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Geochemical Exploration inLateritic Terrains
Paul Agnew
Chief Geochemist
Rio Tinto Exploration Pty. Ltd.
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Presentation outline
The LateriticWeatheringProfile
GeochemicalExploration inLateriticEnvironments
AnalyticalTechniques
TheMicroscopicFuture
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Global Lateritic Weathering
Global Distribution of Lateritic Soils
LateriticWeathering
Potential
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Deep Lateritic WeatheringA process of Landscape Reduction in tropical environments
Multiple periods of extended weathering caused by fluctuating water table Leads to the development of a weathering profile
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Lateritic Landscapes - Australia
Australian Dry Lateritic Terrain - A challenge to early explorers
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Lateritic Landscapes - Australia
Australian Dry Lateritic Terrain - A challenge to early explorers
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Lateritic Landscapes - Australia
Australian Dry Lateritic Terrain - A challenge to early explorers
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Regolith :Everything between Fresh Rock and Fresh Air!
The Lateritic Profile
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Basalt
Saprock
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Regolith :Everything between Fresh Rock and Fresh Air!
The Lateritic Profile
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The Lateritic Profile
Leached
Saprolite
Ferruginous
Saprolite
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Regolith :Everything between Fresh Rock and Fresh Air!
The Lateritic Profile
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Clay Zone Leached Saprolite
Clay Zone Leached Saprolite
Ferruginous Saprolite
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Regolith :Everything between Fresh Rock and Fresh Air!
The Lateritic Profile
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Ferruginous
Saprolite
Mottled Saprolite
Mottled
Saprolite
Mega Mottled
Saprolite
Mottled Saprolite
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Regolith :Everything between Fresh Rock and Fresh Air!
The Lateritic Profile
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Lateritic Duricrist
Lateritic Duricrist
Lateritic Duricrist
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Ferruginous Duricrist
Bauxitic Duricrist
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Regolith :Everything between Fresh Rock and Fresh Air!
The Lateritic Profile
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Lateritic Gravel
Pisolitic Gravel
Lateritic Duricrist
Mottle Fragments
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Pisolitic Lag
Pisolitic Gravel
Lateritic Soil
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Regolith Controls on GeochemicalDispersion
No geochemical Response
Strong anomaly
Wide dispersion
Moderate anomaly
Wide dispersion
Weak / absent anomaly
Narrow dispersion
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Residual Erosional Depositional
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Geochemical Exploration in Lateritic Terrains
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Public DomainGeochemicalSampling
Western Australia:n= >194,000
Ni Deposits and Occurrences
Next SlideGeochemical Sample Location
(soil, pisolitic lag, laterite,
ferruginous saprolite, sediment,
rock)
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Public DomainGeochemicalSampling
Western Australia:n= >194,000
Percentile Sliced Ni(All Media Raw)
Next Slide
Next Slide
Ni Deposits
andOccurrences
Next Slide
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SW Western Australia Regolith Map1:500,000
Depositional
Aeolian ColluviumSheetwash
Residual;Ferruginous,Siliceous,
Calcareous
Duricrust
ErosionalSaprockSaprolite
Fe-Saprolite
DepositionalAlluvial-Lacustrine Deposits
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Public Domain Regolith SamplingSW Western Australia: n= 30,488
Ni Deposits andOccurrences
Percentile Sliced Ni
(All Media Raw)
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Analysis of Lateritic Sample Media
Lateritic weathering leads to intense
leaching and concentration through
residual accumulation
Lateritic materials therefore need both
low detection and strong digestion toreveal the complete geochemical
patterns
They also require matrix matched
standards (leached and ferruginousmatrices)
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Basic Exploration Suite
Strong Total Acid Digest (4 acid)
XRF is a option but may lack low
detection limits required for some ore andpathfinder elements
Inductively Coupled Plasma
Optical Emission Spectroscopy
ICP-OES
Al, Ba, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na,
Ni, P, Sr, Th, Ti, V, Zn, Zr
Inductively Coupled Plasma
Mass Spectroscopy
ICP-MSAg, As, Bi, Cd, Mo, Nb, Pb, Sb, U, W
Fire Assay (Pb collection, ICP-MS finish)
Au, Pt, Pd
Ore and path-finder
elements
Lithochemistry +
Transported Overburden ID
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Basic Exploration Suite - Mineralogy
Portable Spectroscopy (PIMA, Terraspec)
Particularly applied to identification of transported overburden onleached saprolite which can be visually very similar. Illite crystalinity
Potential identification of alteration minerals preserved in
saprolite / saprock
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Our Microscopic Future..
