Geochemistry and the challenge posed by exotic cover

David CohenUniversity of New South Wales

and

Association of Applied Geochemists

Recent Practical Advancesin Mineral Exploration

Technologies

September 11, 2009

Ahmad MokhtariIsfahan University of Technology, Isfehan

Neil RutherfordRutherford Mineral Resource Consultants, Sydney

Simon GatehouseHellman and Schofield, Sydney

David Kelley MMG – Minerals and Metals Group, Denver

Ravi AnandCRCLEME / CSIRO Exploration and Mining, Bentley

William CokerBHP Billiton World Exploration, Vancouver

Iain DalrympleActlabs, Perth

RMRC

SMEDG -AIG ‘09

SMEDG -AIG ‘09

Intro

Models

Mandy

SEx

Conclusions

Data

The problematic terrains for geochemical exploration

Glacial depositsThick gravel and scree depositsThick alluvium or colluvium + deep weatheringAeolian deposits

?IceVolcanic ash

IntroModels

MandySEx

ConcData

SMEDG -AIG ‘09

Data: Australian Minerals Atlas

IntroModels

MandySEx

ConcData

L. Mesozoic to Tertiary basins

Thompson Orogen

Lachlan Fold Belt

SMEDG -AIG ‘09

Significant mineral occurrences

⇒ Systematic approach to identifying regolith materials and landforms

⇒ Link evolution of regolith and its components to geochemical processes and sampling strategies

Weathered profiles typically have RESIDUAL and TRANSPORTED components of varying ages

Several distinct phases of creation and destruction of secondary Fe, Ca, Si and Al minerals – each with total or partial resetting of trace element geochemistry

Anand and Smith, 2005

IntroModels

MandySEx

ConcData

Red-brn sandy clay

Pisolitic ferricrete

Silicd sandy, silty + gravel colluv & alluv

Mega mottle palaeo-channel clays

Transported gravel

Clays

Mottled saprolite

Saprolite

Saprock

Base of sedimentsLateritic residuum

Bedrock

Reg

olith

dev

elop

ed o

n an

d in

prim

ary

litho

logy

Tran

spor

ted

Ove

rbur

den

Tert

iary

Qa

min

eral

isat

ion

?Regolith Classification

unconformity

SMEDG -AIG ‘09

(data: Lawrance, 1999)

MesozoicCover

ProterozoicBedrockCu - Au

Mineralisation

< 15 (ppm)15 - 3030 - 5050 - 200200 - 10001000 - 2000>2000

Cu

Unconformity

< 1 %1 - 22 - 33 - 88 - 1515 - 20> 20

Fe

Unconformity

100 m

40m

Dispersion mechanisms?

Dispersion timing?

Osborne Cu-Au deposit, Qld

IntroModels

MandySEx

ConcData

Does dispersion occur through deep, transported cover?

SMEDG -AIG ‘09

IntroModels

MandySEx

ConcData

CSA Mine Lease, Cobar

C.S.A.

Recent Drainage

Line ALine B

Line D

Line C

1km

U. Amphitheatre Gp.CSA SiltstoneChesney Fmn. (grwk)

Girilambone Gp.Chesney Fmn. (slts)

Palaeo-drainage

Shen and Cohen, 2005

Line DCoarse silt + lagTransported silt + clay

Residual regolith

0 m 150

Chesney Fmn. siltstone Girilambone Gp

3000

600

50m

Pb

Fe

Zn

SMEDG -AIG ‘09

Does dispersion always occur?

McKinnons deposit, Cobar

Intro

MandySEx

ConcData

Models

SMEDG -AIG ‘09

Mixed pediment / alluvium

Au mineralised structure Cohen et al, 1996

Alluvial fillPediment

Saprolite

line 79600N

300 m

205 m

195 m

Dispersion models – vegetation effects

0

0.3ppm

0

15ppm

As

300ppm

0

Cypress pineneedles

Soil total

Lag total

magnetic

non-mag

Dispersion models – gases and aerosols

Deployment arrangement

Collector assembly

polystyrene-coatedglass slide

surface

hole

plastic cap(dust cover)

