Skarn
As skarn a group is extremely heterogeneous rocks metamorphic or metasomatic origin referred to, which by their mineralogical composition with most calcium-rich silicates are distinguished. They often arise from the intrusion of magmas into carbonate-rich rock layers such as limestone or dolomite , but in principle they can also arise in most rocks without magmatic processes. Skarns are frequently occurring rocks, but their extension is usually small and ranges from centimeters to several kilometers measuring bodies. Skarns are often mineralized and can form significant ore deposits for a wide variety of metals . The use of the term skarn in its current meaning goes back to Alfred Elis Törnebohm , who first introduced it to scientific literature in 1875 to describe garnet-pyroxene rocks from the Persberg deposit in Sweden .
Rock description and mineral inventory
Skarns are defined by their mineralogical composition, which is mostly dominated by calcium-rich silicates. These are mainly garnet (mostly grossular or andradite ), pyroxene (e.g. diopside , hedenbergite ), amphibole (e.g. actinolite , hornblende ) or minerals of the epidote group (e.g. epidote , clinozoisite ). Depending on the original rock and the conditions in which it was formed, silicates rich in magnesium , iron or manganese can also play a role. Under certain circumstances, skarns can contain high levels of oxide and sulphide minerals , which makes them important deposits for a wide variety of metals.
Based on the exact mineralogical properties, skarns can vary in color from mostly brown or green to red, yellow, gray or white. The grain size of the rocks is also variable and often ranges from fine to coarse-grained.
Skarns are often very hard rocks and with a high percentage of garnets are very resistant to weathering. Pyroxene-rich skarns, on the other hand, can be broken down more quickly.
Classification of skarns
Skarns can be classified according to different criteria. This depends on the respective question and can include petrological, genetic or economic properties.
Emergence
With regard to their formation , skarns can be divided into calcium silicate rocks , reaction skarns , skarnoids and the most widespread classical metasomatic skarns .
Lime silicate rocks are formed by metamorphosis from impure carbonate rocks such as marl . Lime silicate rocks are fine-grained, when they formed there was no external supply of substances. They are stratiform, which means that they follow the stratification of the original sediments.
Reaction threads arise at the contact between carbonate and silicate layers in finely stratified rock units without any external material supply. However, there is an exchange of substances over a few centimeters in the contact area between the individual layers. Like calcium silicate rocks, they are very fine-grained and follow the stratification of the original sediments. Due to the way they are formed, however, they are very small-scale units and often only a few centimeters thick.
Skarnoids can be viewed as an intermediate stage between calcareous silicate rocks and metasomatic skarns. Like the calcium silicate rocks or reaction karne, they essentially reflect the geochemistry of the parent rock and are fine-grained and poor in iron. However, there was also an influx of fluids from outside (including groundwater) and thus a transport of substances through fine cracks in the rock. In contrast to the calcium silicate rocks or reaction barns, skarnoids can cut layer boundaries.
Metasomatic skarns arise from the interaction of aggressive hydrothermal solutions and carbonate-rich rock units. In contrast to the types mentioned above, metasomatic skarns are usually coarse-grained and have a high substance intake from the outside in their composition. They mostly arise when granitic intrusions intrude into carbonate-rich sediment units. Due to the falling pressure that acts on the molten rock during the ascent, water and other dissolved volatiles separate from the magma, which have a very aggressive effect on neighboring rocks and the solidifying magma due to the high temperatures and dissolved components such as chloride or fluoride ions. This leads to a lively exchange of substances between the hydrothermal solution, the carbonate-rich side rocks and the newly formed igneous rock.
Therefore, metasomatic skarns can be further subdivided according to their location in relation to their parent rocks. Endoscarne denote the skarn that was created by the transformation of the pluton causing it (usually a granitic rock) and exoscarn denotes that which was created by the transformation of the adjacent rocks outside the pluton (usually a limestone). Exoscarns are usually significantly larger than endoscarns. In the case of very large and intensely transformed skarn systems, the demarcation between endo- and exoscarns is often difficult to determine. Exoscarne can be further divided into proximal (close to the pluton) and distal ( further away from the pluton) skarns.
The different types of skar often appear together in the context of intrusions. The formation of calcareous silicate rocks, reaction skarns and skarnoids precede the formation of metasomatic skarns, as these arise from the pure thermal effect of an ascending intrusion before there is a mass transfer between the intrusion and secondary rocks through hydrothermal solutions. These early skarn formations therefore run parallel to the formation of other contact metamorphic rocks such as Hornfels or marble . Under certain circumstances, they can hinder the subsequent formation of metasomatic skarns, as they seal pathways for hydrothermal solutions with their fine-grained and geochemically relatively resistant mineralogy.
