ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...

deposit geology, 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 dept ...

processes. Supergene processes include the predominance of

meteoric water Meteoric water is the water derived from precipitation (snow and rain). This includes water from lakes, rivers, and icemelts, which all originate from precipitation indirectly. While the bulk of rainwater or meltwater from snow and ice reaches the ...

circulation (i.e. water derived from precipitation) with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize 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 ore deposits where the copper sulfide minerals chalcocite (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 (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 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 1013.25 millibars, ...

(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 giv ...

(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 (FeO(OH)·''n''H₂O), which form a porous covering over the oxidized zone known as a gossan cap or iron hat.The Encyclopedia of Gemstones and Minerals (1991). Martin Holden. Publisher: Facts on File Prospectors use gossan as an indication ore reserves.

Leached zone

Groundwater contains dissolved oxygen and carbon dioxide. As it travels downwards it oxidizes primary sulfide minerals, concomitant with forming sulfuric acid and solutions 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), is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of hydrates are known, including the most commonly encountered form of "ferric sulfate". Solutions are used in dyeing as a morda ...

(FeSO₄), which oxidizes pyrite and other sulfide minerals.

Oxidized zone

Above the water table the environment is

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

, 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 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 salts of sulfuric acid and many ar ...

and

carbonates A carbonate is a salt of carbonic acid (H2CO3), 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 g ...

, and limonite, which is a characteristic product in all oxidised zones. In the formation of secondary carbonates, primary sulfide minerals generally are first converted to sulfates, which in turn react with primary carbonates such as

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

(CaCO₃),

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

(CaMg(CO₃)₂) or

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

(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

precipitates In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...

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 (Pb₅(PO₄)₃Cl); copper precipitates like malachite (Cu₂(CO₃(OH)₂),

azurite Azurite is a soft, deep-blue copper mineral produced by weathering of copper ore deposits. During the early 19th century, it was also known as chessylite, after the type locality at Chessy-les-Mines near Lyon, France. The mineral, a basic ca ...

(Cu₃(CO₃)₂(OH)₂), and cuprite (Cu₂O); and smithsonite (ZnCO₃).

Water table

At the water table the environment changes from an

environment to a reducing one.

Enriched zone

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

sulfide Sulfide (British English also sulphide) 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 chemical compounds lar ...

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), chalcocite (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 copp ...

(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 hardness of 3.5 to 4 on the Mo ...

CuFeS₂ (primary) readily alters to the secondary minerals bornite Cu₅FeS₄,

CuS and brochantite 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 cryst ...

PbS (primary) alters to secondary

PbSO₄ and cerussite PbCO₃. Sphalerite ZnS (primary) alters to secondary

hemimorphite Hemimorphite is the chemical compound Zn4( Si2O7)( OH)2 ·H2O, a component of mineral calamine. It is a silicate mineral which, together with smithsonite (ZnCO3), has been historically mined from the upper parts of zinc and lead ores. Both com ...

Zn₄Si₂O₇(OH)₂.H₂O, smithsonite ZnCO₃ and manganese-bearing willemite Zn₂SiO₄.

FeS₂ (primary) alters to secondary melanterite FeSO₄.7H₂O. If the original deposits contain

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...

and phosphorus bearing minerals, secondary

arsenates The arsenate ion is . An arsenate (compound) is any compound that contains this ion. Arsenates are salts or esters of arsenic acid. The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V). Arsenate rese ...

and phosphates 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

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