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.
lower, unaffected part of the orebody is called the hypogene zone.
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).
conditions of time, climate, physiographic development and amenable ores are
necessary for the process of oxidation and supergene enrichment to be effective.
occur in most of the non-glaciated land areas of the world.
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.
enrichment can affect most classes of ore deposit, but it is notably important
in three circumstances:
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.
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.
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 &
of oxidation on mineral deposits are profound - the minerals are altered and the
structure is obliterated.
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.
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:
render barren the upper parts of many ore deposits.
change minerals into more usable or less usable form or to make rich bonanzas.
enrichment may add much where there was little.
parts of the vein may be made rich.
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.
with dissolved and entangled oxygen is the most powerful oxidizing agent, but
carbon dioxide also plays n important role.
chlorides, bromides and iodides also play an important role.
substances react with certain minerals to yield strong solvents, such as ferric
sulfate and sulfuric acid.
acid, in turn, reacting with sodium chloride yields hydrochloric acid, with
which iron yields the strongly oxidizing ferric chloride.
also promote oxidation, they oxidize ferrous iron to ferric iron at low pH.
Oxidation & Solution in the
Zone of Oxidation:
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.
process operates in three stages:
& solution in the zone of oxidation
in the zone of oxidation
two main chemical changes within the zone of oxidation:
solution and removal of the valuable material.
in situ, of metallic minerals into oxidized compounds.
metallic minerals contain pyrite, which rapidly yields sulfur to form iron
sulfate and sulfuric acid:
FeS2 + 7O + H2O →FeSO4 + H2SO4
+ H2SO4 + O → Fe2(SO4)3
ferrous sulfate readily oxidizes to ferric sulfate and ferric hydroxide:
6FeSO4 + 3O + 3H2O → Fe2(SO4)3
ferric sulfate hydrolizes to ferric hydroxide and sulfuric acid:
Fe2(SO4)3 + 6H2O → 2Fe(OH)3 +
sulfate is also a strong oxidizing agent and attacks pyrite and other sulfides
to yield more ferrous sulfate:
Fe2(SO4)3 + FeS2 →3FeSO4
ferric hydroxide changes over to hematite and goethite and forms the ever
present “limonite” that characterizes all oxidized zones;
played by ferric sulfate as a solvent can be seen by the following reactions:
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
PbS + Fe2(SO4)3 + H2O + 3O
→PbSO4 + 2FeSO4 + H2SO4
2Ag + Fe2(SO4)3 →
Ag2SO4 + 2FeSO4
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.
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.
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
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
Silver: acanthite, native silver
Zinc: supergene sphalerite, wurtzite
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