Composition of the parent rocks
The geochemical composition of the parent rock has a major influence on the mineralogy of the resulting skarns.
Calcium carrons
Calcium carns are the most widespread types of skarn and emerged from calcium-rich parent rock, usually limestone. Their mineralogy is determined by garnet, pyroxene, wollastonite and other calcium-rich silicates.
Magnesium carrots
Magnesium carvings emerged from rocks rich in dolomite. Their mineralogical composition is often more complex than that of calcium carmin, since besides calcium, magnesium is another main chemical component of the skarn-forming minerals, and is dominated by olivine, pyroxene, humite and other magnesium-rich minerals.
Manganese carrot
Manganese carnishes are relatively rare and usually occur with calcium carnuts. Their mineral components are manganese-rich silicates such as Spessartine, Johannsenite or Rhodonite.
Mineralogical composition
The mineralogical composition of the skarn can also be used for classification. Skarns can be named after their main components (“garnet carne”, “pyroxene carne” etc.). However, the mineralogy can vary considerably within a body of skarn. Typically, metasomatic skarns have a typical zoning of their mineralogy and are richer in garnets near the pluton causing the problem than at a greater distance from the pluton, where pyroxene often dominates. This zoning can be very important for the exploration of deposits, since different metals can be enriched in the different zones of a skarn. The zoning can also indicate the direction to the pluton if it is not open-minded . The chemical composition of the individual skarn minerals also typically changes with the distance from the pluton. Since this has a direct influence on the color of the typical skarn minerals, garnets become lighter with increasing distance from the pluton, while pyroxenes become darker.
Mineralogy also provides information about the redox character of a skarn. Oxidized skarns with a high ratio of Fe 3+ to Fe 2+ , for example, have a high proportion of iron-rich garnet minerals such as andradite , while the iron-poor grossular is more characteristic of reduced skarns. Magnetite also occurs rarely or not at all in reduced skarns, while it is typical for oxidized skarns. This distinction is of great importance for the exploration of raw materials, since heavily reduced skarns, for example, contain tin , but no copper deposits .
The formation of a skarn takes place in several phases under changing conditions. Mineral associations which form under increasing temperature conditions in the early phase of Skarnentwicklung are as prograde referred to are those which form under decreasing temperature conditions, as retrograde . Prograde mineral formations are usually anhydrous (garnet, pyroxene), while retrograde minerals often contain water (amphibole, epidote). But it can also lead to the formation of several generations of the same mineral under progressive and retrograde conditions, e.g. B. several generations of garnet next to each other. Retrograde minerals often destroy the prograde minerals - garnet, for example, can be displaced by epidote.
Economical meaning
From a raw material point of view, skarns can be classified according to the elements they contain and, with a correspondingly large enrichment of economically viable ores, form skarn deposits. Some types of mineralization are characteristic of certain geological formation conditions. Skarn mineralization occurs almost exclusively in metasomatic skarns, since an external supply of substances is necessary for the economic enrichment of the metals.
Skarn deposits have been used for mining for at least 4000 years and are now among the most important raw material sources for some metals, especially copper and tungsten. In the scientific literature today more than 1,400 different skarn deposits are described.
Copper Carne
Copper scarves are the most common types of skarn deposits. They mostly occur in connection with porphyry copper deposits in the area of active continental margins and form at shallow depths. Assuming the appropriate carbonate parent rock, they form a halo around the actual copper porphyry.
The amount of copper contained in the skarn can in some cases exceed that of the actual copper porphyry. In some cases, copper carnel can contain more than a billion tons of ore. Since the ore content in the skarns is usually significantly higher than that of the copper porphyry, smaller skarn ore deposits around a copper porphyry are also very important for the profitability of such a mining project and are often extracted first.
Copper carns often contain economical additions of gold, silver or zinc. Significant examples of copper skarn porphyry systems are the Ertsberg-Grasberg district in Indonesia, Bingham in the USA and Antamina in Peru.
Iron Carne
Iron karne are the largest deposits of skarn and can contain billions of tons of ore. The main ore mineral is magnetite with often only small amounts of calcium silicate. They are often formed by the intrusion of iron-rich magmas into carbonate rocks in the island arc area. In some cases, the endoscarn is larger than the exoscarn, and there are also transitions to copper candies. In some modern publications, some iron carvings are assigned to the IOCG deposits, although calcium silicate minerals are actually not typical for the latter.
