GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF ACID ROCK DRAINAGE AND THE EVALUATION OF ACID ROCK DRAINAGE AND THE EVALUATION OF

PYRITE OXIDATION RATE ATPYRITE OXIDATION RATE AT MINE DOYON, MINE DOYON, QUÉBEC, CANADAQUÉBEC, CANADA

Ondra SraceOndra Sracekk11, Ren, Renéé Lefebvre Lefebvre22

11PalackPalacký University, Olomouc, Czech ý University, Olomouc, Czech RepublicRepublic

22Université Laval, Québec, CanadUniversité Laval, Québec, Canadaa

• Mine Doyon is a gold mine, located north of Montreal, in Abitibi region,

• Average height of south pile is 30-35 m, rocks are sericite schists with pyrite content up to 7.0 wt % (locally 9.0 wt %), initial calcite content about 2.0 wt % has been generally depleted,

• AMD appeared 2 years after construction of pile and reached steady-state in 1988; typical pH values are about 2,0 and concentrations of sulfate in pore water are > 200 g/L; under such low pH conditions, behavior of Al and Mg can be considered conservative in long term.

Mine Doyon pile - description(1)Mine Doyon pile - description(1)

• Behavior of sulfate in the pile is not conservative due to the precipitation of gypsum (CaSO4*2H2O) and K-jarosite (KFe3(SO4)2(OH)6),

• Precipitation of these minerals occurs within metamorphic foliation, with resulting decomposition of rocks and increased access of oxidants to unoxidized pyrite; this process increases greatly pyrite oxidation rate;

Mine Doyon pile – description Mine Doyon pile – description (2)(2)

0 100

Site TBT

Site 7

Site 6

D-512

D-301

D-511

D-309

D-302

D-510

BH-91-101

BH-102

BH-104

BH-91-105

BH-106

BH-6

BH-3

BH-7

BH-2

BH-1

BH-4

BH-5

BH-107

W3

BH-103

m

Mine Doyon pile: sampling Mine Doyon pile: sampling and mesurement sitesand mesurement sites

Baserock

Waste rock

(1) Oxygen diffusion

(4) Oxygen convection

(3) Infiltrationof water

(2) Production and conduction of heat

Conceptual model of processes in Mine Doyon pileConceptual model of processes in Mine Doyon pile

0

5

10

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20

25

30

0 5 10 15 20 25

Oxygen (% vol.)

De

pth

(m

)

Site 6

Site 7

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

Temperature (oC)

De

pth

(m

)• Convection oxygen transport close to slope (Site 6)

• Diffusion oxygen transport in the core (Site 7)

Two types of oxygen transport are evident in temperature Two types of oxygen transport are evident in temperature and Oand O2 2 profilesprofiles

Profiles of dissolved species concentrationsProfiles of dissolved species concentrations

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5

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30

35

0 2 4 6

pH

De

pth

(m

)

October 1995

April 1996

SaturatedZone

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25

30

35

0 300 600 900

Ca (mg/L)

0

5

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35

0 10000 20000 30000

Mg (mg/L)

0

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30

35

0 10000 20000 30000

Al (mg/L)

0

5

10

15

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25

30

35

0 10000 20000 30000

Fe (mg/L)

0

5

10

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20

25

30

35

0 100000 200000 300000

SO4 (mg/L)

a) Site 6

0

5

10

15

20

25

30

35

300 600 900

Eh (mV)

De

pth

(m

)

0

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30

35

0 10000 20000 30000

Fe3+ (mg/L)

0

5

10

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30

35

0 2 4 6

pH

De

pth

(m

)

October 1995

April 1996

SaturatedZone

0

5

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15

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25

30

35

0 300 600 900

Ca (mg/L)

0

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25

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35

0 10000 20000 30000

Mg (mg/L)

0

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25

30

35

0 10000 20000 30000

Al (mg/L)

0

5

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15

20

25

30

35

0 10000 20000 30000

Fe (mg/L)

0

5

10

15

20

25

30

35

0 100000 200000 300000

SO4 (mg/L)

a) Site 6

0

5

10

15

20

25

30

35

300 600 900

Eh (mV)

De

pth

(m

)

0

5

10

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20

25

30

35

0 10000 20000 30000

Fe3+ (mg/L)

SI values SI values for Kfor K-jarosite -jarosite and Si concentrationand Si concentrationss

0

5

10

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25

30

35

-5 -4 -3 -2 -1 0 1

K-JarositeSaturation Index

Dep

th (

m)

a)

SaturatedZone

0

5

10

15

20

25

30

35

0 25 50 75 100 125

Silica (mg/L)

Dep

th (

m)

SaturatedZone

b)

Isotopes D and Isotopes D and 1818OO

-150

-125

-100

-75

-50

-25

0

-20 -16 -12 -8 -4 0

18Oxygen (per mil.)D

eu

teri

um

(p

er

mil

.)

Local Meteoric Water LineInternal Evaporation LineShallow Lysimeters (<10 m)Deep Lysimeters (>10 m)Snow

D=2.4 18O - 66.0

D=7.02 18O + 7.67

a)

Summer 1995

-150

-125

-100

-75

-50

-25

0

-20 -16 -12 -8 -4 0

18Oxygen (per mil.)

Deu

teri

um

(p

er

mil

.)

