Topic 9 supergene enrichment

Topic 9: Supergene Enrichment
Hassan Z. Harraz
hharraz2006@yahoo.com
2012- 2013
22 November 2015
Prof. Dr. H.Z. Harraz Presentation
Supergene Enrichment
1

22 November 2015
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Outline of Topic 9:
We will explore all of the above in Topic 9.
 Definition
 Weathering Processes and Supergene ore Deposits
 Conditions that Influence Supergene Enrichment
 Ideal Starting Material
 Zones:
 Morphology of Zoning
 Gossans and Cappings
 Oxidation and Solution in Zone of Oxidation
 Effects of Oxidation on Mineral Deposits
 Chemical Changes Involved
 Electrowinning
 Why do we need pyrite?
 Structural Control
 Formation of Copper Oxides

Definition
 Secondary or supergene enrichment where leaching of materials occurs
and precipitation at depth produces higher concentrations.
 Kinds from:
 Concentrating minerals by chemical weathering processes.
 Residual Mineral Deposits
 An existing mineral deposit can be turned in to a more highly
concentrated mineral deposit by weathering in a process.
 Remarkable special case of weathering
 Definition from Evans, 1993: “leaching of valuable elements from the
upper parts of mineral deposits and their precipitation at depth to
produce higher concentrations.”
 Definition from Guilbert and Park, 1985: Supergene enrichment occurs
when oxidizing acids dissolve metal ions from the “protore” and re-
deposits it in more reducing, basic areas, i.e. below the water table. This
results in an oxidized zone on top (gossan), a supergene zone beneath and
the hypogene (protore) beneath that.
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Exploring Sulfide Ore Deposits - Gossan
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Weathering processes and Supergene ore Deposits
 Sulfide ore bodies have been subjected to weathering at or near the
surface of the Earth after eons of erosion removed overlying rocks.
 Sulfide minerals are not stable at the earth’s surface and
breakdown during weathering liberating metallic ions (e.g., Cu2+,
Pb2+, Zn2+, Ag2+).
 The surface waters oxidize many ore minerals and yield solvents that
dissolve other minerals.
 An ore deposit thus becomes oxidised and generally leached of many of its
valuable materials down to the groundwater table, or to a depth where
oxidation cannot take place.
 These ions may precipitate as oxides, carbonates and sulfates
above the water table to form the secondary copper, lead, zinc and
silver deposits.
 Where copper ions reach the water table and react with primary
sulphides, supergene copper deposits form that are dominated by
copper sulphides.
1) Oxidation and reduction enrichment go hand in hand.
2) Without oxidation there can be no supply of solvents from which minerals
may later be precipitated in the zones of oxidation or of supergene sulfides.
3) The process resolves itself into three stages:
(i) Oxidation and solution in the zone of oxidation,
(ii) Deposition in the zone of oxidation, and
(iii) Supergene sulfide deposition.
Each is considered separately
22 November 2015 Prof. Dr. H.Z. Harraz Presentation
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4) Ideal starting material
For this to happen, the rock (starting material)
needs to be:
 Porous and permeable.
 Contains abundant pyrite.
 Contains acid soluble ore-metal-minerals.
 Underlain by precipitative environment.
Can apply to many transition metals but Cu is the
outstanding example.
Acidic oxidizing solutions will dissolve many minerals.
Basic reducing conditions at or below groundwater
table will precipitate.
Conditions that influence supergene enrichment
 Active chemical weathering with ground level lowered by
erosion.
 Weathering under acidic (carbonated water) and
oxidizing conditions.
 Permeability and Porous
 Composition of the ore from the standpoint of chemistry
of the solution, chemical environment;
 Contains abundant pyrite.
 Contains acid soluble ore-metal-minerals.
 Time.
 Deep water table imposing reducing conditions.
 Latitude and altitude, and depth of water level
 Climate and physiographic development
 Restricted to non-glacial terranes.

