Chapter Six
Supergene and Secondary
Enrichment Deposits
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6. Supergene & secondary enrichment deposits
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• Occur near the surface as descending meteoric waters
leached metals from the leached zone and carried
downward and react with hypogene sulfides at the
supergene-hypogene boundary.
 The reaction produces secondary enrichment sulfides with
metal contents higher than those of the primary ore.
• Different zones can be identified at different depths into
gossan cap, leached zone, oxidized zone, water table,
enriched zone (supergene enriched zone) and primary
zone (hypogene zone).

a) Gossan cap (iron hat)
• If pyrite FeS2 is abundant near the surface and oxidizes to
insoluble compounds goethite FeO(OH), forming a porous
covering to the oxidized zone.
b) Leached zone
• If the water contains dissolved oxygen, carbon dioxide,
and as it travels downwards it leaches out the minerals
from the host rocks to form sulfuric acid, and other
solutions that continue moving downwards
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CONT…

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CONT…

c) Oxidized zone
• Above the water table the environment is oxidizing,
and below it is reducing.
• Solutions travelling downward from the leached zone
react with other primary minerals in the oxidized zone
to form secondary minerals.
E.g.,
 Malachite (Cu2(CO3)(OH)2) azurite (Cu3(CO3)2(OH)2)
cuprite Cu2O, pyromorphite Pb5(PO4)3Cl and smithsonite
ZnCO3 are stable in oxidizing conditions.
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CONT…

d) Water table
• At the water table the environment changes from an oxidizing to a
reducing environment.
e) Enriched zone
• Copper ions that move down into the reducing environment form a
zone of supergene sulfides enrichment.
Covellite CuS, Chalcocite Cu2S and native copper 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 than in the unmodified primary zone.
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CONT…

f) Primary zone
• The primary zone contains unaltered primary minerals.
Primary minerals
 Is any mineral formed during the original crystallization
of the host igneous primary rock.
 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.
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CONT…

Mineral alterations
• Chalcopyrite CuFeS2 (primary) alters to the secondary minerals
bornite Cu5FeS4, covellite CuS.
Bornite
• Show brown to copper-red color on fresh surfaces that tarnishes to various
iridescent shades of blue to purple in places.
• Its copper ore mineral.
• Also chalcopyrite and bornite are replaced by chalcocite and covellite in the
supergene enrichment zone of copper deposits.
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CONT…

Covellite (CuS)
• Covellite is a rare copper sulfide
mineral.
• The mineral is generally found in
zones of secondary enrichment
(supergene) of copper sulfide
deposits.
Galena (PbS)
• Galena (primary) alters to secondary
anglesite PbSO4 and cerussite
PbCO3
• It is the ore of lead metal.
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CONT…

Sphalerite (ZnS)
• Sphalerite (primary) alters to secondary
hemimorphite Zn4Si2O7(OH)2.H2O,
smithsonite ZnCO3 and manganese-
bearing willemite Zn2SiO4.
• Is an ore of zinc metal.
• When the iron content is high,
sphalerite is an opaque black variety
called marmatite (Zn,Fe)S.
Pyrite (FeS2) or iron pyrite
• Pyrite (primary) alters to secondary
melanterite FeSO4.7H2O.
• The well-known nickname of fool's
gold. 10
CONT…
Sphalerite - Colorado, USA

a) Residual deposits
Residual deposits
 Caused entirely by surficial chemical weathering over
bedrock or by the alteration of a pre-existing rock.
During chemical weathering bedrock is greatly reduced
in volume by the process of leaching its constituents or
ions have been lost by dissolution and which has
therefore become enriched in aluminum.
Elements that are not leached from the rock, thus occur
in higher concentration in the residual rock.
 Resources from residual mineral deposits: Al (Bauxite), Ni
(Nickeliferous laterite), Co, Ni, Cu, Fe, kaolinite
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CONT…
Bauxite (Al2O3.2H2O)
 IAlsos results from a superficial alteration of rocks in a tropical
climate.
 Is a residual rock rich in aluminum oxides and exploited as
aluminum ore.
 It is composed of a mixture of diaspore (aluminum oxide hydroxide
mineral or α-AlO(OH)), boehmite (aluminium oxide hydroxide (γ-
AlO(OH)) mineral) and gibbsite (aluminum hydroxide mineral of the
oxides and hydroxides group, [Al(OH)3].

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Source Rocks
Bauxite deposits are formed from rocks relatively high in
aluminium silicates.
Examples
 Arkansas, Brazil, and Guinea deposits have been formed
from nepheline syenite,
 French deposits at bauxite from limestone's or clays in
limestone,
 Ghana deposits from clay shales and other aluminous
rocks,
 Indian deposits from basalt, and
 Thailand deposits from clay alluvium.
CONT…

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