Ore Deposits Formed by Oxidation and Supergene Enrichment

·       When ore deposits are exposed to the oxidation zone they are weathered and altered with the country rocks.

·       The surface waters oxidize many ore minerals and yield solvents that dissolve other minerals.

·       An orebody thus becomes oxidized and generally leached of many of its valuable materials down to the groundwater table, or to  depth where oxidation cannot take place.

·       The effects oxidation may, however, extend far below the one of oxidation.

·       As the cold, dilute, leaching solutions trickle downwards, they may lose a part or all of their metallic content within the zone of oxidation to give rise to oxidized ore deposits.

·       The oxidized or near-surface part of an orebody is made colorful due to the oxidation of sulfides to oxides and sulfates.

·       As the down trickling solutions penetrate the water table, their metallic content may be precipitated in the form of secondary sulfides to give rise to a zone of secondary or supergene sulfide enrichment.

·      The lower, unaffected part of the orebody is called the hypogene zone.

·      In some places the supergene zone is absent and in rare cases the oxidized zone may be shallow or lacking (as in some glaciated areas undergoing rapid erosion).

·      Special conditions of time, climate, physiographic development and amenable ores are necessary for the process of oxidation and supergene enrichment to be effective.

·      Such ores occur in most of the non-glaciated land areas of the world.

 

Secondary enrichment

An especially important class of residual deposit is formed by both the removal of valueless material in solution and the solution and redeposition of valuable ore minerals. Because solution and redeposition can produce highly enriched deposits, the process is known as a secondary enrichment.

Secondary enrichment can affect most classes of ore deposit, but it is notably important in three circumstances:

1.      The first circumstance arises when gold-bearing rocks--even rocks containing only traces of gold--are subjected to lateritic weathering. Under such circumstances, the gold can be secondarily enriched into nuggets near the base of the laterite. The importance of secondary enrichment of gold in lateritic regions was realized only during the gold boom of the 1980s, especially in Australia.

2.      The second circumstance involves mineral deposits containing sulfide minerals, especially copper sulfides, that are subjected to weathering under desert conditions. Sulfide minerals are oxidized at the surface and produce sulfuric acid, and acidified rainwater then carries the copper, as copper sulfate, down to the water table. Below the water table, where sulfide minerals remain unoxidized, any iron sulfide grains present will react with the copper sulfate solution, putting iron into solution and precipitating a copper mineral. The net result is that copper is transferred from the oxidizing upper portion of the deposit to that portion at and just below the water table. Secondary enrichment of porphyry copper deposits in the southwestern United States, Mexico, Peru, and Chile is an important factor in making those deposits ores. Lead, zinc, and silver deposits are also subject to secondary enrichment under conditions of desert weathering.

3.      The third circumstance in which secondary enrichment is important involves Banded Iron Formations and sedimentary manganese deposits. A primary BIF may contain only 25 to 30 percent iron by weight, but, when subjected to intense weathering and secondary enrichment, portions of the deposit can be enriched to as high as 65 percent iron. Some primary BIFs are now mined and beneficiated under the name taconite, but in essentially all of these deposits mining actually commenced in the high-grade secondary-enrichment zone. Sedimentary manganese deposits, especially those formed as a result of submarine volcanism, must also be secondarily enriched before they become ores.

 

Effects of Oxidation & Supergene Enrichment:

·      Effects of oxidation on mineral deposits are profound - the minerals are altered and the structure is obliterated.

·      The metallic substances are leached or altered to new compounds which require different metallurgical treatment for their extraction unlike that employed for the extraction for the unoxidized ore.

·      The texture and type of deposits are obscured.  Compact ores are rendered cavernous, ubiquitous limonite obscures everything and imparts to the gossan the familiar rusty color.  The effects are therefore:

1)   To render barren the upper parts of many ore deposits.

2)   To change minerals into more usable or less usable form or to make rich bonanzas.

3)   Supergene enrichment may add much where there was little.

4)   Leaner parts of the vein may be made rich.

5)   Unworkable protore may be enriched to the ore grade. E.g. many of the copper districts would not have come into existence except for the process of enrichment.

 

·      Water with dissolved and entangled oxygen is the most powerful oxidizing agent, but carbon dioxide also plays n important role.

·      Locally chlorides, bromides and iodides also play an important role.