collector

funnel

backfill

50 cm

Rutherford et al., 2005

Clay in droplet

50 μm

20 μm

Amorphous silica

10 μm

NaCl + Na2CO3

Intro

MandySEx

ConcData

Models

SMEDG -AIG ‘09

Cu

Fe

Cl

K

Ore Zoneng cm-2

20

10

0

100

50

0

0.8

0.4

0

20

10

0

0 500 1000 1500 m

Osborne Cu-Au deposit, Qld

Intro

MandySEx

ConcData

ModelsPlus a range of other transition and group I and group II elements

SMEDG -AIG ‘09

Osborne (Kulthor) Cu-Au deposit, Qld

Intro

MandySEx

ConcData

Models+6 +7 +8 +9 +10 +11 +12 +13

chalcopyrite

pyrrhotite

pyrite

gypsum in soil

regional soil background

δ34S

SMEDG -AIG ‘09

Figure 5 - Model of Cameron et al

Cameron et al., 2004

Dispersion models – dilatancy pumping

Intro

MandySEx

ConcData

Models

SMEDG -AIG ‘09

Spence porphyry Cu deposit, Northern Chile

0

15ppm Cu

0

2ppm As

0.6%

0

Eastern Fracture

Zone

Spence Deposit

Na

500 m

Surface regolith samples;weak selective extraction

Vertical fracturewith saline soil

Intro

MandySEx

ConcData

ModelsBurial under 250m of Tertiary gravels

SMEDG -AIG ‘09

Dispersion models – electrochemical

Govett, 1977Smee, 1983

Intro

MandySEx

ConcData

Models

SMEDG -AIG ‘09

Till

Varvedclays

Humus

Sulp

hideCountry

rock

SPSurface SP

responseCationconc.

Cation conc. in humus

Cation migration

Cation migration Current

lines

Equipotential lines

e-

Anode: 4FeS2 + 40H2O →4FeO.OH + 76H+ + 8SO4

2- + 60e-

Cathode: 15O2 + 30H2O + 60e- → 60OH-

Zone of increased cation conc. created by organic chelation of diffusive flux

Ions chelated by humus

Cation diffusion

Zone of increased cation conc. created by electrochemical migration

A horizon

B horizon

Clay

Volcanic Rocks

Water Table

Sulphide

Zone of CaCO3Accumulation

Upper limit of Carbonate

Oxidation ZoneLow pH

Carbonate Loss

Zn Cd

Zone of Metal Accumulation

O2

Zn2+ + Cu2+ + Fe2+

Reduced Column

Hamilton, 1998

Intro

MandySEx

ConcData

Models

Dispersion models – electrochemicalSMEDG -

AIG ‘09

A horizon

B horizon

Clay

Volcanic Rocks

Water Table

Sulphide

Oxidation ZoneLow pH

Carbonate Loss

Zn Cd

Zn2+ + Cu2+ + Fe2+

Reduced Column

Hamilton, 1998

Intro

MandySEx

ConcData

Models

Dispersion models – electrochemical

pHCa, REE

SMEDG -AIG ‘09

Transported Cover

Inherited metal content in regolith

[met

als]

with overprint due to dispersion away from mineralisation OR other features

Minn

A need to isolate this component or signal

which may constitute <0.1% of total signal

A basic premise

BedrockMandy

Models

Conc

Intro

Data

SEx

SMEDG -AIG ‘09

Integration of dispersion over timeProgressive accumulation of metalsStabilisation and fixation of pattern

Progressive degradationReworking of metals

Hydration - dehydration reaction cycling

Acid-base and redox reaction recycling

Increasing age of mineral phase in regolithLess transitory metal contents

Resistate minerals

SilicatesCryst. Fe-oxides

Mn-oxides &

am. Fe-ox.Carbonates

Adsorbed & Exch.species

Solublephases

Organics

Selective Extractions

Mandy

Models

Conc

Intro

Data

SEx

SMEDG -AIG ‘09

Mandy

Models

Conc

Intro

Data

SEx

(Cohen et al, 2001; based on Gray, 1999)

Carbonates Resistate mineralsSilicates

Mn-oxides &

am. Fe-ox.