Local Meteoric Water LineInternal Evaporation LineShallow Lysimeters (<10 m)Deep Lysimeters (>10 m)Snow

D=2.4 18O - 66.0

D=7.02 18O + 7.67

b)

Spring 1996

a) Dry period

(Summer 1995)

b) Recharge period

(Spring 1996)

There are 3 principal possibilities in waste rock piles:

(1) Interpretation of temperature and oxygen profiles (TOP)

(2) Mass balance for sulfate/pyrite (SMB/PMB)

(3) Oxygen consumption method in laboratory (OCM)

Experiments in different scales generally produce different results (Malmström et al., 2000).

Determination pyrite oxidation rate (POR)Determination pyrite oxidation rate (POR)

(1) POR from T and O(1) POR from T and O22 profiles (TOP) profiles (TOP)

• When O2 is dominated by diffusion, O2 concentration profile is more or less exponential,

• Analytical solution of Crank can be applied to determine transport parameters,

• In Mine Doyon pile, O2 profile at Site 7 indicates O2 transport by diffusion,

• At this site analytical solution works for O2 profile, but not for T profile - there probably is lateral heat transport,

• Calculated POR at Site 7 is 4.4x10-10 mol(O2)·kg-1·s-1,

A comparison between calculated and measured A comparison between calculated and measured profiles of T and Oprofiles of T and O22 – Site 7 – Site 7

0

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30

0.00 0.05 0.10 0.15 0.20 0.25

Oxygen (% vol.)

Dep

th (

m)

0

5

10

15

20

25

30

0 10 20 30 40 50

Temperature (oC)

Dep

th (

m)

0

5

10

15

20

25

30

0.0 0.2 0.4 0.6 0.8 1.0

Oxidation (kg Py/m3*y)

Dep

th (

m)

• Oxidation of pyrite for pH < 3.0 is described as:

FeS2 + 3.5O2 + H2O Fe2+ + 2SO42- + 2H+

(oxidation of 1 mol of pyrite produces 2 moles of sulfate),

• When behavior of sulfate is conservative, the amount of sulfate flushed out of the pile can be converted to oxidized pyrite, e.g. sulfate mass balance (SMB),

• As an alternative, pyrite mass balance (PMB) can be used, assuming known initial pyrite content and pyrite content at time zero; calculated POR is an average value for whole oxidation period.

(2) POR from mass balance for sulfate/pyrite (SMB/PMB)(2) POR from mass balance for sulfate/pyrite (SMB/PMB)

•At Mine Doyon, precipitation of gypsum, CaSO4.2H2O, and jarosite, KFe3(SO4)2(OH)6, in the pile affects mass balance for sulfate (SMB) and values of POR are underestimated,

• For this reason, pyrite mass balance (PMB) was a better option.

Role of secondary minerals in mass balance calculationsRole of secondary minerals in mass balance calculations

• Sample of a broken waste rock is placed into a closed chamber, sprinkled with water and changes in headspace oxygen concentration are recorded,

• A graph of voltage as a function of time is plotted and straight line in the graph is used to calculate POR in mol(O2)·kg-1·s-1,

• Scale of the test is small compared to field and POR values are generally high (oxygen transport limitations are not present);

(3) POR from oxygen consumption method(3) POR from oxygen consumption method (OCM)(OCM)

computer for data logging

time

V

Reactor containing sample

voltmeter

O2 sensor

Rock Sample

Mesurement of oxygen consumptionMesurement of oxygen consumption

39.4

39.6

39.8

40

40.2

40.4

40.6

40.8

41

41.2

0 20000 40000 60000 80000 100000

time [s]

volt

age

[mV

]

• Fresh material from Mine Doyon, size of particles 4.5 cm:

Voltage vs. time plotVoltage vs. time plot

Site/method Temperature and O2 profiles

(TOP)

Pyrite mass balance (PMB)&

Oxygen consumption

method (OCM)

Site 6 (slope) 3.9x10-9 2.2x10-9 n.a.

Site TBT (core)

n.a. 1.8x10-9 n.a.

Site 7 (core) 4.4x10-10 2.0x10-9 n.a.

Pile average 1.5x10-9 2.0x10-9 6.6x10-8**

3.4x10-9*

& - based on 5 maximum values of pyrite content in deep unoxidized zone, n.a.-not available, **- fresh material, 5 runs, *- weathered slope material, 4 runs

Comparison of various methods of POR Comparison of various methods of POR [[mol(Omol(O22))··kgkg-1-1··ss-1-1] ]

determinationdetermination

• In Mine Doyon waste rock pile oxygen convection plays an important role,temperature and oxygen concentration profiles indicate convection close to slope and diffusion in the pile core,

• Concentrations of contaminants in pore water (sulfate, iron, aluminum etc.) are extremely high close to slope an in dry period and decrease in recharge, high concentrations are linked to internal evaporation of pore water in the pile due to high temperature (up to 67º C),

• Internal evaporation is also confirmed by enriched values of isotopes D and 18O,

• Principal secondary minerals are gypsum and jarosite, their precipitation within metamorphic foliation breaks rocks and facilitates the access of oxidants to the surface of unoxidized pyrite,

CONCLUSIONSCONCLUSIONS

• POR values obtained by different methods are comparable,

• PMB method with a minimum precision can still distinguish between POR values close to slope (Site 6) from central zone (Sites 7 and TBT),

• When oxidized material from the slope is used, results obtained by OCM are consistent with TOP results from Site 6 close to slope,

• This means that when transport of O2 is not a limiting factor, values of POR obtained from small scale experiments may give representative results; this is different from sites like Aitik in Sweden with oxygen transport dominated by diffusion, where POR values depend very much on the scale of experiments.

CONCLUSIONS (continuation)CONCLUSIONS (continuation)