GEOCHEMICAL CONSEQUENCES OF DIFFERENTIAL SETTLING OF GOLD TAILINGS Barbara L. Sherriff, Nikolay Sidenko University of Manitoba Canada.
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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
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