Iron karne are of great importance for the steel industry in Russia, which has large deposits in the Urals . There are also significant deposits on other continents, including small deposits in the Saxon and Bohemian Ore Mountains .
Zinc carrots
Zinc carrots often form further away from the intrusion. In the case of some deposits, the intrusion that led to the formation of skarn is therefore unknown. Zinc carrots can contain several million tons of ore and often have high ore grades with 10 to 20% zinc and lead. In addition, silver is often contained in significant quantities. Zinc carrots have a mineralogy rich in iron and manganese. In larger ore districts, the amount of typical skarn minerals in the zinc ores decreases with increasing distance from the pluton to massive, skarn-free sulphide mineralizations.
Molybdenum yarn
Molybdenum grains can form small, high grade ore bodies or very large ore bodies with low ore grades. Often other metals are enriched in these skarns and in some cases only the joint extraction of molybdenum and other metals makes the extraction economical. There are also transitions to tungsten or copper karnen. These skarns often arise in silt-rich carbonate rocks or marls, more rarely in dolomite.
Tungsten Carne
Tungsten karne often emerges at a greater depth, which means that they are usually only a few meters thick in direct contact with the intrusion. Nevertheless, there are some large deposits, which are often characterized by very high tungsten contents. Today these are among the most important tungsten producers. The main ore mineral is scheelite ; in some deposits there is also a wide range of other raw materials that can be extracted.
There are significant tungsten carbons in China, Korea, the far east of Russia, north-west Canada, north-east Brazil and Tasmania. There are also smaller tungsten karne in Saxony, for example in the Ore Mountains and near Delitzsch .
There are also some tungsten karne produced by regional metamorphic processes with no reference to magmatic intrusion. But these have hardly any economic significance.
Pewter
Tin carrots are almost exclusively bound to very silicon-rich magmas, which were created by the partial melting of the continental crust. Pewter skarns often appear with old age , which is not the case with other types of skarons.
In contrast to other metals, the typical skarn minerals garnet, titanite or Vesuvian can sometimes incorporate large amounts of tin into their structure. This tin cannot be extracted economically and is therefore lost to a mining operation. For the formation of an economically viable deposit it is therefore important that the original tin-containing skarn minerals are destroyed again by further hydrothermal processes and that the tin contained in them was newly deposited as cassiterite or stannite (stannin).
As with zinc skarn, there is a zoning in large systems from lime silicate rich to lime silicate or completely free ore bodies, which has led to discussions as to whether the latter should still be referred to as skarn deposits. In large tin skarn districts, the “skarn-free” ore bodies are economically more attractive due to the previously described problem of tin incorporation in calcium silicate minerals.
Examples of tin carvings can be found in Tasmania or in the Saxon Ore Mountains.
Gold carrots
For a long time gold was almost exclusively extracted from skarnen as a by-product of the extraction of non-ferrous metals, mostly from copper scarves. It was not until the 1970s that gold was increasingly extracted from skarns, also due to rising gold prices.
Gold carrots form a broad group and can form under quite different conditions. However, the skarns with the highest gold content (5 to 15 g / t) often have a reduced character and are mostly free of other minable metals.
Other skarn deposits
Some skarns can contain significant uranium or rare earth elements, such as the Mary Kathleen deposit in Australia. However, their economic importance is low.
Skarns without mineralization can be coveted building raw materials as well as decorative stones.
literature
- MT Einaudi, DM Burt: A Special Issue Devoted to Skarn Deposits - Introduction Terminology, Classification, and Composition of Skarn Deposits. In: Economic Geology. V77 / 4, Society of Economic Geologists, 1982.
- Teunis AP Kwak: W-Sn Skarn Deposits and related metamorphic skarns and granitoids. (= Developments in Economic Geology. 24). Elsevier, 1987
- Lawrence Meinert, Gregory Dipple, Stefan Nicolecu: World Skarn Deposits. Economic Geology 100th Anniversary Volume. Society of Economic Geologists, 2005
- Lawrence Meinert: Skarns and Skarn Deposits. In: Journal of the Geological Association of Canada. V19 / 4, 1992. (journals.lib.unb.ca)
- GE Ray, ICL Webster: An Overview of Skarn Deposits. Geological Survey of British Columbia, 1991. (empr.gov.bc.ca)
- Dietmar Reinsch: Natural stone studies. An introduction for civil engineers, architects, preservationists and stonemasons. Enke, Stuttgart 1991, ISBN 3-432-99461-3 .
- Roland Vinx: Rock determination in the field. 2nd Edition. Springer-Verlag, Heidelberg 2008, ISBN 978-3-8274-1925-5 .
