Presentation sracek o

• 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 % locall

GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF ACID ROCK DRAINAGE AND THE EVALUATION

OF PYRITE OXIDATION RATE AT MINE DOYON,

QUÉBEC, CANADA

Ondra Sracek 1 , René Lefebvre 2

1 Palacký University, Olomouc, Czech

Republic

2 Université Laval, Québec, Canada

• 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)

• Behavior of sulfate in the pile is not conservative due to the

• 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 (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 and mesurement sites

Waste rock

(1) Oxygen diffusion

(4) Oxygen convection

(3) Infiltration

of water

(2) Production and

conduction of heat

Conceptual model of processes in Mine Doyon pile

0 5 10 15 20 25 30

Oxygen (% vol.)

Site 6 Site 7

0 5 10 15 20 25 30

0 10 20 30 40 50 60 70

Temperature ( o C)

• 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

Profiles of dissolved species concentrations

0

5

10

15

20

25

30

35

pH

October 1995 April 1996

Saturated Zone

0

5

10

15

20

25

30

35

Ca (mg/L)

0

5

10

15

20

25

30

35

Mg (mg/L)

0

5

10

15

20

25

30

35

Al (mg/L)

0 5 10 15 20 25 30 35

0 10000 20000 30000

Fe (mg/L)

0 5 10 15 20 25 30 35

0 100000 200000 300000

a) Site 6

0

5

10

15

20

25

30

35

300 600 900

Eh (mV)

0

5

10

15

20

25

30

35

Fe 3+ (mg/L)

0

5

10

15

20

25

30

35

pH

October 1995 April 1996

Saturated Zone

0

5

10

15

20

25

30

35

Ca (mg/L)

0

5

10

15

20

25

30

35

Mg (mg/L)

0

5

10

15

20

25

30

35

Al (mg/L)

0 5 10 15 20 25 30 35

0 10000 20000 30000

Fe (mg/L)

0 5 10 15 20 25 30 35

0 100000 200000 300000

a) Site 6

0

5

10

15

20

25

30

35

300 600 900

Eh (mV)

0

5

10

15

20

25

30

35

Fe 3+ (mg/L)

SI values for K-jarosite and Si concentrations

0 5 10 15 20 25 30 35

K-Jarosite Saturation Index

a)

Saturated Zone

0 5 10 15 20 25 30 35

Silica (mg/L)

Saturated Zone

b)

Isotopes D and 18O

-150 -125 -100 -75 -50 -25 0

18 Oxygen (per mil.)

Internal Evaporation Line Shallow 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

18 Oxygen (per mil.)

Internal Evaporation Line Shallow 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)

(1) POR from T and O2 profiles (TOP)

is more or less exponential,

• Analytical solution of Crank can be applied to determine

transport parameters,

by diffusion,

T profile - there probably is lateral heat transport,

A comparison between calculated and measured

0

5

10

15

20

25

30

0.00 0.05 0.10 0.15 0.20 0.25

Oxygen (% vol.)

0

5

10

15

20

25

30

Temperature ( o C)

0

5

10

15

20

25

30

0.0 0.2 0.4 0.6 0.8 1.0

Oxidation (kg Py/m 3 *y)

• 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)

•At Mine Doyon, precipitation of gypsum, CaSO4.2H2O, and

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 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

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

computer for data logging

time V

Reactor containing sample

voltmeter

O2 sensor

Rock Sample

Mesurement 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]

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

Voltage vs time plot

Site/method Temperature

profiles (TOP)

Pyrite mass balance

Oxygen consumption method (OCM)

Site TBT

zone, n.a.-not available, **- fresh material, 5 runs, *- weathered slope material, 4 runs

determination

• 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

• 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,

CONCLUSIONS

• 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,

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)