ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

deposit

geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, supergene processes or enrichment are those that occur relatively near the surface as opposed to deep

hypogene In ore deposit geology, hypogene processes occur deep below the Earth's surface, and tend to form deposits of primary minerals, as opposed to supergene processes that occur at or near the surface, and tend to form secondary minerals. At great d ...

processes. Supergene processes include the predominance of

meteoric water Meteoric water, derived from precipitation such as snow and rain, includes water from lakes, rivers, and ice melts, all of which indirectly originate from precipitation. The journey of meteoric water from the atmosphere to the Earth's surface is a ...

circulation (i.e. water derived from

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

) with concomitant

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

and chemical

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

. The descending meteoric waters

oxidize Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

the primary (hypogene) sulfide ore minerals and redistribute the metallic ore elements. Supergene ''enrichment'' occurs at the base of the oxidized portion of an ore deposit. Metals that have been leached from the oxidized ore are carried downward by percolating groundwater, and react with hypogene sulfides at the supergene-hypogene boundary. The reaction produces secondary sulfides with metal contents higher than those of the primary ore. This is particularly noted in

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

ore deposits where the copper sulfide minerals

chalcocite Chalcocite (), copper(I) sulfide (Cu2S), is an important copper ore mineral. It is opaque and dark gray to black, with a metallic luster. It has a hardness of 2.5–3 on the Mohs scale. It is a sulfide with a monoclinic crystal system. T ...

(Cu₂S),

covellite Covellite (also known as covelline) is a rare copper sulfide mineral with the formula CuS. This indigo blue mineral is commonly a secondary mineral in limited abundance and although it is not an important ore of copper itself, it is well known ...

(CuS),

digenite Digenite is a copper sulfide mineral with formula: Cu9S5. Digenite is a black to dark blue opaque mineral that crystallizes with a trigonal–hexagonal scalenohedral structure. In habit it is usually massive, but does often show pseudo-cubic fo ...

(Cu₁₈S₁₀), and djurleite (Cu₃₁S₁₆) are deposited by the descending surface waters.Guilbert, John M. and Charles F. Park Jr (1986) ''The Geology of Ore Deposits'', W. H. Freeman, All such processes take place at essentially atmospheric conditions, around room temperature (25 °C) and standard

atmospheric pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...

(1 atm).Manual of Mineralogy (1993) Klein and Hurlbut. Wiley

Zones

Distinct zones of supergene processes can be identified at various depths. From the surface down they are the gossan cap, leached zone, oxidized zone, water table, enriched zone (supergene enriched zone) and primary zone (hypogene zone).Understanding Mineral Deposits (2000). Kula C Misra. Kluwer Academic Publishers

Gossan cap

Pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

(FeS₂) has oxidised to form

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

(FeO(OH)) and

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...

(FeO(OH)·''n''H₂O), which form a porous covering over the oxidized zone known as a

gossan Gossan (eiserner hut or eisenhut) is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the ''classic'' gossan or iron cap all that remains is iron oxides and quartz, often ...

cap or iron hat.The Encyclopedia of Gemstones and Minerals (1991). Martin Holden. Publisher: Facts on File Prospectors use gossan as an indication of ore reserves.

Leached zone

Groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...

contains dissolved

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

. As it travels downwards it oxidizes primary

sulfide minerals The sulfide minerals are a class of minerals containing sulfide (S2−) or disulfide () as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the ar ...

, concomitant with forming

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

and

solutions Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solutio ...

of oxidized metals.Field Guide to North American Rocks and Minerals (1992) The Audubon Society. Alfred A Knopf For example, groundwater commonly interacts with pyrite (FeS₂) to form an oxidized iron (FeO(OH)) and sulfuric acid (H₂SO₄), portrayed in the idealized chemical reaction below (intermediate steps omitted): : An intermediate in this process is

ferric sulfate Iron(III) sulfate or ferric sulfate (British English: sulphate instead of sulfate) is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of water of crystallization, hydrates are known, including the most commonly encount ...

(Fe₂(SO₄)₃), which oxidizes pyrite and other sulfide minerals.

Oxidized zone

Above the water table the environment is

oxidizing Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

, and below it is reducing.John Rakovan (2003) Rocks & Minerals 78:419 Solutions traveling downward from the leached zone react with other primary minerals in the oxidised zone to form secondary minerals such as

sulfates The sulfate or sulphate ion is a Polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...

and

carbonates A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group . ...

, and

, which is a characteristic product in all oxidised zones. In the formation of secondary carbonates, primary

sulfide mineral The sulfide minerals are a class of minerals containing sulfide (S2−) or disulfide () as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenide mineral, selenides, the tell ...

s generally are first converted to sulfates, which in turn react with primary carbonates such as

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

(CaCO₃), dolomite (CaMg(CO₃)₂) or

aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

(also CaCO₃, polymorphic with calcite) to produce secondary carbonates. Soluble salts continue on down, but insoluble salts are left behind in the oxidised zone where they form. Examples of insoluble salts that are commonly found in the oxidized zone include

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

precipitates In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...

anglesite Anglesite is a lead sulfate mineral with the chemical formula PbSO4. It occurs as an oxidation product of primary lead sulfide ore, galena. Anglesite occurs as prismatic orthorhombic crystals and earthy masses, and is isomorphous with barite and ...

