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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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North MRS

South MRS

Open pit ROM

Process plant

TMA

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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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Detour Lake Mine at Closure

WRS4 WRS3

WRS1

WRS2

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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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•  Summary of Key Findings

Summary of Investigation

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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

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•  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: tdesormeaux@detourgold.com Phone: 647.847.2089

Marie-Helene Turgeon Environment Permitting, & Sustainability Manager Email: mhturgeon@detourgold.com Phone: 647.847.2089

www.detourgold.com

Contact Information

Jeff Bain Hydrogeochemist, UW Earth & Envir. Sciences Email: jbain@uwaterloo.ca Phone: 519.888.4567