Zones:
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Ideally, them, there are three fundamental zones:
1)Oxidized zone  leached
2) Supergene zone  enriched
3) Hypogene zone {or Protore (parent)}
+ Gossans and Cappings

Morphology of Zoning
 Zone of oxidation :
 The oxidized part .
 The region above the water-table in an ore deposit is known as the oxidised zone as it is the zone of oxidation of the primary ore minerals.
 The effects of oxidation may extend far below the zone of oxidation (The zone of oxidized ore is generally above the water table).
 As the cold, dilute, leaching solutions trickle downward they may lose a part or all of their metallic content within the zone of oxidation and
give rise to oxidized ore deposits.
 This oxidised zone is primarily composed of mixtures of iron oxides/hydroxides and quartz which we call gossan.
 Most primary ore minerals (particularly the sulfide minerals) are only stable in anaerobic dry environments. With the rise and fall of the
water-table and downward percolating rainwater (containing dissolved oxygen), these minerals dissolve and new minerals (oxide zone
minerals) are precipitated in the gossan. With the dissolution of sulfide minerals, the water becomes acidic, further enhancing the
dissolution of the ore.
 Most of the spectacular minerals we see from ore deposits are those formed in the oxidised zone. 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 in the past because they formed outcrops of easily
identifiable stained gossans
 Zone of secondary or supergene sulfide enrichment:
 Immediately below the oxidised zone is sometimes a zone known as the supergene zone where metals are deposited by fluids percolating
downwards from the oxidised zone and concentrating in a narrow band just below the water table.
 The supergene zone is the richest part of an ore deposit but in many instances, is either only very thin or not developed at all.
 The zone of supergene sulphides is in general below the water level deposits owe their economic success to this process.
 Reducing zone.
 If 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.
 Best sulfide ores (Covellite and Chalcocite)
Primary or hypogene zone :
 Reducing zone
 The lower part of the deposit.
 Protore (parent or original) part of the deposit.
 Unaltered, primary, and disseminated sulfide minerals (pyrite, chalcopyrite, sphalerite, Bornite).
Note:
 This zonal arrangement is characteristic of many mineral deposits that have undergone long-continued weathering.
 In places the supergene sulfide zone may be absent, and in rare cases the oxidized zone is shallow or lacking, as in some glaciated areas or
regions undergoing rapid erosion.
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The most common minerals found in oxidised zones are:
Copper: malachite, azurite, chrysocolla
Gangue minerals: quartz (usually cryptocrystalline),
barite, calcite, aragonite
Iron: goethite, hematite
Lead: anglesite, cerussite
Manganese: pyrolusite, romanechite, rhodochrosite
Nickel: gaspeite, garnierite
Silver: native silver, chlorargyrite
Zinc: smithsonite
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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Morphology of Zoning

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Replacement Textures

Gossans and Cappings
 The leached, oxidized surface exposures of weathered
sulphide deposits.
 is a heavy concentration of "limonitic" material, derived
from massive sulfide minerals or from their iron yielding
gossan, which has been leached in place and
transported downward.
 are not limited to the surface but nay extend some
distance below the surface (In some cases the gossan
has contained sufficient economic mineralization to
warrant mining).
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Gossan (16 cm x 9 cm) BHP mine, Broken
Hill, NSW. Photo: S Humphreys ©
Australian Museum.

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Effects of Oxidation on Mineral Deposits:
 The minerals are altered and the structure is obliterated.
 The metallic substances are leached or are altered to new compounds
that require metallurgical treatment for their extraction quite unlike
that employed for the unoxidized materials.
 The texture and the type of deposit are obscured  Compact ores are
made cavernous.
 Water (H2O) with dissolved and entangled oxygen is the most
powerful oxidising reagent, but carbon dioxide (CO2) also plays an
important role (Locally chlorides, iodides, and bromides play a part).
 These substances react with certain minerals to yield strong solvents
(such as ferric sulfate {Fe2(SO4)3} and sulfuric acid {H2SO4 }).
 Sulfuric acid {H2SO4 }, in turn, reacting with sodium chloride (NaCl)
yields hydrochloric acid (HCl), which with iron yields the strongly
oxidizing ferric chloride.
 Bacteria also promote oxidation; they oxidise ferrous iron at low pH to
ferric sulfate {Fe2(SO4)3 }.

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2 FeS2 + 7O2+ 2H2O  2FeSO4 (aq) + 2H2SO4
FeSO4 (aq) + 6H2O  2Fe(OH)3 + 3H2SO4
Cu2S (Chalcocite )+SO2 + 4H+  Cu2+ +2(SO4)2- + 2H2O
5 FeS2 + 14 Cu +2 + 14 SO4
-2 + 12 H2O 7 Cu2S + 5 Fe+2 + 24 H+ + 17 SO4
-2
Cu +2 + ZnS  CuS + Zn+2
pyrite + water + oxygen  hydroxide + acids
sulfate + sulfide  sulfate + sulfide
Chalcocite Cu2S
element + oxygen oxide
Sulfide + water  sulfate + sulfuric acid
sulfide + acid  sulfate
Copper
(ppm)
Iron
(%)
Depth(m)
Water Table

