Skarn is a contact-metasomatic rock that occurs near intrusion , if the host rocks sharply differ from the intrusive rocks in chemical composition. Skarn is a reaction product of carbonate and aluminosilicate rocks in contact with each other with the participation of high-temperature post-magmatic solutions under conditions of heating by the introduced silicate (most often acidic) magma . The word skarn comes from the Swedish skarn (literally - dirt, garbage).
According to the mineralogical and chemical composition, two types of skarns are distinguished: calcareous and magnesian. Limestone skarns are formed in the zone of contact of silicate rocks with limestones, magnesian - with dolomites.
Lime Skarn
Mineral composition
Calcium-containing minerals are characteristic of calcareous skarns. The main minerals are monoclinic pyroxene and calcium garnet , often in association with magnetite , hematite and epidote . Calcite is contained . Pyroxene typically has a diopside-hedenbergite composition. Diopside in calcareous skarns can form large crystals or a sugar-like aggregate of a dull green or even white color. Garnet - grossular - andradite . Sometimes vesuvian or wollastonite skarns are found. Also calcareous skarns may contain sulfides ( galena , sphalerite , arsenopyrite , chalcopyrite , molybdenite ), boron minerals ( datolite , danburite , axinite ), rhodonite .
Structure: crystalline, granular, blast. Texture: massive, heterogeneous, spotty.
Conditions of education and location
They form deposits, lenses, bodies of irregular shape, zones, in direct contact of silicate and carbonate rocks ( limestones ).
Magnesian Skarns
Mineral composition
Magnesian skarns are characterized by magnesium-containing minerals. First of all, these are diopside , spinel , forsterite , phlogopite and dolomite . Of the accessory ones, apatite is most often found, often forming large crystals of bluish-green color. Sphene and radioactive orthite may occur. Exposure to radioactive radiation from orthite changes the color of sphene to black. Scapolite , which replaces crystals of potassium feldspar, and hornblende , which develops according to diopside, is also characteristic. In more ferruginous rocks, instead of pure forsterite , olivine with a fayalite mineral content is present.
Conditions of education and location
They form zones, deposits, lenses, vein bodies at the contact of granitoids with dolomites and dolomite marbles , or with igneous rocks of ultrabasic composition ( dunites , harzburgites ).
Skarn Minerals
Industrial skeletons of ores and non-metallic minerals are often confined to skarn. In this regard, a special genetic type of deposits is distinguished - skarn, which is of great industrial importance.
Deposits of iron , copper , tungsten - molybdenum ( Tyrnyauz in the North Caucasus), lead and zinc , and boron are associated with calcareous skarn. Deposits of skarn type iron are usually characterized by not very large reserves (although there are exceptions, such as Magnitnaya in the Urals) and high quality ore, usually composed of magnetite , less often hematite . Lead-zinc mineralization is associated not so much with skarning as with the subsequent hydrothermal study of skarn bodies. Large deposits of phlogopite (for example, Slyudyanka in the southern Baikal region) and magnetite (Toyskoye in Mountain Shoria) are associated with magnesian skarns.
Literature
- Einaudi MT, & Burt DM, 1982. Introduction, terminology, classification and composition of skarn deposits. Economic Geology, 77 , pp. 745–754.
- Evans, AM, 1993. Ore Geology and Industrial Minerals: An Introduction 3rd ed. Blackwell Publishing ISBN 0-632-02953-6
See also
- Skarna