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Geochemistry and Microbiology of Historic Waste Rock Piles at the Detour Lake Gold Mine, Ontario
Jeff Bain, Brayden McNeill, Mark Steinepreis, Lianna Smith, David Blowes, University of Waterloo Richard Amos – Carleton University
Aileen Cash, G. Ward Wilson - University of Alberta Jim Robertson and Marie-Helene Turgeon – DGC
23rd Annual BC-MEND ML/ARD Workshop Vancouver, Nov 30 – Dec 1, 2016
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BACKGROUND information ! Located in Northern Ontario ! Mine site built 2010 to 2012 ! Open pit ! In 4th year of production ! Mine life of 22+ years ! Mill process rate ~60,000
tonnes per day (tpd) ! Mine rate ~250,000 tpd ! Projected to manage 1.6 BT
of waste rock
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1. Site waste rock has median sulfur level of 0.2 wt.%, on the lower end of scale of ARD risk (range from 0.01 to 7 wt.%)
! Primary minerals are pyrite, pyrrhotite, chalcopyrite 2. The neutralization minerals in the waste rock are primarily calcite,
averaging 4 wt.% 3. Trace metals in the rock are below reference levels of concern (10x
basalt reference) with exception sulfur, but are being monitored. 4. With the low sulfides, low trace metals, and available carbonates,
the Detour site is in the lower area of ML-ARD risk when compared to global case histories.
Geochemistry Ranking of DGC site
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1. The site had 17 years of operation by Placer Dome (1983-1999) and 10 years of closure before Detour Gold commenced development
2. Five waste rock piles deposited by Placer Dome between 1983 and 1999 did not appear to be reactive
! Soil cover was added to all waste rock piles at closure in 2000 to reduce future AMD
! Monitoring showed only minor seasonal seepages ! No serious water quality detected during the 17 years of
mine operation or in the 13 year post-closure period
Detour Lake Gold Mine - Site History
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• Many ARD research projects ask ‘How did ARD develop in waste rock piles and how can it be stopped?’
In support of their closure planning and risk management programs, Detour Gold reversed the question and asked: • Why is ARD NOT occurring at this site? • What conditions need to be created in the new waste rock piles to
prevent ARD from developing? • What can we learn from the historic waste rock piles to minimize ARD
risk in the future?
Focus of Current Research
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• Phase 1: In 2011, DGC started waste rock studies with University of Waterloo (UW). The project received a NSERC CRD grant to match DGC funds.
• Coapplicants University of Alberta and Carleton U • 2 MSc students have graduated, 1 MSc near completion
• Phase 2: In 2015, UW with DGC support received a new NSERC CRD grant for five more years
• 4 more graduate students and some coop students • These research studies are based on 30+ years of waste rock
history and will have 20 years going forward, which is unusual for closure planning
DGC Waste Rock Research Program
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Historic Waste Rock Investigation 1. Two original rock piles needed to be moved:
• Examined the interior while relocating the rock to new locations
• Waste rock sampling and analysis
2. Two original rock piles were not moved: • Instrumented for long term internal
observations • Borehole instruments
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Pore-gas tubing
Suction sampler
Thermistor string
Moisture, EC, T probes
Pore-gas sampler
Tensiometers
Air-permeability testing points
Borehole Instrumentation
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BH4-2
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Soil Cover Characteristics • The cover is a 0.3-1 m thick mix of overburden silt, sand and boulders • Air permeability measurements:
