into which they can migrate. It is assumed that only the reactions in the pore water cause the formation of petroleum hydrocarbons.
The short-chain hydrocarbons of the natural gas apparently form in the remaining petroleum mother rock ( bitumen ). The natural gas then also migrates into the storage rock. However, there are also other theories about the origin of natural gas. Even an inorganic formation from carbon monoxide , hydrogen , sulfur dioxide and other gases from the upper mantle is still being discussed.
Often these are simply igneous or sedimentary deposits that were transported into the depths in the course of mountain formation ( orogenesis ), where they came under great pressures and temperatures. Such metamorphosed deposits show characteristic changes in the mineral stock, such as recrystallization and new mineral formation. Through a regional metamorphosis , certain metals such as zinc, lead or manganese are mobilized in the rocks. If they then encounter hydrothermal solutions that are also driven out by the metamorphosis, an ore concentration can occur in suitable structures, such as faults or shear zones . In many quartz or quartz - calcite veins (e.g. in the metabasites in the Yellowknife gold field in the Northwest Territories of Canada), it is assumed that the silica required for the formation of the quartz (together with the metallic components) is from migrated to the surrounding rock in the opening corridors. This process is known as lateral secretion .
When a magma body is seated, contact metamorphosis often occurs ; H. on mineral reactions and material exchange between intrusion and host rock. Typical contact metamorphic or pyrometasomatic deposits are skarns and greisen.
Originally, skarn was not the name of a type of deposit, but a Swedish miner's term for silicate gangue (dead rock). Even today, most of the skarn formations cannot be mined. In most cases, these are almost pure limestones or dolomites, which have been converted into often very irregular rock bodies by the addition of large quantities of silicon , aluminum, magnesium and iron ( metasomatosis or displacement). Skarns can be confused with lime silicate rocks , which, however, were not created by displacement, but rather by a transformation from clayey-sandy (impure) limestone or pebbly dolomites, which had the same chemical composition as the end products ( isochemical transformation). As is so often the case in deposit science, the origin of the metallic components in minable skarns is controversial. Some investigators suspect the source to be in the intruding magmas themselves. Others believe in an origin from circulating hydrothermal solutions.
Some skarns are important iron deposits, such as in Magnitogorsk and Sarbai (Kazakhstan), in Marmoraton (Ontario / Canada), or in the Cornwall Mine (Pennsylvania), the oldest continuously operated mine in the USA. There are also copper and tungsten carnation deposits.
Similar to copper porphyries, old man's deposits usually form in the roof region of granite intrusions and are sometimes accompanied by mineralization. However, the greisen are not impregnations, but irregular, but massive rocks. Mostly they consist of uniformly granular ( granoblastic ) aggregates of quartz and muscovite with subordinate topaz , tourmalines and fluorite . As a rule, they are mined because of their tin content, like the deposits in the Ore Mountains , occasionally for tungsten.
Formation of deposits in the course of the earth's history
The earth , and especially the earth's crust, has undergone many profound changes over the course of its 4.5 billion year history. For this reason, there are different types of deposits that are limited to a very specific epoch in the history of the earth. In the preceding or following periods of time the conditions for their formation were no longer given.
The Archean period spanned the period from 3.8 to 2.5 billion years ago. The tectonic situation of this early epoch is characterized by two basic elements: the highly metamorphic " migmatite - gneiss - granulite areas", which represent the first solid cores of the lithosphere that forms, and the surrounding mobile " green stone belt ". While only a few stratified chromite deposits are important in the former areas, many significant deposits are found in the greenstone belts, e.g. B .:
- Orthomagmatic deposits of nickel and copper sulphides bound to basic and ultra- basic lavas ( Kalgoorlie Belt in southwestern Australia, Abitibi Belt in Canada, in the Baltic Shield , and in Zimbabwe). Since these metallogenetic provinces are clearly delimited, the metals and their host rocks likely originate from local anomalies in the Upper Mantle.
- In the edge zones of the greenstone belt, close to the adjacent granite intrusions, there are many gold-bearing vein deposits. In fact, the search for gold used to be the main reason for exploring and mapping the greenstone belts.
- Volcanogenic massive sulphide deposits of copper and zinc, especially in the Abitibi orogen.
In the early and middle Proterozoic (about 2.5 to 1.6 billion years before today) the first stable, albeit small, lithospheric plates developed. This created the basic requirement for crustal movements in the sense of plate tectonics . Now, for the first time, sedimentary basins were formed, platform sediment deposits and geosynclines formed on the continental margins. Characteristic of this epoch are sedimentary and sedimentary-exhalative deposits, which could only form under reducing conditions, in the absence of oxygen in the atmosphere:
- The unique gold-uranium conglomerates from the Witwatersrand in South Africa.
- The first banded iron formations date back to the Archean, but they were most widespread between 2.6 and 1.8 billion years ago. It is believed that iron-precipitating bacteria played an important role in their deposition in intra-continental basins and in the shelf areas of the young continents.
- Various sedimentary or sedimentary-exhalative deposits of manganese, lead and zinc (e.g. Mount Isa in Australia).
- Diamond bearing kimberlites and lamproites appear for the first time. Previously, the crust was not thick enough to generate the enormous pressures required to form diamonds.
- The presence of lithospheric plates also enabled the formation of regional fracture systems on which huge dike-like bodies and magmatic complexes could rise. The formation of the large stratified chromite deposits in southern Africa is likely due to chromite anomalies in the Upper Mantle. The intrusions of many anorthosite plutons with ilmenite mineralization in Norway and Canada also represent a magmatic event that has never been repeated.
- The extensive absence of orthomagmatic sulphide deposits in later times is attributed to the depletion of sulfur in the upper mantle in the course of plate tectonic processes.
Many researchers already suspect the existence of a supercontinent for the middle and late Proterozoic . This period is characterized by an unusually high copper concentration in sedimentary rocks, such as the “Red Bed” deposits of Katanga. In addition, three distinct belts of tin deposits formed in Africa, and another in Brazil. The formation of BIFs continued to decline, which is attributed to the creation of an oxygen-rich atmosphere through plant photosynthesis .
Towards the end of the Proterozoic the plate tectonic situation arose as it still exists today. The shifting of the continents led to the subduction of oceanic crust and the formation of fold mountains . Here, especially in the continental fold belts and the offshore arches of the islands, the "copper porphyries" are formed, which belong to the largest metal enrichments of the Phanerozoic (0.57 billion years until today). One example is Chuquicamata in Chile, the largest open-cast mine in the world. Salt deposits worldwide show a noticeable accumulation in certain geological epochs, such as in the period from the Permian to the Triassic , or in the Tertiary . In other words, they preferably follow the major mountain formation phases when there are enough sub-basins, but the differences in relief are no longer so great that the depressions are simply filled with rubble from the mountains. Coal deposits, on the other hand, go back to times with increased production of biomass , such as the eponymous Carbon Age.
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