Ravi Anand
May 2014

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Transported cover
In situ regolith
 Regolith masks mineral deposits. BUT
 Weathering produces many secondary deposits: Al, Nb, Ni, Co, Au, Mn, Fe, P, Li, U
 Subtle dispersion patterns in regolith; important geochemical sampling medium
 Mineral industry in Australia (and world wide) is vitally concerned with locating new deposits under cover.
 This need for new mineral discoveries has been the driving force behind regolith research in Australia.

• What useful information can be obtained from the regolith?
• How can we distinguish between residual and transported regolith?
• What is the geochemical/mineralogical ‘fingerprint’ of a concealed ore deposit in deeply weathered terrain? How reliable is this? What media should be sampled?
• Do ore deposits, buried under transported overburden have a surface or near-surface geochemical expression?
• Can we distinguish between null and negative result?
• How can we predict what sample media works where and why?

 Deeply weathered profiles, ferruginous or bauxitic towards the surface are widespread
 Commonly overlain by transported cover
 Regionally continuous over large areas
 The regolith has been forming continuously for over 100 my
 Continue to evolve under savanna, rainforest and arid climates and a variety of landscape processes

Modification of regolith by climatic conditions: we need to understand these variations
Savanna, West Africa Rainforest, Latosol, Amazon
 Different climatic conditions produce modifications to pre existing profiles
 Modifications give rise
to new geochemical
parameters that will affect
general procedures for geochemical exploration
in various climatic regimes.
Claudio Porto
Adriana Horbe

Climate conditions influence the distribution of metals (e.g,
Au) and hence sample media for exploration
Compiled from several sources

Development of complex weathering profiles by landscape processes and multiple weathering
 Weathered profiles have residual and transported components
 Several phases of Fe, Ca, Si and Al
minerals-each with total or partial resetting of geochemistry
 Systematic approach to identifying regolith
materials
 Link evolution of regolith to
geochemical processes and sampling strategies
Ferricrete (transported)
Lateritic residuum (residual)

Complete profiles
(R)
Truncated
(E)
Depositional
(D)
Arid
Inverted landscape
Savanna
Rainforest Costa, 1993
Adriana Horbe

Factual regolith-landform map

Interpretative regolith-landform map
(Sampling strategy map)
Regime Sample
Relict
Lateritic duricrust and/or gravel
Ferruginous lag & saprolite
Erosional
Depositional
Soil – note colluvial & aeolian input)
Establish depth of overburden nature of residual profile:-
Buried lateritic residuum preferred if present
If cover <2m thick: soil
If cover >2m thick: Vegetation
Termite mounds
Calcrete
Gases
Interface
Saprolite
groundwater
termite mounds

Dispersion model, Erosional regime (Truncated profile)
Regolith profile: Erosional regime
Residual soil
Saprolite
Anomaly in soil and lag due to:
 Bioturbation, Residual and Chemical dispersion
 Dispersion halo is narrow (50-100 m)
Sample media:
 Soil
 Lag
 Saprolite

Dispersion model: Relict regime (Complete profile preserved)
Lateritic residuum (Residual nodules and pisoliths)
 Dispersion halo is much larger than
ore deposit itself
 Residual, biological and mechanical dispersion
 Goethitic cortices are important carrier of metals
 Large proportion of Au is biogenic
Biogenic Au

Anand and Smith

The Challenge - Seeing through transported cover in a cost effective manner
Australia Brazil
Paleochannel clays, sand and gravel
Belterra clay
Mottled clays
Mineralisation
 Surface techniques have tremendous advantages for mineral exploration
 Partial extractions had limited success
 Poor understanding of vertical metal migration processes
Adriana Horbe

Understanding mechanisms that can form anomalies through transported cover in various climatic zones

Cross Lake VMS deposit, northern Ontario, Canada
 Glaciated terrain
 High water table
 Transported overburden
(30-50 m) overlying sulphide mineralisation
0-10 cm soil
10-20 cm
VMS mineralisation
Cameron et al. (2004)