• gas permeability of the waste rock is high • cover is much less permeable and restricts advective gas transport • cover should reduce the rate of sulfide oxidation
Waste rock
Average Air Permeability Coefficient (m2) 1e-9 1e-8 1e-7 1e-11 1e-10
Dep
th (m
BG
S)
0
5
10
15
20
25
Cover Waste Rock
Diffusion dominant
below 1x10-10 m2
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Waste Rock Pore Gas • Soil gas measurements confirm that sulfide oxidation and pH neutralization by
carbonate dissolution are occurring in the waste rock piles • Sulfide oxidation consumes O2
• O2 depletion at depth indicates that the cover restricts replenishment of atmospheric O2
• Carbonate mineral dissolution releases CO2 to the pore gas • CO2 increase at depth indicates that the cover traps CO2
CaCO3 + 2H+ " Ca2+ + CO2 + H2O
• Cover restricts (but does not stop) gas transport, limits rate of oxidation
Waste rock
Saturated WR
Atmospheric
Depletion Buildup
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Waste Rock Acid-Base Analysis
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NPR = NP/AP NPR < 1.5 = PAG
Neutralization Potential(kg CaCO3/tonne)
0 50 100 150 200 250
Aci
d Pr
oduc
tion
Pote
ntia
l(k
g C
aCO
3/to
nne)
0
50
100
150
200
250WRS#1 PEWRS#1 TPWRS#2 PEWRS#2 TPWRS#3 DCWRS#4 PEWRS#4 DC
DetourcutoffvalueforPAGrock
NPR<1.5poten:allyacidgenera:ng
NPR>1.5unlikelyto
generateacid
NPR1.5
NAG
PAG
#ofSamples
%PAG(NPR<1.5)
WRS1 42 62 % WRS2 29 72 % WRS3 16 0 % WRS4 31 47 %
New rock 20 %
• ABA measurements indicate that historic WRS 1, 2 and 4 are unsegregated mixtures of PAG and NAG rock
• Have higher PAG content than new waste rock (see table) • This historic waste rock is similar to the new PAG piles
• WRS 3 has lower S content, mostly NAG; similar to new NAG piles
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Potential Reactivity vs Grain Size
Showing average and 1 StdDev NPR = NP/AP
Grain Size (mm)0.01 0.1 1 10
NP
(kgC
aCO
3/to
nne)
0
20
40
60
80
100
Grain Size (mm)0.01 0.1 1 10
APP
(kgC
aCO
3/to
nne)
0
20
40
60
80
100
120NP vs. Grain Size AP vs. Grain Size
Grain Size (mm)0.01 0.1 1 10
NP
R (N
P/A
P)
0
4
8
12
16
20
• Analysed carbon (NP) and sulfur (AP) content versus grain size to identify relationships between reactivity of the waste rock particles and grain size
• Fine fractions in the waste rock are enriched in both carbon and sulfur • Resulting neutralization potential ratio (NPR) is consistent across the range of
grain sizes, indicating no preferential reactivity vs grain size
Average NPR vs. Grain Size
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Waste Rock Distribution and Paste pH • PastepHiden,fies
• LocalizedPAGzoneswithreac,vesulfideoxida,onandacidicpH• ExtensivezonesofneutralpH,duetoac,vecarbonatemineraldissolu,on
• NopaAerntothesezones–thePAGandNAGrockareintermixed• ThehighNPofthewasterockmixturehaslimitedthedevelopmentandextentof
acidiccondi,onsformorethan30years
1M 34/30 (1.1)
pH7.7
1D 36/39 (0.9)
pH5.7
2M* 14/43 (0.3)
pH3.7
2D 72/28 (2.5)
pH6.4
3M 36/67 (0.5)
pH5.9
3D 45/47 (0.9)
pH5.9
4D 68/32 (2.1)
pH6.3
4M 64/39 (1.6)
pH5.6
5M 107/18 (5.7)
pH6.2
5D 85/38 (2.2)
pH6.4
6M 67/35 (1.8)
pH6.7
6D 62/31 (1.9)
pH6.5
7M-1 100/15 (6.6)
pH6.6
8M-1 35/15 (2.2)
pH5.8
7M-2* 40/15 (2.6)
pH5.3
8M-2* 6/69 (0.1)
pH2.9
9C 66/1
(34.2) pH7.3
10C 65/1
(38.3) pH7.5
11C 67/5
(12.4) pH6.7
12C 61/4
(15.2) pH7.4
10S* 18/19 (0.9)
pH3.8
12S* 12/40 (0.3)
pH3.0
9S* 19/29 (0.6)
pH4.3
11S* 11/74 (0.1)
pH2.5 PAG <1.5
NAG >1.5
Sample NP/AP (NPR)
Paste pH
Till cover
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pH6 7 8 9
Dep
th (m
BG
S)
0
5
10
15
20
25
Eh0 200 400 600
Alkalinity(mg/L as CaCO3)0 1000 2000
Sulfate (mg/L)0 200 400 600 800
Calcium (mg/L)100 200 300 400
Manganese (µg/L)0 500 1000 1500
Dep
th (m
BG
S)
0
5
10
15
20
25
Iron (µg/L)0 50 100 150
Copper (µg/L)0 10 20 30 40
Nickel (µg/L)0 5 10 15
Zinc (µg/L)0 100 200 300
Porewater Chemistry - Waste Rock Matrix
