geochemical behaviour of adit drainage under saturated and...
TRANSCRIPT
Geochemical behaviour of adit drainage under saturated and unsaturated conditions at two
underground mines in Northern British Columbia
John Dockrey & Alison Shaw
22nd annual BC MEND ML/ARD workshopDecember 2-4, 2015
Overview
1. Two geologically distinct mine sites with similar mining histories2. Similarities in geochemical trends
o Sulphate concentrations increase when mine workings are unsaturated
o Metal cation (Zn and Cd) concentrations decrease when mine workings are unsaturated
3. Identify mechanisms that could result in decline in metal cation solubility between saturated and unsaturated time periods
4. Results show that metal cation behaviour is controlled by pH related solubility constraints in neutral mine drainage.
Site Naming Conventions
Site Background: GeologySite Ao Epithermal Au-Ag deposit hosted in stockwork
veining intruding a primarily volcanic host rock.o Existing excavations are primarily in hydrothermally-
altered PAG rock consisting of volcanic and volcaniclastic sedimentary rock units
Site B: aka the tunnel of loveo Ag-Pb-Zn carbonate replacement deposit hosted in a
limestone formationo Existing excavations are primarily in non-PAG
limestone with minor amounts of mudstone and ore excavated as part of bulk sampling program.
1984 1990 1995 2001 2006 2012 2017
SaturatedUnsaturated
Site Background: Mining History
Site B:Total Excavations: 2400 m
• Nearly all mine workings are below adit elevation in both sites• No backfilled waste rock or tailings are present at either site
Site A:Total Excavations: 7200 m
Site A Timeline
Site B Timeline
Conceptual Model of Geochemical Loading: Unsaturated
O2
GW
GW
GW
Wall RockMuck Rock
GW Seepage Shotcrete
Unsaturated Geochemical Load = Wall Rock Oxidation + Rinsing of Freshly Exposed Surfaces + Additives + Groundwater
Conceptual Model of Geochemical Loading: Saturated
GW
GW
Short-Term Geochemical Load = Flushing Load + Groundwater
Long-Term Geochemical Load = Groundwater
Flushing of Rock Surfaces
0
500
1,000
SO4
mg/
L
SaturatedUnsaturated
1984 1990 1995 2001 2006 2012 2017
0
200
400
SO4
mg/
L
Sulphate Results
Site A
Site B
o Sulphate concentrations reflect impact of oxygen ingress and sulphide mineral oxidation during unsaturated time periods at both minesites.
1984 1990 1995 2001 2006 2012 2017
0
200
400
SO4
mg/
L
Site A
0
500
1,000
SO4
mg/
L
SaturatedUnsaturated
Sulphate Results
1984 1990 1995 2001 2006 2012 2017
Site B
o Sulphate concentrations return to the range observed in background groundwater when mine workings are saturated.
1984 1990 1995 2001 2006 2012 2017Background Groundwater
0.00
0.10
0.20
0.30
D-Zn
mg/
L
0.0
4.0
8.0
12.0
D-Zn
mg/
L
SaturatedUnsaturated
Zinc Results
Site A
1984 1990 1995 2001 2006 2012 2017
o Zinc concentrations tend to be lower when mine workings are unsaturated.
Site B
1984 1990 1995 2001 2006 2012 2017
0.0
4.0
8.0
12.0
D-Zn
mg/
L
SaturatedUnsaturated
Zinc Results
Site A
1984 1990 1995 2001 2006 2012 2017
o Zinc concentrations decline to below background groundwater when mine workings are drained, indicating that zinc is being attenuated
0.00
0.10
0.20
0.30
D-Zn
mg/
L
Site B
1984 1990 1995 2001 2006 2012 2017Background Groundwater
Potential Attenuation Mechanisms
1. Secondary Mineral Precipitationo solution chemistryo equilibrium concentrations can be
calculatedo thermodynamic data only
available for end-memberso potential rate limitations
2. Sorption to Particle Surfaceso solution chemistryo availability of surfaceso mineralogy of surfaceso must be empirically determined
powellite precipitation on calcite
The pH Dependence of Metal Sorption on Fe-oxide (After Stumm, 1992)
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
6 8 10 12
D-Zn
mg/
L
pH
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
6 8 10 12D-
Zn m
g/L
pH
• Site A: sorption controlled• Site B: secondary mineral control
Potential Attenuation Mechanisms
Smithsonite (ZnCO3)
ZnCO3:H2O
Zn(OH)2
Site A Site B
Smithsonite (ZnCO3)
ZnCO3:H2O
Zn(OH)2
Factors influencing sorption and solubility during active mining
1. Elevated TSS: increased particle surfaces available for sorption
2. Cement in additives: dissolution of CaO
3. Cation exchange: increase in carbonate mineral equilibrium pH
4. Aeration of mine water: leading to degassing of carbon dioxide
Elevated TSS
• During active mining TSS is observed to increase at both minesites
• Blasting and ground disturbance releases and suspends silt and clay particles
Cement in Additives
CaO + H2O → 2OH- + Ca2+
Cement Dissolution:
• Dissolution of cement in grout or shotcrete can drive up pH
Cation Exchange
Ca2+ + Na2:Ex → 2Na+ + Ca:Ex
2CaCO3 + 2H2O →2HCO3- + 2OH- + Ca2+
Cation Exchange:
0
40
80
120
2009 2010 2012 2013 2014 2016
D-N
a
0
20
40
60
80
2009 2010 2012 2013 2014 2016
D-Ca
• Water quality trends provide evidence that Na:Ca exchange reactions are occurring and potentially driving up pH at site A.
• Similar trend observed in early humidity cell data from freshly crushed drill core samples
Site A
Aeration of Mine Drainage
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
6 7 8 9
log_
pCO
2(g
)
pH
Saturated Mine Drainage
Unsaturated Mine Drainage
Site A
atmospheric pCO2
(1) CO32- + H2O = HCO3
- + OH-
(2) HCO3- + H2O = H2CO3 + OH-
(3) H2CO3 = H2O + CO2(g)
Carbon Dioxide Degassing:
• A number of reactions are caused by mine water aeration
• Mine drainage at unsaturated time points is closer to atmospheric CO2 equilibrium, indicating that degassing has occurred.
7
7.5
8
8.5
-4
-3.5
-3
-2.5
-2
0 25 50
pH
Log_
pCO
2
Weeks
atmospheric pCO2
pCO2
pH
Aeration of Mine Drainage
Ageing Experiment of Saturated Portal Drainage (Site B)
Portal Drainage Stored in 20L
Jug at 2°C
• Aeration of saturated drainage and degassing of carbon dioxide leading to an increase in pH and attenuation of Cd and Zn.
0
0.01
0.02
0.03
0.04
0
1
2
3
4
0 25 50
Cd m
g/L
Zn m
g/L
Weeks
Zn
Cd
Conclusions
• Geology is not the dominant control of metal cation behaviour.
• Solubility controls dictate metal cation behaviour in neutral pH settings. Solubility decreases when mine workings are drained due to:– Elevated TSS– Dissolution of cement– Cation exchange reactions – Aeration
• Interpreting neutral mine drainage datasets and predicting future water quality must take into account geochemical reactions which are highly sensitive to pH in addition to the intrinsic metal leaching potential of mine rock.
Questions?