Dispersion mechanism: Seismic pumping in neotectonic active areas
Spence Cu deposit, Northern Chile
Vertical fracture in saline soil
 Over 250 m of gravel overlying mineralisation
 Earthquake prone area
 Movement of metals along vertical fractures
Cameron et al 2004; Kelley et al, (2006)

 Metal uptake by deep tap root system and laterals

Mapping of ore-related elements by PIXE and Synchrotron in leaves and roots from various deposits
Cu-Zn-Ag
Au

Biogenic particulate Au in Eucalyptus leaves: varies from 2 to 68 ppb in a single tree
45
5
2
68 4
6
1
2
7
2
Organic
Gold particles (red) within
leaves

Dispersion mechanism: Termites
Moolart Well Au deposit, Yilgarn Craton  5-15 m of transported cover
 Response in termite mounds but not in soil using aqua regia
( Aqua regia ) or partial extractions
 Response in termite mounds
in shallow cover only
Jaguar
VMS deposit:
Termite mandibles
Mn n
M n
Br n

Why soil anomaly not always formed despite anomaly in vegetation, termite mounds or gas collectors?
Normal environment Dust storm
 Erosion by sheetwash, flooding
and wind
 Erosion > Input = No anomaly in soil

North Miitel Ni deposit,
Yilgran Craton
Ni on activated carbon
 15 m transported cover
 Highly saline water
 Anomaly only in gas collectors but not in soil or vegetation

Mineralised
Microbial processes are important in anomaly formation: example from VMS (Cu-Zn-Ag) deposit
Background
% similarity
Bands associated with ‘mineralisation’ were DNA sequenced
 Generated a library of ~100 DNA sequences:
1) Target for exploration
2) Provide insights into microbial species associated with mineral interaction in regolith

Pit experiments: Anomalies can form quite quickly
Water extraction
Pit Experiment
 Six pits were dug
 Ores (VMS-Cu-Zn-Ag, Au) and salts buried under stagnant and nonstagnant environments
 Elevated concentrations of Zn, Cu and Au in soil after 7 months in stagnant environments.
 Seasonal variations in metal migration
Column experiments
Ore was placed on tray

Summary of dispersion mechanisms at sites investigated in Australia (AMIRA P778)

• Understanding terrain evolution is of more than academic interest.
• Select geochemical methodologies to suit the regolith terrain and interpret the results appropriately
• Lateritic residuum (relict regime) and soil and lag are effective sample media in erosional regime
• In depositional regime, more than one mechanism of vertical metal migration is likely to operate in a given setting.
• Electrochemical dispersion, seismic pumping, vegetation, termites, gaseous and capillary are important mechanisms for vertical transport of metals.
• Geochemical anomalies can form quite quickly.
• Research is required in Brazilian and African environments.

• MINERALS DOWN UNDER FLAGSHIP

Key objectives: proposed project (AMIRA /ADIMB 1123):
• Develop consistent and uniform system for identifying, describing and naming of regolith materials.
• Determine the processes of formation of regolith materials (regolith mapping) and metal dispersion in various climatic regimes.
• Determine the suitability of regolith materials as geochemical sample media in various climatic regimes
• Investigate the effect of provenance and properties of transported overburden and soil, on metal migration.
• Design pit experiments to verify the existence of specific metal migration mechanisms.
• Develop geochemical dispersion models and guidelines
• Prepare an atlas of various regolith types
• Translate research results into more cost-effective mineral exploration

• Participation in collaborative research with significant funding leverage
• Access to extensive experience and knowledge of world class regolith team
• New/improved cost-effective and practical exploration methods for exploring relict, erosional and depositional environments.
• Advanced characterisation of materials
• Guidelines for how, where and why to use regolith materials
• Better distinction between the negative and null result
• Training and workshops