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Environmental Geology of Mine Waste Dr. Rob Bowell SRK Consulting (UK)

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Page 1: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

Environmental Geology

of Mine Waste Dr. Rob Bowell – SRK Consulting (UK)

Page 2: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

Introduction

• Characterization of

mine water

• What effects mine

water chemistry?

• Preliminary study

includes mineralogy,

geology

• Groundwater

chemistry

• Chemistry of inflows

etc.

Page 3: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 4: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

Mineralogy: evidence of hydrogeochemistry

Page 5: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 6: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 7: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 8: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 9: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 10: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

Generation of Acid Rock

Drainage

Driven by mineral

stability or instability

Sulfide or acid sulfate

source

Limitation on

carbonate buffering

Page 11: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 12: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 13: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 14: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

Area of coal fires

pH ~ 3.3

Fe~ 80 mg/L

Al ~ 450 mg/L

Sulfate ~ 2200 mg/L

Page 15: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 16: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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+

Page 17: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 18: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 19: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 20: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

)

Page 21: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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)

Page 22: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 23: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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.

Page 24: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 25: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 26: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches
Page 27: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 28: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 29: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 30: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 31: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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

Page 32: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

• 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

Page 33: Environmental Geology of Mine Waste · • GARD manual has excellent range of methodologies that cover >90% of requirements • Consider site specific or problem specific approaches

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