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GEOCHEMICAL CONSEQUENCES OF DIFFERENTIAL SETTLING OF GOLD TAILINGS

GEOCHEMICAL CONSEQUENCES OF DIFFERENTIAL SETTLING OF GOLD TAILINGS. Barbara L. Sherriff, Nikolay Sidenko University of Manitoba Canada. Central Manitoba Gold Mine 1924-1937 Rice Lake Archean Greenstone Belt, SE Manitoba

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GEOCHEMICAL CONSEQUENCES OF DIFFERENTIAL SETTLING OF GOLD TAILINGS

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

  2. Central Manitoba Gold Mine 1924-1937 Rice Lake Archean Greenstone Belt, SE Manitoba Gold associated with pyrite and chalcopyrite in quartz carbonate veins in metavolcanics XCentral Manitoba

  3. Central Manitoba Tailings 0 500 m WASTE ROCK PR 204 GREEN POND pH 7-8 BLUE POND pH 4.4 Points of Discharge of Tailings MINE BUILDINGS N

  4. Green pH 5 Burgundy pH 7-8 Orange Brown pH 3-4

  5. ACID MINE DRAINAGE Oxidation of sulphides exposed to water and oxygen in waste rock piles or mine workings Oxidation of pyrite by oxygen FeS2 + 7/2 O2 + H2O = Fe2+ + 2SO42- + 2H+ Oxidation of Fe2+ to Fe3+ Fe2+ + 1/4 O2 + H+ = Fe3+ + ½H2O Further oxidation of pyrite FeS2 +14Fe3+ + 8H2O = 15Fe2+ + 2SO42- + 16H+

  6. Neutralization of Acid RockDrainage Carbonate dissolution: CaCO3 + H+→ Ca2+ + HCO3- pH 7-8 Aluminum hydroxide dissolution: Al(OH)3 + 3H+ → Al3+ + 3H2O pH 4

  7. Discharge Point CO3:S 1:2 CO3:S 2:1 Blue Pond pH 4.4 Green Pond CO3:S 3:1 pH 6-8 At Central Manitoba Mine, Carbonate:Sulphide ratio of unoxidized tailings varies due to initial differential settling

  8. Net Neutralizing Capacity vs Distance from Discharge Point 20 10 0 -10 Net Neutralizing Potential -20 -30 -40 -50 -60 South North 20 50 100 130 140 200 250 Distance from discharge point (m)

  9. 3G 1G 2G Green Burgundy 7.1 4.0 4.0 1.6 Orange Brown 3430 3470 7.1 2.6 4.8 8.9 531 198 2.8 7.3 307 7.1 2.1 0.0 Blue Grey 7.6 7.3 7.5 0.0 0.0 0.0 Vertical Variation in Colour & Pore Water Geochemistry pH Cu (ppm) South North

  10. PPL X polars 50μm Photomicrographs of brochantite (Cu4(SO4)(OH)6) from the green stripe A B A 50μm Photomicrographs of blue mineral (possibly Fe cyanide) from the blue stripe (A) PPL (B) X polars

  11. Vertical Variation in Mineralogy calculated from Saturation Indices of Pore Water Advance of acidity Purple Brown pH 7- 8 Goethite, Ferrihydrite, Malachite Calcite Green pH 5 Orange Brown pH 2 - 4 Jarosite, Schwertmannite, Goethite Brochantite Pyrite, Chalcopyrite Fe cyanide complexes Blue Grey

  12. Why are there such variations in surface water chemistry? pH 4 pH 4 pH 8 pH 8 pH 4

  13. 4.4 7.5 pH 4.4 7.2 0.0 163 HCO3 0.0 107 1930 143 SO4 1540 433 4.7 0.0 Al 6.6 0.0 550 77 Ca 450 170 109 0.1 Cu 116 0.2 71 0.0 Fe 0.2 0.0 Composition of Stream & Pond water(ppm) Green Stream Green Pond Blue Stream Blue Pond

  14. Green Pond Geochemistry 3.2 3.0 Ferrihydrite 2.8 4 pSO 2.6 Schwertmannite 2.4 2.2 2.0 3 4 5 6 7 8 9 pH Tailings Stream to Blue Pond Blue Pond Stream to Green Pond Green Pond 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

  15. Blue Pond Geochemistry 8Fe3+ + SO4-2 + 14H2O → Fe3+8O8(OH)6(SO4) + 22H+ (schwertmannite) Acidity Buffered at 4.4 by Al: 3H+ + Al(OH)3 → Al3+ + 3H2O Cu2+,left in solution gives blue colour Schwertmannite in precipitate from unacidified water from the Blue Pond But why is there solid Al(OH)3 in the Blue Pond?

  16. 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

  17. The relationship between the Erosion Edge, Oxidized & Reduced tailings, the Blue Pond & Stream. Oxidized Tailings (pH 2.7, 330 ppm Al) N Scale 2 m 0.2 m Erosion Edge Redox Boundary Blue Stream (pH 4.4, 212 ppm Al) Solid Surface Water table Water Flow Reduced Tailings (pH 7-8, <0.1ppm Al) Blue Pond (pH 4.4, 6 ppm Al) Al-Cu slimes

  18. 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.

  19. ACKNOWLEDGEMENTS Funding from: Manitoa Sustainable Development and Innovation Fund Manitoba Conservation NSERC Discovery Grant NSERC//NATO Fellowship Thanks to many students including: Kristin Salzsauler Dr David Teertstra Dana Johnson

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