What is a deposit in geology ?

Deposit : definition

A deposit is a site of concentration of one or more elements or objects (deposit of minerals, oil, fossil...).

However, in geology, the term deposit is now often taken in a restrictive economic sense and denotes a concentration of substances which can be exploited profitably. We then speak of deposits of gold, copper, vanadium, diamond, petroleum, barite, etc...

Deposit types are very different :

Contact deposit : designates various types of mineral concentrations located in contact or near a granular (granite in particular) or microgranular rock, and formed by thermal and chemical action of the fluids resulting from the magma on the surrounding rocks. This term includes veins, impregnations, and especially large metalliferous clusters formed by replacement of carbonate rock : skarns. The term "contact deposit" is often used more restrictively as a synonym for pyrometasomatic deposit.
Eluvial deposit : consists of the on-site accumulation of certain very resistant minerals (rutile, cassiterite, beryl, platinum, etc...), resulting from the destruction by erosion of the source rocks. An alluvial deposit can be derived from it, by transporting these minerals in running water and then accumulating in alluvial sediments.
Epithermal deposit : mineralized deposit linked to aerial volcanism. The hot fluids (150°C to 280°C), salty, and rich in metals that accompany the volcanic phenomenon will deposit the metals contained in the foundations of the volcano and in the surrounding rocks, creating veins or epithermal deposits rich in gold, silver, lead, zinc and / or copper. Many of the gold veins of the Andes, the Rocky Mountains (USA), the Philippines or Indonesia are formed in this way.
Hydrothermal deposit : deposit formed by the action of hydrothermal fluids. The process that generates hydrothermalism can be of various origins, mainly volcanism and plutonism, but also metamorphism, basin fluids, etc...
Pneumatolytic deposit : mineral deposit generated at the end of the crystallization of a magma, between 374°C (critical water temperature), and around 600°C, for example when the water of the mineralizing fluids was in the supercritical state. These ultimate magmatic fluids concentrate most of the gases (water vapor, CO2, H2S, etc...) and rare magma metals (B, Be, F, Li, La, Ce, Nb, Sn, Mo, Bi, etc...) which are expressed in the form of pneumatolytic minerals, mainly silicates and oxides (beryl, tourmaline, topaz, cassiterite, lepidolite, etc...), more rarely phosphates (monazite, amblygonite, etc...) and sulphides and sulphosalts (molybdenite, arsenopyrite, löllingite, etc...). The pneumatolytic deposits mainly include pegmatites, greseins or veins, frequently located in fractures on the immediate periphery of the generating granite. The hydrothermal stage follows the pneumatolytic stage by a simple drop in temperature. Other minerals will then precipitate (sulphides of lead, zinc, copper, etc...).
Pyrometasomatic deposit : designates certain contact deposits around granites. Useful minerals were formed at high temperature by recrystallization and partial replacement of rocks in the surrounding area under the action of mineralized fluids from granite magma. A limestone context favors the formation of such deposits which then take the name of skarns or tactites. The pyrometasomatic deposits can constitute important deposits of iron (magnetite), tungsten (scheelite), copper, etc... The pyrometasomatic deposit is one of the contact deposits.
Residual deposit : is a geological formation resulting from the alteration of a pre-existing rock, which has lost by dissolution a significant part of its constituents. The remaining residual rock is therefore relatively enriched in certain minerals or insoluble elements, an enrichment which may be sufficient to constitute exploitable mineral concentrations, called "residual deposits". Bauxite, for example, is an aluminum ore formed by on-site weathering of rocks rich in aluminous silicates (mainly feldspars and feldspathoids). Aluminum released by weathering reprecipitates on site as insoluble aluminum hydrates and concentrates, while silicon, calcium and potassium are removed. Other minerals that are completely insoluble under surface conditions (zircon, cassiterite, columbite, etc...) will concentrate mechanically on site, while the other minerals are dissolved and the salts formed are evacuated. The content of these minerals can therefore increase until they form exploitable residual deposits.
Sedimentary deposit : designates the metal concentrations generated by sedimentary processes (red-beds, kupferschiefer, BIF, etc...).


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