GEOCHEMICAL CONSEQUENCES OF

DIFFERENTIAL SETTLING OF GOLD TAILINGS

Barbara L. Sherriff,

Nikolay Sidenko

University of Manitoba

Canada

X Central Manitoba

Central Manitoba Gold Mine1924-1937Rice Lake ArcheanGreenstone Belt,SE Manitoba

Gold associated with pyrite and chalcopyrite in quartz carbonate veins in metavolcanics

0 500 m

BLUE PONDpH 4.4

GREEN PONDpH 7-8

N

MINEBUILDINGS

WASTEROCK

Central Manitoba Tailings

Points of Discharge of Tailings

PR 204

Orange BrownpH 3-4

GreenpH 5

BurgundypH 7-8

ACID MINE DRAINAGE

Oxidation of sulphides exposed to water and oxygen in waste rock piles or mine workings

Oxidation of pyrite by oxygenFeS2 + 7/2 O2 + H2O

= Fe2+ + 2SO42- + 2H+

Oxidation of Fe2+ to Fe3+

Fe2+ + 1/4 O2 + H+ = Fe3+ + ½H2O

Further oxidation of pyriteFeS2 + 14Fe3+ + 8H2O = 15Fe2+ + 2SO4

2- + 16H+

Neutralization of Acid Rock

Drainage

Carbonate dissolution:

CaCO3 + H+ → Ca2+ + HCO3- pH 7-8

Aluminum hydroxide dissolution:

Al(OH)3 + 3H+ → Al3+ + 3H2OpH 4

At Central Manitoba Mine,Carbonate:Sulphide ratio of unoxidized tailings

varies due to initial differential settling

pH 4.4

pH 6-8

Green Pond

Blue Pond

CO3:S 1:2

CO3:S 2:1

CO3:S 3:1

Discharge Point

Distance from discharge point (m)

20 50 100 130 140 200 250

Net

Neu

tral

izin

g P

oten

tial

-60

-50

-40

-30

-20

-10

0

10

20

Net Neutralizing Capacity vs Distance from Discharge Point

South North

4.0

2.6

2.8

7.6

3430

531

307

0.0

7.1

7.3

7.1

7.5

1.6

8.9

2.1

0.0

3470

4.0

4.8

7.1

7.3

198

0.0

0.0

OrangeBrown

Green

Blue Grey

Burgundy

1G 2G 3G

Vertical Variation in Colour & Pore Water Geochemistry

pHCu (ppm)

South North

Photomicrographs of brochantite (Cu4(SO4)(OH)6) from the green stripe

PPL X polars

50μm

Photomicrographs of blue mineral (possibly Fe cyanide) from the blue stripe (A) PPL (B) X polars

50μm

A

A

B

Vertical Variation in Mineralogy calculatedfrom Saturation Indices of Pore Water

OrangeBrownpH 2 - 4

Green pH 5

Purple BrownpH 7- 8

Blue Grey

Jarosite, Schwertmannite, Goethite

Goethite,Ferrihydrite, MalachiteCalcite

Advance of acidity

Brochantite

Pyrite, ChalcopyriteFe cyanide complexes

pH 4 pH 8

pH 4

pH 4

pH 8

Why are there such variations in surface water chemistry?

BlueStream

BluePond

Green Stream

GreenPond

4.4 4.4 7.5 7.2

0.0 0.0 163 107

1930 1540 143 433

4.7 6.6 0.0 0.0

550 450 77 170

109 116 0.1 0.2

pHHCO3

SO4

AlCaCu

Fe 71 0.2 0.0 0.0

Composition of Stream & Pond water (ppm)

5Fe3+ + 12 H2O → Fe3+5(OH)8.4H2O (ferrihydrite) + 8H+

Buffered at pH 7-8 by

CaCO3 + H+ → Ca2+ + HCO3-

Cu precipitates as malachite (Cu2CO3(OH)2

and chalcanthite (CuSO45H2O) at neutral pH

Green Pond Geochemistry

Blue Pond Stream to Green PondGreen Pond

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3 4 5 6 7 8 9

Tailings Stream to Blue Pond

pSO

4

pH

Ferrihydrite

Schwertmannite

8Fe3+ + SO4-2 + 14H2O → Fe3+

8O8(OH)6(SO4) + 22H+

(schwertmannite)

Blue Pond Geochemistry

Acidity Buffered at 4.4 by Al:3H+ + Al(OH)3 → Al3+ + 3H2O

Cu2+,left in solution gives

blue colourSchwertmannite in precipitate from unacidified water from the Blue Pond

But why is there solid Al(OH)3 in the Blue Pond?

Streams containing Blue Slime only occur where oxidized and unoxidized Tailings are being eroded together.

Pore water from oxidized tailings at pH 2.7, with 330 ppm Al, 3000 ppm Cu +

Pore water from unoxidized tailings at pH 7, with 0 ppm Al, 0 ppm Cu

Blue Stream water at pH 4, with 5 ppm Al, 100 ppm Cu+

Blue slime precipitate with < 30 wt.% Al, < 35 wt.% Cu

Blue Pond pH 4.4, 6 ppm Al, 116 ppm Cu

Blue Pond (pH 4.4, 6 ppm Al)

ReducedTailings(pH 7-8,<0.1ppm Al)

Blue Stream (pH 4.4, 212 ppm Al)

N

Erosion Edge

Oxidized Tailings (pH 2.7, 330 ppm Al)

Redox BoundarySolid SurfaceWater tableWater Flow

2 m

0.2 m

Scale

Al-Cu slimes

The relationship between the Erosion Edge, Oxidized & Reduced tailings, the Blue Pond & Stream.

CONCLUSIONS

• Differential settling of carbonate and sulphide minerals caused the tailings close to the discharge point, to become acidic while the distal portion stayed neutral.

• The acidic front is advancing across the tailings from south to north as the carbonate in the oxidized zone becomes exhausted by acid neutralization and the pH drops below 7

• When the pH is reduced to 5, brochantite precipitates producing a green stripe. This redissolves as the pH is reduced below 4. Fe-oxyhydroxides precipitate to give the orange brown colour.

• Green Pond: acid produced by the precipitation of ferrihydrite is buffered at about pH 7 by carbonate dissolution

• Blue Pond: acid produced by the precipitation of schwertmannite is buffered at pH 4.5 by solid Al(OH)3 dissolution.

ACKNOWLEDGEMENTS

Funding from:Manitoa Sustainable Development and Innovation FundManitoba ConservationNSERC Discovery GrantNSERC//NATO Fellowship

Thanks to many students including:Kristin SalzsaulerDr David TeertstraDana Johnson