(PbSO₄) and

pyromorphite Pyromorphite is a mineral species composed of lead chlorophosphate: Pb5( P O4)3 Cl, sometimes occurring in sufficient abundance to be mined as an ore of lead. Crystals are common, and have the form of a hexagonal prism terminated by the basal p ...

(Pb₅(PO₄)₃Cl);

precipitates like

malachite Malachite () is a copper Carbonate mineral, carbonate hydroxide mineral, with the chemical formula, formula Basic copper carbonate, Cu2CO3(OH)2. This opaque, green-banded mineral crystallizes in the monoclinic crystal system, and most often for ...

(Cu₂(CO₃(OH)₂),

azurite Azurite or '' Azure spar'Krivovichev V. G.'' Mineralogical glossary. Scientific editor A. G. Bulakh. — St.Petersburg: St.Petersburg Univ. Publ. House. 2009. — 556 p. — ISBN 978-5-288-04863-0. ''(in Russian)'' is a soft, deep-blue copp ...

(Cu₃(CO₃)₂(OH)₂), and

cuprite Cuprite is an oxide mineral composed of copper(I) oxide Cu2O, and is a minor ore of copper. Its dark crystals with red internal reflections are in the isometric system hexoctahedral class, appearing as cubic, octahedral, or dodecahedral forms, ...

(Cu₂O); and

smithsonite Smithsonite, also known as zinc spar, is the mineral form of zinc carbonate ( Zn CO3). Historically, smithsonite was identified with hemimorphite before it was realized that they were two different minerals. The two minerals are very similar in a ...

(ZnCO₃).

Water table

At the

water table The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...

the environment changes from an

environment to a reducing one.

Enriched zone

Copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

that move down into this reducing environment form a zone of supergene

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

enrichment.

Covellite Covellite (also known as covelline) is a rare copper sulfide mineral with the formula CuS. This indigo blue mineral is commonly a secondary mineral in limited abundance and although it is not an important ore of copper itself, it is well known ...

(CuS),

(Cu₂S) and

native copper Native copper is an uncombined form of copper that occurs as a natural mineral. Copper is one of the few metallic elements to occur in native form, although it most commonly occurs in oxidized states and mixed with other elements. Native coppe ...

(Cu) are stable in these conditions and they are characteristic of the enriched zone. The net effect of these supergene processes is to move metal ions from the leached zone to the enriched zone, increasing the concentration there to levels higher than in the unmodified primary zone below, possibly producing a deposit worth mining.

Primary zone

The primary zone contains unaltered primary minerals.

Mineral alterations

Chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a Mohs scale, hardness of 3.5 to 4 ...

CuFeS₂ (primary) readily alters to the secondary minerals

bornite Bornite, also known as peacock ore, is a sulfide mineral with chemical composition that crystallizes in the orthorhombic system (pseudo-cubic). It is an important copper ore. Appearance Bornite has a brown to copper-red color on fresh surfa ...

Cu₅FeS₄,

CuS and

brochantite Brochantite is a sulfate mineral, one of a number of cupric sulfates. Its chemical formula is Cu4SO4(OH)6. Formed in arid climates or in rapidly oxidizing copper sulfide deposits, it was named by Armand Lévy (mineralogist), Armand Lévy for his ...

Cu₄SO₄(OH)₆.

Galena Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver. Galena is one of the most abundant and widely distributed sulfide minerals. It crysta ...

PbS (primary) alters to secondary

PbSO₄ and

cerussite Cerussite (also known as lead carbonate or white lead ore) is a mineral consisting of lead carbonate with the chemical formula PbCO3, and is an important ore of lead. The name is from the Latin ''cerussa'', white lead. ''Cerussa nativa'' was ...

PbCO₃.

Sphalerite Sphalerite is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in Sedimentary exhalative deposits, sedimentary exhalative, Carbonate-hoste ...

ZnS (primary) alters to secondary

hemimorphite Hemimorphite is the chemical compound Zinc, Zn4(Pyrosilicate, Si2O7)(Hydroxide, OH)2Water of crystallization, ·H2O, a component of mineral Calamine (mineral), calamine. It is a silicate mineral which, together with smithsonite (ZnCO3), has bee ...

Zn₄Si₂O₇(OH)₂.H₂O,

ZnCO₃ and

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

-bearing

willemite Willemite is a zinc silicate mineral () and a minor ore of zinc. It is highly fluorescent (green) under shortwave ultraviolet light. It occurs in a variety of colors in daylight, in fibrous masses and apple-green gemmy masses. Troostite is a vari ...

Zn₂SiO₄.

FeS₂ (primary) alters to secondary

melanterite Melanterite is a mineral form of hydrous iron(II) sulfate: FeSO4·7H2O. It is the iron analogue of the copper sulfate chalcanthite. It alters to siderotil by Dehydration, loss of water. It is a secondary sulfate mineral which forms from the oxida ...

FeSO₄.7H₂O. If the original deposits contain

arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...

and

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

bearing minerals, secondary

arsenates The arsenate is an ion with the chemical formula . Bonding in arsenate consists of a central arsenic atom, with oxidation state +5, double bonded to one oxygen atom and single bonded to a further three oxygen atoms. The four oxygen atoms orien ...

and

phosphates Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphor ...

will be formed.

Etymology

The word ''supergene'' is derived from the Latin root meaning 'above' and the Greek root ''-gene'' () meaning 'born' or 'produced'. The terms ''supergene'' and ''hypogene'' refer to the depth at which they occur.

References

{{Authority control Economic geology