WRS 3 • Mostly NAG rock, little PAG • Gases show that sulfide oxidation is occurring • Low PAG content = Neutral pH, attenuation leading to low concentrations of metals & SO4 • Cover contributes to low rate of oxidation & low release of metals • Composition of WRS 3 is similar to new NAG dumps at the mine • New NAG dumps may have similar chemical behaviour
Saturated waste rock
Waste rock
Aquifer
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Porewater Composition - Waste Rock Matrix
WRS 4 • Mostly NAG, some PAG • Elevated SO4 (=oxidation) • Abundant NP leads to neutral pH and metal removal reactions within the stockpile • Limited evidence of ML-ARD release to aquifer • Composition of WRS 4 is similar to new PAG rock • New PAG dumps may have similar chemical behaviour
pH6 7 8 9
Dep
th (m
BG
S)
0
5
10
15
20
25
Eh0 200 400 600
Alkalinity(mg/L as CaCO3)
0 500 1000 1500
Sulfate (mg/L)0 1000 2000
Calcium (mg/L)0 200 400 600
Manganese (µg/L)0 100 200 300
Dep
th (m
BG
S)
0
5
10
15
20
25
Iron(µg/L)0 2000 4000
Nickel (µg/L)0 20 40 60 80
Zinc (µg/L)0 50 100 150
Copper (µg/L)0 20 40 60
Peat
Aquifer
Waste rock
Sulfate reduction
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• Several techniques were used to identify the role of microbial activity in the waste rock
• DNA analysis of bacteria and fungus at genus level • Enumerations of iron and sulfur oxidizing bacteria (SOB)
• SOBn; neutrophilic sulfur oxidizers • SOBa; acidophilic sulfur oxidizers • FeO; acidophilic iron oxidizers
• Summary: • Common AMD-related iron and sulfur oxidizing bacteria are present throughout
the waste rock piles • Acidic conditions promote the dominance of AMD-generating populations
• SOBn major population at WRS#1 • FeO also numerous in acidic microenvironments • SOBa at low numbers throughout • Enrichment of iron and sulfur oxidizing bacteria DNA at low paste pH
Microbial Characterization
1g
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Summary • Our soil gas, porewater chemistry and microbial characterization
measurements in the historic waste rock piles indicate • Common AMD-related iron and sulfur oxidizing bacteria are present
throughout the waste rock piles • Sulfide oxidation is occurring • The cover and heterogeneous structure of the historic waste rock piles
restrict gas transport and sulfide oxidation • Metal attenuation reactions are occurring in the waste rock piles • The high NP of the historic waste rock mixtures has limited the
development and extent of acidic conditions and metal leaching for more than 30 years
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Phase 1 Outcomes • New NAG waste rock is similar to historic WRS 3 • New PAG waste rock piles appear to be similar to WRS1, 2 and 4
• Expect the new NAG and PAG dumps to behave similar to the old dumps
• In support of their risk management and closure planning, this research indicates that careful management of the new waste rock will help Detour to limit or prevent future ML-ARD releases from their waste rock dumps, for example,
• by using dump construction methods to disrupt gas transport in the waste rock piles
• through use of covers that have lower permeability, to limit advective gas transport and downward migration of sulfide oxidation products
• Phase 2 research is moving forward, e.g. • Grain-scale understanding of the sulfide oxidation and neutralization
pathways and rates, to improve predictability of ML-ARD • The role of soil covers on water balance, gas transport and ML-ARD
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Acknowledgements • Detour Gold Corp. • NSERC • Graduate and coop students and staff from the Universities of Waterloo and
Alberta, and Carleton University • Staff in the Detour Gold Environment Department
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Travis Desormeaux EH&S Superintendent Email: [email protected] Phone: 647.847.2089
Marie-Helene Turgeon Environment Permitting, & Sustainability Manager Email: [email protected] Phone: 647.847.2089
www.detourgold.com
Contact Information
Jeff Bain Hydrogeochemist, UW Earth & Envir. Sciences Email: [email protected] Phone: 519.888.4567