Solublephases

Cryst. Fe-oxides

Adsorbed & Exch.species

Organics

SMEDG -AIG ‘09

Acetate + HOAc

HF / fusion

Aqua regia

Mixed acids (Canadian wine)

Weakly acidified NH2OH

EDTA / H+

Water (Qld Beer)

Strongly acidified NH2OH

Regoleach

HCl

Ammonium acetate

MMI

Guinness

Enzyme Leach / H2O2

Na-pyro / H2O2t

Selective Extractions

Mandamah, NSW

Lower Saprolite

50 m

30 mMHACD 228

MACD 312

206m @ 0.51 g/t Au0.37 % Cu

+ Mo

Ser-py altn

Andesite

Ser-c

hl-m

t-alb

altn

SEx

Conc

ModelsIntro

DataMandy

Mandamah

NSW Transported Regolith

SMEDG -AIG ‘09

Incr conc of some trace elementse.g. Cu, Ni and Znin SEx

Increasing Cs U Be Th in all SEx

Ba accum.

Higher leachableFe and Al

Na, S accum.

NaCl + SO42-

Increase in gypsum

Organic matter accumulation

Fe-enriched mottling zoneAccumulation of Cu, U & REE

Carbonate formationdol, gyp and Na2CO3

0 cm

25

50

75

100

125

150

200

SEx

Conc

ModelsIntro

DataMandy

5 6 7 8 9 10pH

SMEDG -AIG ‘09

mineralisation

pH 5 K-acetate

0.01

0.1

1

10

100

1,000

10,000ppm

Northern block Central block Southern block

CaMg

Ce

Cu

Mo

30 cm depth,

mineralisation

SEx

Conc

ModelsIntro

DataMandy

SMEDG -AIG ‘09

Mineralization

0

50

100

150m0.2123 % Cu

0 100 200 300 400 500 600 700mE

0

1.8m

CEC18

28 cmolc/kg

23

S0

240 ppm

120

Na0

1400 ppm

700

Ca0

1.0 %

0.5

pH

5.5

9.5

7.5

CEC

S

Na

Ca

pH

SEx

Conc

ModelsIntro

DataMandy

SMEDG -AIG ‘09

Mineralization

0

50

100

150m0.2123 % Cu

0 100 200 300 400 500 600 700mE

0

1.8m

Cu by aqua regia

8

20 ppm

14

Cu by NH2OH

2

6

4

Cu by NH4.OAc

0.0

0.24

0.12

Dep

th b

elow

su

rfac

e (m

)

ppm

ppm

SEx

Conc

ModelsIntro

DataMandy

SMEDG -AIG ‘09

0

40

80

120

160

200

ECa(mS/m)

EM38 vertical coil modeEM38 horizontal coil mode

240

CEC18

28 cmolc/kg

23

Ca0

1.0 %

0.5

Mineralization

0

50

100

150m0.2123 % Cu

0 100 200 300 400 500 600 700mE

0

1.8m

CEC

Ca

SEx

Conc

ModelsIntro

DataMandy

SMEDG -AIG ‘09

Initial weathering phase (period of elevated rainfall)

Dispersion model for Mandamah? archaeo-electrochemical

Ore body

Host rock

Weathering of mineralization and H+ production due to oxidn of sulfide minerals

In-situ30 m

Leaching of elements

Oxidn of mineralization and production of H+

and ore-related ionsSEx

Conc

ModelsIntro

DataMandy

Movement of Fe2+ and hydrolysis generating large amounts of H+

SMEDG -AIG ‘09

Ore body Host rock

In-situ

Transported

Migration of H+ into transported regolith and clay alteration

Pedogenic carbonate

General loss of pedogenic carbonate or limited formation

“Prograde” Phase(still high rainfall)

50 m

30 m

SEx

Conc

ModelsIntro

DataMandy

SMEDG -AIG ‘09

Ore body Host rock

In-situ

Transported

Addition of H+ to surface ceases or

diminishes

Pedogenic carbonate Zone of (palaeo) clay modification

with low CEC+

Non-carbonate alkalinity

Recent migration of mobile alkalis (e.g. Na).

“Retrograde” Phase(Post onset of aridity)

SEx

Conc

ModelsIntro

DataMandy

SMEDG -AIG ‘09

Data Analysis

Mul

tivar

iate

dis

tanc

e

Probability function

Objective – detecting samples whose geochemistry appears “anomalous”

101101001101011001010100010010010101100100101100101011001001011000101010010101010010101011010100101011010110111111010101011001010101111001101011100111111011010111001101110111100111110101011110100110001011101011011010010100101011101001011010110100001011010100011010111101011010111001100011101010110101101010110101000101010101001100011110011011011011001110110011100011010110110101010110101101100111011010110100010101001010

