environmental geology of mine waste · • gard manual has excellent range of methodologies that...
TRANSCRIPT
Environmental Geology
of Mine Waste Dr. Rob Bowell – SRK Consulting (UK)
Introduction
• Characterization of
mine water
• What effects mine
water chemistry?
• Preliminary study
includes mineralogy,
geology
• Groundwater
chemistry
• Chemistry of inflows
etc.
Processes active in weathering
DISPERSION
Mineral weathering
• Sulfide oxidation
• Salt dissolution
• Mineral buffering
Desorption
Cation Exchange
ATTENUATION
Mineral precipitation
• Solubility control
• Trace element
incorporation
Adsorption
• Surface effects
Absorption
• Cation Exchange
• Metal Scavenging
Mineralogy: evidence of hydrogeochemistry
Case study: Tsumeb, Namibia
• Polymetallic
pipe-like deposit
• Precambrian age
• 1908-1993 operation
• 5Mt Cu, 9.5 Mt Pb
2.1 Mt Zn
• Ag, Au, Cd, Ge, As,
Sn, W, V, Mo, Co,
Hg, Ga, In, Sb
• Current resource
~5Mt @ 4.3% Cu,
7% Pb, 2% Zn,
3 opt Ag, + Ge
Eh-pH Groundwaters
Upperoxide zone
SurfaceS N
Sulfide ore
Lower oxide zone
Nor
th B
reak
Fra
ctur
e Zon
e
0 1000Metre
2 4 6 8 10 12
-0.2
0
0.2
0.4
0.6
0.8
1.0
H O
H O
O
H2
2
2
2
pH
E(V
)
First oxidation zoneSecond oxidation zone
First sulfide zoneSecond sulfide zone
Processes active in weathering
DISPERSION
Mineral weathering
• Sulfide oxidation
• Salt dissolution
• Mineral buffering
Desorption
Cation Exchange
ATTENUATION
Mineral precipitation
• Solubility control
• Trace element
incorporation
Adsorption
• Surface effects
Absorption
• Cation Exchange
• Metal Scavenging
Major Issues:
Hydrogeochemistry
Acid Rock Drainage
• Metal release
• Acid Generation
• Salination of water
resources
Radioactivity
• Release of
radionuclide
• Long term exposure
to radiation
• Low dilution effect
How long does ARD last?
Days or many years
Can last many years
In time the rate
will slow as
• the reactive sulfides
are oxidised
• pH increases
• ambient water
is buffered
Generation of Acid Rock
Drainage
Driven by mineral
stability or instability
Sulfide or acid sulfate
source
Limitation on
carbonate buffering
Acid Generation Process:
Sulfide oxidation
Stages in oxidation
of pyrite
1. FeS2 + 7/202 + H2O =
Fe2+ + 2SO42- + 2H+
2. Fe2+ + 1/402 + H+ =
Fe3+ + 1/2H2O
3. Fe3+ + 3H2O =
Fe(OH)3 + 3H+
4. FeS2 + 14Fe3+ + 8H2O
= 15Fe2+ + 2SO42- +
16H+
Pyrite + oxygen + water +
catalyst
Case Study: Coal mine impacts
• Pyrite oxidation in inter-burden
• Fine grained, porous pyrite
• Rapid kinetics – oxidation
• No buffering
• Exothermic reaction
• Burn coal
• Approx. 75kt lost pa
• Impact water resources – ARD
Explanation
Identify source
components
Identify susceptible
seams and inter-burden
Alter mining schedule
• Reduce exposure time
• Reduce oxidation
• Preserve coal
Net benefit –
environmental &
economic
Pyrite
Fluid flow- waterCarbon in shale
Heat from oxidation
reaction burns carbonOxygen diffuses
along fractures
Area of coal fires
pH ~ 3.3
Fe~ 80 mg/L
Al ~ 450 mg/L
Sulfate ~ 2200 mg/L
Release of secondary acidity
Acid Sulfate Salts
Dissolution of highly
soluble salts
• E.g. Melanterite
FeSO4.7H2O
Formation
of extremely acid
conditions
Examples:
• Aquas Teindas, Spain
• Pascua Lama, Chile
• Furtei, Sardinia
Case Study: Furtei, Sardinia
High Sulfidation Epithermal Au-
Ag-Cu deposit
Pyrite, Enargite
• Fine grained
• Poorly crystalline
High E/T
Seasonal rainfall
High acidity > 2 g/L H2SO4
High Cu ~ 0.5 g/L;
Fe ~ 2 g/L; pH < 2 (lowest < 0)
Secondary salts drive pH < 0 –
high solubility;
super-saturation of H+
Metal Mobilization
• metal leaching processes in
mine waste piles are complex
• dependent on the mineralogy
of the waste rock
• solubility of most metals
increase with decrease in pH
• conversely metals precipitate
from solution with increase in
pH
• contaminated drainage can
serve as a leachate promoting
mobilization of metals
Flicklin plot
0.01
0.1
1
10
100
1000
10000
100000
0 2 4 6 8 10 12
pH (su)
(Co
+N
i+C
u+
Zn+
Cd+
Pb),
mg/L
High sulfide-Au
Porphyry
Low sulfide-Au
Carlin-type
VMS
SEDEX
Tin veins
Summary of studies
at Sa Dena Hes, Yukon
Water chemistry:
• Atypical zinc geochemistry
from adit interacting with
marble
• Typical zinc geochemistry
for tailings pore water
Polymetallic mantos style
deposit in marble/phyllite
Missing Zinc Load at 1380 Portal
• Zinc load at springs
feeding Camp Creek
is much lower than
discharged from
1380 Portal.
• Sulfate load in Camp
Creek in contrast is
(at peak) about 10
times 1380 Portal.
