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

X Central Manitoba Central Manitoba Gold Mine Rice Lake Archean Greenstone Belt, SE Manitoba Gold associated with pyrite and chalcopyrite in quartz carbonate veins in metavolcanics

0500 m BLUE POND pH 4.4 GREEN POND pH 7-8 N MINE BUILDINGS WASTE ROCK Central Manitoba Tailings Points of Discharge of Tailings PR 204

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

ACID MINE DRAINAGE Oxidation of sulphides exposed to water and oxygen in waste rock piles or mine workings Oxidation of pyrite by oxygen FeS 2 + 7/2 O 2 + H 2 O = Fe SO H + Oxidation of Fe 2+ to Fe 3+ Fe /4 O 2 + H + = Fe 3+ + ½H 2 O Further oxidation of pyrite FeS Fe H 2 O = 15Fe SO H +

Neutralization of Acid Rock Drainage Carbonate dissolution: CaCO 3 + H + → Ca 2+ + HCO 3 - pH 7-8 Aluminum hydroxide dissolution: Al(OH) 3 + 3H + → Al H 2 OpH 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 CO 3 :S 1:2 CO 3 :S 2:1 CO 3 :S 3:1 Discharge Point

Distance from discharge point (m) Net Neutralizing Potential Net Neutralizing Capacity vs Distance from Discharge Point South North

Orange Brown Green Blue Grey Burgundy 1G 2G 3G Vertical Variation in Colour & Pore Water Geochemistry pH Cu (ppm) SouthNorth

Photomicrographs of brochantite (Cu 4 (SO 4 )(OH) 6 ) from the green stripe PPL X polars 50 μm Photomicrographs of blue mineral (possibly Fe cyanide) from the blue stripe (A) PPL (B) X polars 50 μm A A B

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

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

Blue Stream Blue Pond Green Stream Green Pond pH HCO 3 SO 4 Al Ca Cu Fe Composition of Stream & Pond water (ppm)

5Fe H 2 O → Fe 3+ 5 (OH) 8.4H 2 O (ferrihydrite) + 8H + Buffered at pH 7-8 by CaCO 3 + H + → Ca 2+ + HCO 3 - Cu precipitates as malachite (Cu 2 CO 3 (OH) 2 and chalcanthite (CuSO 4 5H 2 O) at neutral pH Green Pond Geochemistry Blue Pond Stream to Green Pond Green Pond Tailings Stream to Blue Pond pSO 4 pH Ferrihydrite Schwertmannite

8Fe 3+ + SO H 2 O → Fe 3+ 8 O 8 (OH) 6 (SO 4 ) + 22H + (schwertmannite) Blue Pond Geochemistry Acidity Buffered at 4.4 by Al: 3H + + Al(OH) 3 → Al H 2 O Cu 2+, 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?

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) Reduced Tailings (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 Boundary Solid Surface Water table Water 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 Fund Manitoba Conservation NSERC Discovery Grant NSERC//NATO Fellowship Thanks to many students including: Kristin Salzsauler Dr David Teertstra Dana Johnson