Microscopic view of skarn under crossed polarizersHand sample of skarn containing serpentinite from the edge of the Alta Stock, Little Cottonwood Canyon, Utah
Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-hydrothermal fluids.[1] Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminiumsilicate minerals, which are also referred to as calc-silicate minerals.[2][3][4][5] These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a graniticpluton found in and around faults or shear zones that commonly intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional or contact metamorphism and therefore form in relatively high temperature environments.[2][3][4][5] The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; magmatic, metamorphic, meteoric, marine, or even a mix of these.[4] The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.[4]
If a skarn has a respectable amount of ore mineralization that can be mined for a profit, it can be classified as a skarn deposit.[2][3][4]
Etymology
Skarn is an old Swedish mining term originally used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits apparently replacing Palaeoproterozoic age limestones in Sweden's Persberg mining district.[6]
A skarn is formed by a variety of metasomatic processes during metamorphism between two adjacent lithologic units. Skarns can form in almost any rock type such as shale, granite, or basalt but the majority of skarns are found in carbonate rocks containing limestone or dolomite. It is common to find skarns near plutons, along faults and major shear zones, in shallow geothermal systems, and on the bottom of the sea floor.[4] The specific mineralogy of skarns are highly related to the mineralogy of the protolith.[7]
Skarns can be subdivided depending on specific criteria. One way to classify a skarn is by its protolith. If the protolith is of sedimentary origin, it can be referred to as an exoskarn and if the protolith is igneous, it can be called an endoskarn.[3][4]
Further classification can be made based on the protolith by observing the skarn's dominant composition and the resulting alteration assemblage. If the skarn contains minerals such as olivine, serpentine, phlogopite, magnesium clinopyroxene, orthopyroxene, spinel, pargasite, and minerals from the humite group, it is characteristic of a dolomitic protolith and can be classed as a magnesian skarn. The other class, called calcic skarns, are the replacement products of a limestone protolith with dominant mineral assemblages containing garnet, clinopyroxene, and wollastonite.[3]
Rocks that contain garnet or pyroxene as major phases, and that are also fine-grained, lack iron, and have skarn-like appearances, are generally given the term "skarnoid". Skarnoid is therefore the intermediate stage of a fine-grained hornfels and a coarse-grained skarn.[3][4]
Skarn ore deposits
Metal ore deposits that have skarn as gangue are called skarn deposits and can form by any combination of closed metamorphism or open system metasomatism, although most skarn deposits are thought to be related to magmatic-hydrothermal systems.[1] Skarn deposits are classified by their dominant economic element, e.g., a copper (Cu) skarn deposit or a molybdenum (Mo) skarn deposit.[2][3][5]
Fe (Cu, Ag, Au) skarn deposits
The tectonic setting for calcic Fe skarns tends to be the oceanic island arcs. The host rocks tend to range from gabbro to syenite associated with intruding limestone layers. The tectonic setting for magnesium Fe skarns tends to be the continental margin. The host rocks tend to be granodiorite to granite associated with intruding dolomite and dolomitic sedimentary rocks. Magnetite is the principal ore in these types of skarn deposits which its grade yields from 40 to 60%. Chalcopyrite, bornite and pyrite constitute minor ores.[9][10]
Cu (Au, Ag, Mo, W) skarn deposits
The tectonic setting for Cu deposits tends to be the Andean-type plutons intruding older continental-margin carbonate layers. The host rocks tend to be quartz diorite and granodiorite. Pyrite, chalcopyrite and magnetite are typically found in higher abundances.[9][10]
Formation
Generally, there are two types of skarns that form, exoskarns and endoskarns.[11]
Exoskarns are more common and form on the outside of an intrusive body that comes into contact with a reactive rock unit. They are formed when fluids left over from the crystallisation of the intrusion are ejected from the mass at the waning stages of emplacement, in a process called boiling. When these fluids come into contact with reactive rocks, usually carbonates such as limestone or dolomite, the fluids react with them, producing alteration (infiltration metasomatism).[4]
Endoskarns form within the intrusive body where fracturing, cooling joints, and stockworks have been produced, which results in a permeable area. This permeable area can be altered by fluids originally sourced from the intrusion itself, after interacting with surrounding rocks (protolith). Thus, both the composition and the textures of protoliths strongly play a role in the formation of the resulting skarn. Endoskarns are considered to be rare. [4]
Reaction skarns are formed from isochemical metamorphism occurring on thinly interlayered sedimentary units, via small-scale[lower-alpha 1] metasomatic exchange between adjacent units.[4][12]
Skarnoids are calc-silicate rocks that are fine-grained and iron poor. Skarnoids tend to be found between hornfels and coarse-grained skarn.[13][14][15] Skarnoids commonly reflect the composition of the protolith.[4]
Most large skarn deposits experience a transition from early metamorphism—which forms hornfels, reaction skarns, and skarnoids—to late metamorphism, which forms relatively coarser grained, ore-bearing skarns. The magma intrusion triggers contact metamorphism in the surrounding region, forming hornfels as a result. The recrystallization and phase change of hornfels reflects the composition of the protolith. After the formation of hornfels, metasomatism occurs involving hydrothermal fluids from a source that is magmatic, metamorphic, marine, meteoric, or even a mix of these. This process is called isochemical metamorphism, and can result in the production of a wide range of calc-silicate minerals that form in impure lithology units and along fluid boundaries where small-scale metasomatism occurs (argillite and limestone, and banded iron formation).[2][3]
The skarn deposits that are considered economically important for containing valuable metals are a result of large-scale metasomatism, where the composition of fluid controls the skarn and its ore mineralogy. They are relatively coarser grained and do not strongly reflect the composition of protolith or surrounding rocks.[3][4]
Uncommon types of skarns are formed in contact with sulfidic or carbonaceous rocks such as black shales, graphite shales, banded iron formations and, occasionally, salt or evaporites. Here, fluids react less via chemical exchange of ions, but because of the redox-oxidation potential of the wall rocks.[4]
Ore is natural rock or sediment that contains one or more valuable minerals concentrated above background levels, typically containing metals, that can be mined, treated and sold at a profit. The grade of ore refers to the concentration of the desired material it contains. The value of the metals or minerals a rock contains must be weighed against the cost of extraction to determine whether it is of sufficiently high grade to be worth mining, and is therefore considered an ore. A complex ore is one containing more than one valuable mineral.