CHEMICAL CHANGES INVOLVED
Most metallic mineral deposits contain pyrite. This mineral under attack readily yields
sulfur to form iron sulfate acid; pyrrhotite does the same. The following reactions are
suggested to indicate, without intermediate steps, their general trend:
(1) 2 FeS2 + 7O2+ 2H2O  2FeSO4 (aq) + 2H2SO4
(2) 2FeSO4 (aq) + H2SO4 + 0.5 O2 Fe2(SO4)3 (aq) + H2O
Reaction 2 passes through intermediate stages during which S, SO2, and FeSO4, may form.
The sulfur may oxidize to sulfuric acid. The ferrous sulfate readily oxidizes to ferric sulfate
and ferric hydroxide:
(3) 6FeSO4 + 3O + 3H2O  2Fe2(SO4)3 + 2Fe(OH)3
The ferric sulphate hydrolyses to ferric hydroxide and sulfuric acid:
(4) FeSO4 (aq) + 6H2O  2Fe(OH)3 + 3H2SO4
Ferric sulfate is also a strong oxidising agent and attacks pyrite and other sulfides to yield
more ferrous sulfate.
(5) FeSO4 (aq) + FeS2  3FeSO4 (aq) + 2S
Ferric sulfate, in addition, changes to various "basic sulfates.
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There are two main chemical changes within the zone of oxidation:
i) the oxidation, solution, and removal of the valuable minerals, and
ii) the transformation in situ of metallic minerals into oxidized compounds.

 See figures:
 Gossan
 May be subdued expression of topography
 Development related to phyllic altered zone
 Shifting water tables
 Hydrolysis, hydration accompany oxidation
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Cu2+
Cu2+

Cations
H+
Fe2+
Zn2+
Cu2+
Anions
OH-
Negative
Neutral
“Self Potential”
effect
e-
e-
Elements Cations
H+
Fe2+
Zn2+
Cu2+
Anions
OH-
Electrowinning
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CHEMICAL CHANGES INVOLVED (Cont.)
22 November 2015
Supergene Enrichment 16
 The above reactions indicate the importance of pyrite, which
yields the chief solvents, ferric sulfate and sulfuric acid, and
also ferric hydroxide and basic ferric sulfates (Reactions 1 and 4).
 Moreover, ferric sulfate is continuously being regenerated not
only from pyrite, but also from chalcopyrite and other sulfides.
 The ferric hydroxide changes over to hematite and goethite
and forms the ever-present "limonite" that characterizes all
oxidised zones.
 The basic ferric sulfates, of which there are several, may be
deposited as such, but; generally limonite is the end product.

Leaching reactions
2FeS2 + 7O2+ 2H2O  2FeSO4 (aq) + 2H2SO4
2FeSO4 (aq) + H2SO4 + 0.5 O2  Fe2(SO4)3 (aq) + H2O
 2FeS2 + 7.5O2+ 4H2O  Fe2O3 + 4H2SO4
2Fe+2 (aq) + ½ O2 + 2H2O  Fe2O3 + 4H+
 2CuFeS2 + 8.5O2 +2H2O Fe2O3 + 2Cu+2 + 4SO4
-2 + 4H+
Or
2CuFeS2 + 8Fe2(SO4)3 + 8H2O CuSO4 + 17FeSO4 + 8H2SO4
During the precipitation phase, the pyrite is again important,
because Cu replaces Fe.
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In general:
5FeS2 + 14Cu +2 + 14SO4
-2 + 12H2O 7Cu2S + 5Fe+2 + 24H+ + 17SO4
-2