Conc

SExModels

Intro

Mandy

Data

Traditional approach – satellite spotting

SMEDG -AIG ‘09

Data transformed and standardised

-3

-2

-1

0

1

2

3

-3 -2 -1 0 1 2 3

Sm

Y

-3

-2

-1

0

1

2

3

-3 -2 -1 0 1 2 3

Fe

Al

Weak hydroxylamineAqua regia

Reality for SEx data – very complex multivariate signals

Mandamah101101001101011001010100010010010101100100101100101011001001011000101010010101010010101011010100101011010110111111010101011001010101111001101011100111111011010111001101110111100111110101011110100110001011101011011010010100101011101001011010110100001011010100011010111101011010111001100011101010110101101010110101000101010101001100011110011011011011001110110011100011010110110101010110101101100111011010110100010101001010

Conc

SExModels

Intro

Mandy

Data

SMEDG -AIG ‘09

Sample from above mineralised sites

Sample from above non-mineralised sites

1.2ppm

Cu

As1.0ppm

0.0

0.0

0.0

Ave

0.8

Enzyme Leach Aqua Regia 800

0

Chimborazo porphyry Cu, Chile

As average7-point moving As variance

gravel

0 500 1000 1500 2000 2500 3000 3500m

2500m

2300

Supergeneenrichment

Monzonite porphyry

Porphyryticandesite

Libra zone Virgo zone

101101001101011001010100010010010101100100101100101011001001011000101010010101010010101011010100101011010110111111010101011001010101111001101011100111111011010111001101110111100111110101011110100110001011101011011010010100101011101001011010110100001011010100011010111101011010111001100011101010110101101010110101000101010101001100011110011011011011001110110011100011010110110101010110101101100111011010110100010101001010

Conc

SExModels

Intro

Mandy

Data

SMEDG -AIG ‘09

SExModels

Data

Intro

Mandy

Conc

ConclusionsSMEDG -

AIG ‘09

RED scheme (Anand et al, 1998)

Summary of various forms of interactions of deeply buried mineral deposits with hydrosphere and biosphere

GW Table

Higher pHHigher pH

Dec

reas

ing

Eh

Transported cover

Bedrock

20 m

In-situregolith

Low pH

Sulphides

Reduced column

Cu2+

Vapor orgroundwater

dispersion

Microbial Redox

Cyclical dilatancypumping alongstructural zone

Biologicalaccumulation

SExModels

Data

Intro

Mandy

Conc

ConclusionsSMEDG -

AIG ‘09

SExModels

Data

Intro

Mandy

Conc

Seeing Geochemical Dispersion Haloes

OBJECTIVE:

Separate processes that have affected the elemental and mineralogical composition of regolith (both gain and loss)

METHODS:

Isolate signals that can be related to the effects or presence ofmineralisation through:

PHYSICAL (the extraction) and/or

NUMERICAL (data processing) methods

ConclusionsSMEDG -

AIG ‘09

SExModels

Data

Intro

Mandy

Conc

research into fundamental processes of geochemical dispersion development of new approaches to definition and detection of geochemical patterns related to the effects of underlying mineralisation

SEx not (yet) the silver bullet for thick transported or deeply weathered cover, but has prompted:

The bests tool for detecting deeply buried, sulphide mineralisation, may yet prove to be a simple pH meter and a bottle of dilute HCl

Developments in media and analysis, based on process modellingApplication of isotopes to refine understanding of dispersion mechanisms and regolith evolution modelsAbandoning “baselines”, “backgrounds” and “anomalies” in favour of new data processing methods that recognise geochemical signals related to processes of interest

Future Research Directions

Conclusions SMEDG -

AIG ‘09

Advance!

Dispersion models

Analytical improvements

General Ponsonby leads the Scots Greys against the

Polish lancers at Waterloo

Retreat!Old thinking on

defining geochem“anomalies”

Emperor Napoleon leads his remaining troops back

from Moscow

War of Attrition

Selective extractions

Exploring in deep transported

cover

Data modelling

Field Marshal Haig leads his troops

nowhere in particular

SExModels

Data

Intro

Mandy

Conc

SMEDG -AIG ‘09

Phys Geog GeologyFirst year (excludes engineering servicing)

Mineralogy Ore deposits Geology honsUpper years

Students

Students

0

50

100

150

200

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 20090

10

20

30

2 43

1 1

40

SExModels

Data

Intro

Mandy

Conc

SMEDG -AIG ‘09

People