·      These substances react with certain minerals to yield strong solvents, such as ferric sulfate and sulfuric acid.

·      Sulfuric acid, in turn, reacting with sodium chloride yields hydrochloric acid, with which iron yields the strongly oxidizing ferric chloride.

·      Bacteria also promote oxidation, they oxidize ferrous iron to ferric iron at low pH.

 

Oxidation & Solution in the Zone of Oxidation:

·      Supergene oxidation and reduction enrichment go hand in hand.  Without oxidation there can be no supply of solvents from which minerals may later be precipitated in the two zones.

·      The process operates in three stages:

1)   Oxidation & solution in the zone of oxidation

2)   Deposition in the zone of oxidation

3)   Supergene sulfide deposition

 

Chemical Changes:

·      There are two main chemical changes within the zone of oxidation:

a)    Oxidation, solution and removal of the valuable material.

b)   Transformation, in situ, of metallic minerals into oxidized compounds.

 

·      Most metallic minerals contain pyrite, which rapidly yields sulfur to form iron sulfate and sulfuric acid:

                        FeS2 + 7O + H2O →FeSO4 + H2SO4

                                2FeSO4 + H2SO4 + O → Fe2(SO4)3 + H2O

 

·      The ferrous sulfate readily oxidizes to ferric sulfate and ferric hydroxide:

                        6FeSO4 + 3O + 3H2O → Fe2(SO4)3 + 2Fe(OH)3

 

·      The ferric sulfate hydrolizes to ferric hydroxide and sulfuric acid:

                        Fe2(SO4)3 + 6H2O → 2Fe(OH)3 + 3H2SO4

 

·      Ferric sulfate is also a strong oxidizing agent and attacks pyrite and other sulfides to yield more ferrous sulfate:

                        Fe2(SO4)3 + FeS2 →3FeSO4 + 2S

 

·      The ferric hydroxide changes over to hematite and goethite and forms the ever present “limonite” that characterizes all oxidized zones;

·      The part played by ferric sulfate as a solvent can be seen by the following reactions:

            Pyrite               FeS2 + Fe2(SO4)3 → 3FeSO4 + 2S

            Chalcopyrite    CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S

            Chalcocite       Cu2S + Fe2(SO4)3 →CuSO4 + 2FeSO4 + CuS

            Covellite          CuS + Fe2(SO4)3 →2FeSO4 + S

            Sphalerite         ZnS + 4Fe2(SO4)3 + H2O →ZnSO4 + 8FeSO4 + 4H2SO4

            Galena              PbS + Fe2(SO4)3 + H2O + 3O →PbSO4 + 2FeSO4 + H2SO4

            Silver                2Ag + Fe2(SO4)3  Ag2SO4 + 2FeSO4

 

·      Most of the sulfates formed are readily soluble, and these cold dilute solutions slowly trickle downwards through the deposit till the proper Eh-pH conditions are met to cause deposition of their metallic content.

·      If pyrite is absent in deposits undergoing oxidation, only minor mounts of solvents are formed, and the effects are mild.  This is illustrated in the New Cornelia Mine, Ajo, Arizona.

·      A country rock of limestone tends to inhibit migration of some sulfate solutions.

 

Deposits in the zone of oxidation: When the oxidised zone is well developed and the secondary minerals sufficiently concentrated, it is a highly profitable zone to mine as the processing is much cheaper and easier and the metals more concentrated. However, most oxidised zones have been mined because they formed outcrops of easily identifiable gossans. The most common minerals found in oxidised zones are: 

Copper: malachite, azurite, chrysocolla

Gangue minerals: quartz (usually cryptocrystalline), baryte, calcite, aragonite

Iron: goethite, hematite

Lead: anglesite, cerussite

Manganese: pyrolusite, romanechite, rhodochrosite

Nickel: gaspeite, garnierite

Silver: native silver, chlorargyrite

Zinc: smithsonite

Deposits in the zone of supergene enrichment:  In the supergene zone metals are concntrated in a narrow band just below the water table. This is the richest part of an ore deposit but in many instances, is either only very thin or not developed at all. The most common minerals found in supergene zones are:

Copper: chalcocite, bornite

Lead: supergene galena

Nickel: violarite

Silver: acanthite, native silver

Zinc: supergene sphalerite, wurtzite

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