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.
Pentlandite is an iron–nickel sulfide with the chemical formula (Fe,Ni)9S8. Pentlandite has a narrow variation range in nickel to iron ratios (Ni:Fe), but it is usually described as 1:1. In some cases, this ratio is skewed by the presence of pyrrhotite inclusions. It also contains minor cobalt, usually at low levels as a fraction of weight.
Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.
Wollastonite is a calcium inosilicate mineral (CaSiO3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolomite is subjected to high temperature and pressure, which sometimes occurs in the presence of silica-bearing fluids as in skarns or in contact with metamorphic rocks. Associated minerals include garnets, vesuvianite, diopside, tremolite, epidote, Plagioclase feldspar, pyroxene and calcite. It is named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.
Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are caused by fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.
Carbonatite is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geochemical verification.
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.
Talc carbonates are a suite of rock and mineral compositions found in metamorphosed ultramafic rocks.
Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate formations and which share a common genetic origin.
The Broken Hill Ore Deposit is located underneath Broken Hill in western New South Wales, Australia, and is the namesake for the town. It is arguably the world's richest and largest zinc-lead ore deposit.
Iron oxide copper gold ore deposits (IOCG) are important and highly valuable concentrations of copper, gold and uranium ores hosted within iron oxide dominant gangue assemblages which share a common genetic origin.
The Cornubian batholith is a large mass of granite rock, formed about 280 million years ago, which lies beneath much of Cornwall and Devon in the south-western peninsula of Great Britain. The main exposed masses of granite are seen at Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land's End and the Isles of Scilly. The intrusion is associated with significant quantities of minerals particularly cassiterite, an ore of tin which has been mined since about 2000 BC. Other minerals include china clay and ores of copper, lead, zinc and tungsten.
Millerite is a nickel sulfide mineral, NiS. It is brassy in colour and has an acicular habit, often forming radiating masses and furry aggregates. It can be distinguished from pentlandite by crystal habit, its duller colour, and general lack of association with pyrite or pyrrhotite.
Phyllic alteration is a hydrothermal alteration zone in a permeable rock that has been affected by circulation of hydrothermal fluids. It is commonly seen in copper porphyry ore deposits in calc-alkaline rocks. Phyllic alteration is characterised by the assemblage of quartz + sericite + pyrite, and occurs at high temperatures and moderately acidic conditions.
A calc–silicate rock is a rock produced by metasomatic alteration of existing rocks in which calcium silicate minerals such as diopside and wollastonite are produced. Calc–silicate skarn or hornfels occur within impure limestone or dolomite strata adjacent to an intruding igneous rock.
Hydrothermal mineral deposits are accumulations of valuable minerals which formed from hot waters circulating in Earth's crust through fractures. They eventually create metallic-rich fluids concentrated in a selected volume of rock, which become supersaturated and then precipitate ore minerals. In some occurrences, minerals can be extracted for a profit by mining. Discovery of mineral deposits consumes considerable time and resources and only about one in every one thousand prospects explored by companies are eventually developed into a mine. A mineral deposit is any geologically significant concentration of an economically useful rock or mineral present in a specified area. The presence of a known but unexploited mineral deposit implies a lack of evidence for profitable extraction.
1 2 3 4 5 6 7 8 9 10 Ray, G.E., and Webster, I.C.L. (1991): An Overview of Skarn Deposits; in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera; McMillan, W.J., compiler, B. C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 213-252.
↑ Korzhinskii, D.S. (1948). "Petrology of the Tur'insk skarn deposits of copper". 68 (10). Ser. Rundnykh Mestorozhdenii. Academy nauk SSSR: Institute of Geology Nauk Trudy: 147.{{cite journal}}: Cite journal requires |journal= (help)
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