• Loss of zinc load
cannot be explained
by precipitation of
zinc carbonate
(smithsonite).
1380 Portal
Camp Creek0
10
20
30
40
50
60
70
80
90
100
04-May-00
14-May-00
24-May-00
03-Jun-00
13-Jun-00
23-Jun-00
03-Jul-00
13-Jul-00
23-Jul-00
02-Aug-00
12-Aug-00
Zn
(m
g/s
)
Attenuation Column Residues
Upper part of column
weakly cemented
Cement contains
60% zinc
Carbonate & silicate –
Zn phases identified
Hemimorphite type
mineral
Mineralogy of sediments
confirmed presence
of same phase
ZincCadmium
0
20
40
60
80
100
120
0.1 1 10 100 1000
Zn, Cd, Pb (mg/kg)
Ap
pro
x C
olu
mn
De
pth
(c
m)
Investigation of Tailings Beach
North Dam seepage contains high Zn (~ 15 mg/L)
Pore water samples from above water table
• Indicated zinc up to 56 mg/L in pore water.
Drive point piezometers
• Mostly lower than pore water but up to 41 mg/l
Mineralogy
• Confirmed presence of smithsonite, gypsum and ferric hydroxide in tailings
Explanation
Investigation at the 1380 portal
indicated formation of a
hemimorphite with very low
solubility
• Explained disappearance of
zinc load and indicated
attenuation capacity.
Conventional zinc behavior
indicated for tailings
• High solubility due to soluble
secondary minerals
(smithsonite)
Both studies showed importance
of predicting and understanding
mineralogical controls.
Radioactivity
• Radiation of natural waters
is related to release
energy as electromagnetic
waves
• The energy release is
related to particle release
from unstable mass
chemical elements
• Major indicators in natural
mine waters are uranium,
radium and radon gas
• All are metals and conform
to metal behaviour so can
be predicted
Uranium geochemistry
Species dependent in aqueous environment
• U (IV) dominant in low Eh environment
Solubility high in alkaline high carbonate waters
In acid drainage UO2 to uranyl (VI) sulfate
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
1 2 3 4 5 6 7 8pH
Su
lph
ate
(p
pm
), U
ran
ium
(p
pb
)SO4 UU (VI) chelates
U (IV) carbonates
Geochemical behaviour of U
Buffering
• Consumption of protons
• Typically viewed as a
reaction with cabonates:
CaCO3 + H2SO4 + H2O =
CaSO4. 2H2O + CO2
• Silicates and hydroxides
can also buffer
• Typically carbonates
in range 7–9;
silicates 3–7; and
hydroxides from 2–5
• As pH increases,
metals precipitate
Buffering rates
0
5
10
15
20
25
30
0 20 40 60 80 100
Mineral concentration (wt%)
We
ath
eri
ng
ra
te k
eq
/Ha
/yr
Dissolving
Fast
Intermediate
Slow
Very slow
Inert
Dissolving
Fast weathering
IntermediateSlow weathering/Very
Younger plot
PUMPED DEEP
GROUND WATERS
BRINES
NET ALKALINE
NET ACID
ALKALINITY100%
ACIDITY100%
SO
100%4
2- Cl100%
-
100
60
40
20
0
0 20 40 60 80 100
80
% t
ota
l as m
g/l C
aC
O 3
%S (SO +Cl ) meq/l4
2- -
High Sulfidation
Porphyry
CarbonatePb-Zn Clay pitsLow Sulfidation
CarlinShear zone Au
Fate of metals/metalloids
Predictions from
mineralogy as to mine
water chemistry
Precipitation reactions
• Secondary minerals
• Co-precipitated with more
abundant minerals
Adsorption reactions
• Surface adsorption
• pH dependent
• Example of Arsenic
• Arsenic occurrence in
nature
- As (III), sulfides, reducing
- As (V), ambient, oxide
- MMAA/DMAA/organic-rich
environments
• Arsenic mobility
- reducing
- low Al/Fe
- very acidic (pH <2) or
alkaline (pH > 8.5)
• Strong adsorption onto iron
oxyhydroxides
Metalloid geochemistry
Take Home Points
Environmental Issues
• Release of metals, oxyanions, sulfate, acidity, radioactivity
• Potential impacts to soil, water, sediments, air and vegetation in receiving
environment
Mineralogy
• Screening approach
• Provide informed sampling approach
Secondary Processes
• Data verification
• Use of laboratory QA to confirm valid analysis
Baseline data
• Account for variability
• Identify trends
Sampling
• Internal factors e.g. particle size, mineralogy, mineral reactions in cell, biota activity
• External factors e.g. aeration, sample size, frequency of flush
Protocols
• GARD manual has excellent range of methodologies that cover >90% of
requirements
• Consider site specific or problem specific approaches
• Greater use of mineralogy & selective extraction
• Kappa approach as an alternative to humidity cells
• Assess directly potential toxicity – useful where direct receptors can be identified