• Special
•
•
•
Overview
weathering
process
Requires initial
ore body
Oxidizing
groundwater
leaches metals
Percolates down
to water table
and deposits ore
minerals
Supergene Features
•Acidic oxidizing solutions
are very reactive
-
high solubility for many
metals
•Groundwater generally
basic/neutral, reducing
solution
-
•-
lower solubility
Three zones form:
-
Oxidized zone (leached,
gossan)
Supergene zone (enriched)
Protore (primary, unaltered)
Supergene Enrichment
Supergene Features
• Ideal starting material:
-
must be porous, permeable
contain abundant pyrite
contains acid-soluble ore-metal minerals
• Many metals can be enriched; Cu is prime
example
• What sort of deposits will benefit most?
Replacement textures
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Supergene Features
•-Gossan/Leached zone
-
subdued expression of
topography
• water table
related to phyllic alteration
zone
• hydrolysis accompanies
oxidation
•Water table shifts over
time
-
pattern may be complex
Schürmann’s Solubility Series
•Palladium
•Mercury
•Silver
•Copper
•Bismuth
•Cadmium
•Antimony
•Tin
•Lead
•Zinc
•Nickel
•Cobalt
•Iron
•Arsenic
•Thallium
•Manganese
•Higher elements have
greater sulfur affinity
-
can replace elements lower
down in sulfides
•Sulfides of higher elements
have lower solubilities
•Would you expect to see
Chalcocite (Cu2S) replacing
galena (PbS), or vice-versa?
Reactions
• Balance a reaction between pyrite and
oxygenated groundwater forming dissolved
species:
-
FeS2 + O2 + H2O
-
Balanced:
FeSO4(aq) + ?
• 2FeS2 + 7O2 + 2 H2O
2FeSO4(aq) + 2H2SO4(aq)
• Still need to oxidize the ferrous Fe to ferric:
-
FeSO4(aq) + H2SO4(aq) + O2
-
2FeSO4(aq) + H2SO4(aq) + 0.5O2
Fe2(SO4)3(aq) + ?
Fe2(SO4)3(aq) +
H2O
Mineral abundance & timing
Cu/Fe Oxides, Sulfates
Cu/Fe Sulfides
Example: Chalcopyrite Oxidation
aCuFeS2
zFe2O3
aCuFeS2 + bO2 + cH2O
What other reactants?
What other products
and required
stoichiometry?
zFe2O3 + ?
a/2Fe2O3 + aCu+2 +
aCuFeS2 + bO2 + cH2O
2aSO4-2 + 2cH+
2CuFeS2 + bO2 + cH2O
1Fe2O3 + 2Cu+2 +
4SO4-2 + 2cH+
2CuFeS2 + bO2 + 2H2O
4SO4-2
+
1Fe2O3 + 2Cu+2 +
4H+
2CuFeS2 + 8.5O2 + 2H2O
1Fe2O3 + 2Cu+2 +
4SO4-2 + 4H+
Now start balancing.
Deal with any other
cations.
Balance oxygens and
check charges.
Result: Copper in
solution, red hematite
gossan remains
Reactions 2
• Pyrite may be dissolved as ferrous ion:
-
2FeS2 + 7O2 + 2H2O
2FeSO4(aq) + 2H2SO4(aq)
• and then oxidized to ferric ion:
-
2FeSO4(aq) + H2SO4(aq) + 0.5O2
Fe2(SO4)3(aq) +
H2O
• or converted directly to hematite:
-
•
2FeS2 + 7.5O2 + 4H2O
Fe2O3 + 4H2SO4(aq)
or how about:
-
2Fe+2(aq) + 0.5O2 + 2H2O
Fe2O3 + 4H+