CHEMICAL CHANGES INVOLVED (cont.)
The part played by ferric sulfate as a solvent may be seen in the following equations (Although the
end products are obtained, it is not established in all cases that the following reactions are those that
actually take place):
(6) Pyrite : FeS2 + Fe(SO4)3  3FeSO4 + 2S
(7) Chalcopyrite: CuFeS2 + 2Fe2(SO4)3  CuSO4 + 5FeSO4 + 2S
(8) Chalcocite: Cu2S + Fe2(SO4)3  CuSO4 + 2FeSO4 + CuS
(9) Covellite: CuS + Fe2(SO4)3  2FeSO4 + CuSO4 + S°
(10) Sphalerite: ZnS + 4Fe2(SO4)3 + 4H2O  ZnSO4 + 8FeSO4 +4H2SO4
(11) Galena: PbS +Fe2(SO4)3 + H2O + 3O  PbSO4 + 2FeSO4 +H2SO4
(12) Silver: 2Ag + Fe2(SO4)3  Ag2SO4 + FeSO4
If pyrite is absent from deposits undergoing oxidation, only minor amounts of the
solvents are formed; little solution occurs, the sulfides tend to be converted in situ
into oxidized compounds, and the hypogene sulfides are not enriched.
A country rock of limestone tends to inhibit migration of some sulfate solutions; it immediately reacts
with copper sulfate, for example, to form copper carbonates, thus precluding any
supergene sulfide enrichment.
During the oxidation processes:
Alumina-silicate minerals are leached of silica and the oxidized material becomes clay by
hydrogen ion metasomatism.
The leached silica may exist as a gel or a cryptocrystalline material incorporated with various
amounts of iron oxide dispersed through the silica.
This material is jasper; jasperoid is a prime prospecting tool used in function with the study
of gossans and alteration.
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Step 1:
 FeS2 (Pyrite) +7.5O2 + 3.5H2O  Fe(OH)3 (Iron hydroxide) + 2(SO4)2- + 4H+
 Cu2S (Chalcocite )+ SO2 + 4H+  2Cu2+ + 2(SO4)2- + 2H2O
 Cu2+ +ZnS (Sphalerite)  CuS (Covellite) + Zn2+
 14Cu2+ + 5FeS2 (Pyrite) +12H2O  7Cu2S (Chalcocite ) + 5Fe2+ + 3(SO4)2- + 24H+
 FeS2 (Pyrite) + O2 + H2O + CO2  Fe(OH)3 + H2SO4 + H2CO3 (Carbonic acid)
 CuFeS2 (Chalcopyrite) + O2 + H2O + CO2 Fe(OH)3 + CuSO4 (Copper sulfate) + H2SO4 +
H2CO3
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Step 2:
 Galena (PbS)  Covellite (CuS) + Anglesite
 Pyrite (FeS2)  Chalcocite (Cu2S)
 Chalcopyrite  Covellite (CuS)
Examples:

Formation of Copper Oxides
 Copper ore bodies have been subjected to weathering at or near the surface of the Earth
after eons of erosion removed overlying rocks.
 Oxygenated groundwater, derived from rainwater, trickles through fractures in the rock and
forms a leaching zone where chalcopyrite, a primary sulfide mineral, is dissolved and
oxidized.
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Formation of Copper Oxides

 The iron hydroxide (called limonite) forms a residual deposit known as a gossan (or iron hat) typically rust-red
in color, used by prospectors as indicators of underlying mineralization.
FeS2 (Pyrite) + 7.5O2 + 3.5H2O  Fe(OH)3 (Iron hydroxide )+ 2(SO4)2- + 4H+
CuFeS2 (Chalcopyrite) + O2 + H2O + CO2  Fe(OH)3 + CuSO4 (Copper sulfate) + H2SO4 + H2CO3 (Carbonic acid)
2CuFeS2 + 8.5O2 + 2H2O ↔ Fe2O3 + 2Cu+2 + 4SO4
-2 + 4H+
CuFeS2 + 8Fe2(SO4)3 + 8H2O ↔ CuSO4 + 17FeSO4 + 8H2SO4
Result: Copper in solution, red hematite gossan remains
 In Leached Zone (carbonates, including malachite, azurite): leaching of copper are provided according to the
following reaction:
CuFeS2 + O2 + H2O + CO2 Fe(OH)3 + CuSO4 + H2SO4 + H2CO3
Cu2S (Chalcocite) + SO2 + 4H+  2Cu2+ + 2(SO4)2- + 2H2O
The copper sulfate and carbonic acid continue trickling through the fractures and react with the chalcopyrite to form
copper oxides, for example:
CuS (Covellite) + 2O2 + H2O  CuO (Tenorite) + H2SO4
CuFeS2 + 2O2 + H2CO3  CuO (Tenorite) + FeCO3 (iron carbonate) + H2SO4
 These oxides form an oxidation zone, 0 - 200 m in thickness above the water table.
 Below the water table is a zone of supergene enrichment in which secondary sulfide minerals such as covellite
and chalcocite form from chalcopyrite and copper sulfate in solution:
CuFeS2 + CuSO4  2CuS (Covellite) + FeSO4
 These kinds of reactions in the supergene zone greatly increase the concentration of copper.
 Native copper might also occur in this zone.
 A lot of the Bagdad and Miami (Arizona) mine production is from copper oxides. Very little of the sulfides
covellite and chalcocite are in the Bagdad ore.
 Groundwater circulation can lead to re-distribution of metals above the water table.
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Formation of Copper Oxides (cont.)

