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  • 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

    Upper oxide zone

    SurfaceS N

    Sulfide ore

    Lower oxide zone

    N or

    th B

    re ak

    F ra

    ct ur

    e Zo

    ne

    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

    H 2

    2

    2

    2

    pH

    E (V

    )

    First oxidation zone Second oxidation zone

    First sulfide zone Second 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+ + 2SO4 2- + 2H+

    2. Fe2+ + 1/402 + H + =

    Fe3+ + 1/2H2O

    3. Fe3+ + 3H2O =

    Fe(OH)3 + 3H +

    4. FeS2 + 14Fe 3+ + 8H2O

    = 15Fe2+ + 2SO4 2- +

    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- water Carbon 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)

    (C o

    + N

    i+ C

    u +

    Z n +

    C d +

    P b ),

    m g /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-M ay-00

    14-M ay-00

    24-M ay-00

    03-Jun-00

    13-Jun-00

    23-Jun-00

    03-Jul-00

    13-Jul-00

    23-Jul-00

    02-A ug-00

    12-A ug-00

    Z n

    ( 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)

    A p

    p ro

    x C

    o lu

    m n

    D e

    p th

    ( 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 environme