Field [mineral deposit]

In geology, the term “field” refers to a mineral deposit containing a metal or other valuable resources in a cost-competitive concentration. It is usually used in the context of a mineral deposit from which it is convenient to extract its metallic component. The deposits are exploited by mining in the case of solid mineral deposits (such as iron or coal) and extraction wells in case of fluids (such as oil, gas, etc.).

Any rock can be understood as a deposit (general deposit), but mostly only those deposits including useful minerals are considered, that is, susceptible to be conveniently exploited (special deposits). A deposit is defined as syngeneic or epigenetic according to whether the minerals that make it up were formed at the same time or after the rock, while it is called primary if the conditions of the deposit have not changed since its origin, secondary if it is the result of the concentration of minerals from the disintegration of other deposits. Secondary deposits often have a higher concentration of useful minerals than the relative primary deposits because of the mechanical selection operated by the running waters during transport; the minerals of these deposits (for example, gold, platinum, cassiterite, etc.) have remarkable characteristics of resistance to degradative actions.

Classification

The classification of the fields can be variously set, but it is particularly important the genetic one, that is linked to the processes responsible for the formation of the minerals constituting the fields themselves and that therefore presents close analogies with the subdivisions of the petrographic systematics. In this regard, fields are divided into magmatic, sedimentary and metamorphic. Magmatic fields derive from consolidation inside the earth’s crust, at considerable depth (plutonic fields) or at a weak depth (subvolcanic fields), or outside (volcanic fields), of magmatic masses.

According to their belonging to a particular stage of magmatic consolidation we distinguish the fields in orthomagmatic or liquidomagmatic, syngenetic fields connected with processes of separation and concentration of minerals at high temperatures, above 750 ºC, such as for example spinels (chromite and magnetite), iron oxides, metal sulfides and sulfosals (pyrrhotine, chalcopyrite), and some primary fields of platinum, diamond and graphite; pegmatitic, fields represented by apophyses or strands branching off from sialic masses and characterized by the presence of crystalline individuals of considerable and sometimes enormous dimensions, as for example occurs in the deposits of mica, beryl, noble tourmalines, topaz, etc.. pegmatitic-pneumatolitic, fields represented mainly by strands, among which typical are those of cassiterite and wolframite; pneumatolitic, fields resulting from the direct crystallization of supracritical solutions, as for example molybdenite, graphite, auriferous and lead-argentiferous fields, or from metasomatosis phenomena operated by such solutions on the incasing rocks, as observed in many leadzinciferous, stanniferous, auriferous, platiniferous fields, etc. hydrothermal, very widespread fields, consequent to the filling of fractures in the embedding rock by minerals deposited by superheated aqueous solutions and that therefore appear in the form of strands; most of the metalliferous mineral deposits exploited on a worldwide scale are of this type.

To the pneumatolitic-hydrothermal phenomenology of weak depth can refer also the exhalation fields: solfataras, geysers, boriferous blowholes, etc.. Based on the dependence relationships between plutons and relative constituent magmatic masses, the fields can be subdivided into endomagmatic, perimagmatic, apomagmatic and telemagmatic, terms that indicate a progressive departure from the intrusive feeder body and thus a gradual decrease in temperature and pressure values during the relative genetic processes, identifiable respectively in the orthomagmatic-pegmatitic, pegmatitic-pneumatolitic, high-temperature hydrothermal, and medium-temperature hydrothermal consolidation phases.

Sedimentary fields lend themselves to subdivisions similar to those of the corresponding rocks and therefore we can distinguish exogenous or mechanical or clastic or placer fields, which include, for example, diamond, auriferous, platiniferous, stanniferous alluvium and other minerals characterized by insolubility, high hardness and high specific weight; fields of chemical origin, such as evaporitic, gypsum-sulphur, sideritic, limonitic, manganesiferous, etc.; fields of surface alteration, such as those of the sedimentary rocks. ; surface or eluvial alteration fields, such as in particular bauxite; organogenic deposits, formed by the remains of animal or vegetable organs fixative of certain minerals and including most limestones, characteristic siliceous formations such as kieselguhr, tripoli radiolarites, and phosphoritic deposits; biochemical fields, formed by the contribution of various microorganisms (limonitic deposits, solfare); organic-biochemical fields, of fundamental economic importance because they include fossil carbons and natural hydrocarbons.

The metamorphic fields can be part of the broad framework of mineralogical transformations operated by regional metamorphism, such as deposits of talc, graphite, asbestos, magnesite, steatite, etc.. ; or depend on metamorphic recrystallization of organogenic rocks, as occurs with crystalline marbles and quartzites, or be the result of thermal metamorphism or dimetasomatism, and in that case the fields present characters analogous to those of magmatic deposits of hydrothermal genesis (e.g., many lead-zinciferous deposits and many of the most important magnetite, hematite, and even pyrite deposits). In terms of the emplacement and distribution of useful minerals, fields can be classified as veins, impregnations, lenses, clusters, and metasomatic and contact deposits.

Again, fields can be differentiated according to the environment of formation or in relation to the depth at which they are located in the earth’s crust: there are thus underwater or subaerial fields, which can be further divided into aeolian, glacial, lacustrine, marine, etc., and epicrostal deposits, hypoabyssal and abyssal. Finally, we can mention the practical classification, based on the possibility of use of extractable minerals and that therefore distinguishes fields in metalliferous and non-metalliferous, the former divided into ferrous and non-ferrous (heavy, light, precious, rare metals, radioactive), the latter, depending on their use, in construction materials (building stones, aerial and hydraulic binders), in materials for the ceramic and glass industry, in abrasive materials, refractory materials, fluxes, dyes, bleaches, degreasers, absorbents, insulators, dry lubricants, in fertilizers, in basic raw materials for the large chemical industry, in fuels (solid, liquid and gaseous), in precious stones, gems and semi-precious stones, etc.. For techniques relating to the identification of useful fields, see prospecting; for techniques for the exploitation of deposits, see cultivation, logging, quarrying, mining.

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