2-5 Geophysical SurveyingEND OF TOPIC 9

Questions
Secondary enrichment process: Supergene enrichment:
• See thought map puzzle:
• Go through thought map puzzles.
 Where oxides and carbonates of Cu {tenorite CuO,
cuprite Cu2O}, azurite Cu(CO3)2(OH)2 and malachite
Cu2(CO3(OH)2}, and chrysocolla (silicate)?
 Where sulfides (reduced, chalcocite, bornite (Cu5FeS4),
native Cu)?
 Talk about Gossan, the iron hat. also Cu oxide-
carbonate minerals as prospecting tool.
 Talk about electrowinning, throwing old metal in for
autoelectrowinning.
22 November 2015
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Grab a problem:
• Chalcocite (Cu2S) to ions
• Chalcocite (Cu2S) to cuprite (Cu2O) and Tenorite
(CuO).
• Cuprite (Cu2O) and native Cu to Tenorite (CuO)
• Tenorite(CuO) to azurite{Cu(CO3)2(OH)2} or
malachite {Cu2CO3(OH)2}
• Galena (PbS) to anglesite (PbSO4)
• Sphalerite (ZnS) to ions
• Acanthite (Ag2S) to ions
• ZnS + limestone to gypsum + smithsonite
22 November 2015
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Start with Pyrite:
 Balance a reaction between pyrite and oxygenated groundwater
forming dissolved species:
FeS2 + O2 + H2O ↔ FeSO4(aq) + ?
2FeS2 + 7O2 + 2H2O ↔ 2FeSO4(aq) + 2H2SO4(aq)
Still need to oxidize the ferrous Fe to ferric:
FeSO4(aq) + H2SO4(aq) + O2 ↔ Fe2(SO4)3(aq) + ?
2FeSO4(aq) + H2SO4(aq) + 0.5O2 ↔ Fe2(SO4)3(aq) + H2O
• 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 ↔ Fe2Ο3 + 4H2SO4(aq)
• or how about:
2Fe+2(aq) + 0.5O2 + 2H2O ↔ Fe2O3 + 4H+
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Ex: Chalcopyrite Oxidation
aCuFeS2 ↔ zFe2O3
What other reactants?
aCuFeS2 + bO2 + cH2O ↔ zFe2O3 + ?
What other products and required stoichiometry?
aCuFeS2 + bO2 + cH2O ↔ a/2Fe2O3 + aCu+2 + 2aSO4
-2 + 2cH+
Now start balancing.
2CuFeS2 + bO2 + cH2O ↔ 1Fe2O3 + 2Cu+2 + 4SO4
-2 + 2cH+
Deal with any other cations.
2CuFeS2 + bO2 + 2H2O ↔ Fe2O3 + 2Cu+2 + 4SO4
-2 + 4H+
Balance oxygens and check charges.
-2 + 4H+
• Result: Copper in solution, red hematite gossan remains
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Chalcopyrite Oxidation - 2
CuFeS2 + ? ↔ CuSO4(aq) + ?
Reactants? Products? Redox pairs? Balance?
aCuFeS2 + bFe2(SO4)3 + cH2O ↔ zCuSO4 + yFeSO4 + xH2SO4
…..
CuFeS2 + 8Fe2(SO4)3 + 8H2O ↔ CuSO4 + 17FeSO4 + 8H2SO4
Compare to previous slide:
-2 + 4H+
22 November 2015
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Bornite Oxidation (Cu5FeS4)
Cu5FeS4 + ? ↔ Fe2O3 + ?
2Cu5FeS4 + ? ↔ Fe2O3 + 10Cu+2 + 8SO4
-2 + ?
charge balance: +20 -16
2Cu5FeS4 + 4H+ ↔ Fe2O3 + 10Cu+2 + 8SO4
-2 + 2H2O
balance oxygens:
2Cu5FeS4 + yO2 + 4H+ ↔ 3 32 2
finally
2Cu5FeS4 + 18.5O2 + 4H+ ↔ Fe2O3 + 10Cu+2 + 8SO4
-2 + 2H2O
Cu5FeS4 + 4H+ + 9O2 ↔ 5Cu+2 + Fe+2 + 4SO4
-2 + 2H2O
• Compare to chalcopyrite:
-2 + 4H+
22 November 2015
28

Topic 9 supergene enrichment

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