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Mineral Liberation Analyser (MLA)
Approx30,000
Grains
per block
(75-250um)
16 blocks
per
automated run
Modal Mineralogy
Mineral
Chemistry
High quality data
>1% detection
limit
Mineral associations
Chemical mapping
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Mineral Liberation Analyser (MLA)
Minerals identified by comparison of ED spectra with standards
Reference high quality high Mg ilmenite spectrum
Rapid (200ms) spectrum
Produced by high Mg ilmenite grain
Mg
peak
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Mineral Liberation Analyser (MLA)
Mineral classification mapReference
Mineral Area_%
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Mineral Liberation Analyser (MLA)
Cu Anomalous -80# Stream Sediment Sample 120ppm Cu
(Heavy Mineral Concentrate)
Quartz
Rutile
Other
Pyroxene
Apatite
Aluminosilicates
Amphibole
Augite
Feldspar
Phlogopite
Chlorite
Clinozoisite
Bornite
Chalcopyrite
Pyrite
IronOxide
IronOxide_Ti
IronOxide_Altered
IronOxide_Other
Ilmenite
100um
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Mineral Liberation Analyser (MLA)
Stream sediment sample from a deep lateritic weathering terrain in
NW Queensland, Australia.
500m
Aluminous Fe Oxides
Gibsite
Gibsite_FeTi
Fe Oxides
Clay
Other
Weipa
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Mineral Liberation Analyser (MLA)
Some of the resistate Fe-oxides associated with known bauxite
occurrences have distinctive chemical compositions
Darling Range Bauxite
Huntley Deposit
Ironstone
l b l ( )
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Mineral Liberation Analyser (MLA)
Direct detection of bauxite profiles - Curua Bauxite Discovery, Brasil
BR500129
65Mineralised at
surface
BR500129680-50cm
Bauxite
Mineralised
Auger Hole
BR50012416
0-50cm
Barren
Auger Hole
i l ib i l ( )
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Mineral Liberation Analyser (MLA)
Direct detection of bauxite profiles - Curua Bauxite Discovery, Brasil
BR50012416
Unmineralised
Auger Hole
BR50012968 - Mineralised BR50012416 - Barren
500
m
Aluminous
Fe Oxides
Gibsite
Gibsite_FeTi
Fe Oxides
Clay
Other
C l i
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Conclusions 1
Lateritic weathering impacts
exploration globally
Lateritic weathering processes
lead to the progressive volumetric
reduction of the regolith profile,
the total destruction of rock fabric,
and dramatic modification of
geochemical composition,presenting a significant challenge
to mineral explorers
Pioneering work by CSIRO in
the early 90s demonstrated that
geochemical explorationtechniques can be effectively
applied in lateritic terrains
C l i
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Conclusions 2
Basic regolith mapping
(Residual-Erosional-Depositional)
provides essential context forgeochemical exploration surveys
The more ferruginous Residual
(relict) materials are excellent sample
media which generate a broad, multi-
element geochemical halo
Ferruginous components of the
Erosional zones are also viable sample
media but most saprolite is intensely
leached and anomalism may be
completely absent, low level or in highlyimmobile elements
There are no reliable surface
geochemical techniques to see
through Depositional cover
C l i
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Conclusions 3
A strong digest and a low
detection limit multi-elementanalytical suite is required
PIMA or Terraspec mineralogy is
very useful in lateritic terrains,
particularly for identifyingdepositional materials
The MLA offers exciting new
analytical capacity to identify
minerals in geochemical samples
very rapidly and investigate the
chemistry of selected phases if
required.
Ob i d !
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Obrigado!