GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF
ACID ROCK DRAINAGE AND THE EVALUATION OF
PYRITE OXIDATION RATE AT MINE DOYON,
QUÉBEC, CANADA
Ondra Sracek1, René Lefebvre2
1Palacký University, Olomouc, Czech
Republic
2Université Laval, Québec, Canada
Mine Doyon pile - description(1)
• 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 (2)
• 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: sampling and mesurement sites
D-302
D-512
D-510
BH-91-101
W3
5000
BH-107
50 10
50
10
00
50
BH-2
20
50
Site TBT
BH-102
BH-5
BH-3
BH-4
BH-106
20
50
Site 6
BH-7
50 00
BH-1
30
50
BH-6
D-301
Site 7
BH-103
D-309
D-511
BH-104
BH-91-105
100
0
m
Conceptual model of processes in Mine Doyon pile
(1) Oxygen diffusion
Waste rock
Baserock
(2) Production and
conduction of heat
(3) Infiltration
of water
(4) Oxygen
convection
Two types of oxygen transport are evident in temperature
and O2 profiles
• Convection oxygen transport close to slope (Site 6)
• Diffusion oxygen transport in the core (Site 7)
Temperature (oC)
0
10 20 30 40 50 60 70
0
0
5
5
10
10
15
Depth (m)
Depth (m)
0
Oxygen (% vol.)
5
10
15
20
15
20
20
25
25
Site 6
Site 7
30
30
25
Profiles of dissolved species concentrations
0
2
a) Site 6
Ca (mg/L)
pH
4
0
6
300
600
0
900
Mg (mg/L)
10000
20000
Al (mg/L)
30000
0
0
0
5
5
5
10
10
10
10
15
15
15
15
20
20
20
20
25
25
25
25
30
30
30
35
35
35
0
0
10000
0
10000
20000
30000
October 1995
April 1996
Depth (m)
5
30
Saturated
Zone
35
Eh (mV)
300
0
5
Depth (m)
10
600
0
0
5
10
100000 200000 300000
3+
Fe (mg/L)
SO4 (mg/L)
900
0
10000
20000
Fe
30000
0
0
5
5
10
10
15
15
15
15
20
20
20
20
25
25
25
25
30
30
30
30
35
35
35
35
(mg/L)
20000
30000
SI values for K-jarosite and Si concentrations
a)
b)
K-Jarosite
Saturation Index
-4
-3
-2
-1
0
1
0
0
0
5
5
10
10
15
75
20
25
35
50
15
20
30
25
Depth (m)
Depth (m)
-5
Silica (mg/L)
25
Saturated
Zone
30
35
Saturated
Zone
100 125
a) Dry period
(Summer 1995)
a)
Deuterium (per mil.)
Isotopes D and 18O
0
Local Meteoric Water Line
Internal Evaporation Line
Shallow Lysimeters (<10 m)
Deep Lysimeters (>10 m)
Snow
-25
-50
D=7.02
18
O + 7.67
-75
D=2.4
-100
-125
18
O - 66.0
Summer 1995
-150
-20
-16
-12
18
b) Recharge period
(Spring 1996)
Deuterium (per mil.)
b)
0
-8
-50
0
Oxygen (per mil.)
Local Meteoric Water Line
Internal Evaporation Line
Shallow Lysimeters (<10 m)
Deep Lysimeters (>10 m)
Snow
-25
-4
D=7.02
18
O + 7.67
-75
D=2.4
-100
-125
18
O - 66.0
Spring 1996
-150
-20
-16
-12
18
-8
Oxygen (per mil.)
-4
0
Determination pyrite oxidation rate (POR)
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).
(1) POR from T and O2 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
profiles of T and O2 – Site 7
0
5
5
5
10
10
10
15
10
20
30
40
50
Depth (m)
0
15
0
0.0
0.00 0.05 0.10 0.15 0.20 0.25
0
Depth (m)
Depth (m)
Oxidation (kg Py/m3*y)
Temperature (oC)
Oxygen (% vol.)
15
20
20
20
25
25
25
30
30
30
0.2
0.4
0.6
0.8
1.0
(2) POR from mass balance for sulfate/pyrite (SMB/PMB)
• 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.
Role of secondary minerals in mass balance calculations
•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.
(3) POR from oxygen consumption method (OCM)
• 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);
Mesurement of oxygen consumption
O2 sensor
V
time
voltmeter
computer for data logging
Reactor
containing
sample
Rock
Sample
Voltage vs. time plot
• Fresh material from Mine Doyon, size of particles 4.5 cm:
voltage [mV]
41.2
41
40.8
40.6
40.4
40.2
40
39.8
39.6
39.4
0
20000
40000
60000
time [s]
80000
100000
Comparison of various methods of POR [mol(O2)·kg-1·s-1]
determination
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.
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*
Site TBT
(core)
&
- 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
CONCLUSIONS
• 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,
CONCLUSIONS (continuation)
• 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.