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.
Why regolith research for mineral exploration
In situ regolith
Transported cover
Needs of the mineral industry
• 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?
Distribution of regolith and present climatic zones
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
Savanna, West Africa Rainforest, Latosol, Amazon
Rainforest, Stone line, Amazon
Arid, calcrete, Yilgarn, WA
Modification of regolith by climatic conditions: we need to understand these variations
Different climatic conditions produce modifications to pre existing profiles
Claudio Porto
Adriana Horbe
Modifications give rise to new geochemical parameters that will affect general procedures for geochemical exploration in various climatic regimes.
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
Lateritic residuum (residual)
Ferricrete (transported)
Complete profiles Truncated
Depositional
Inverted landscape
Costa, 1993 Rainforest
Savanna
(E) (R)
(D)
Arid
Mapping regolith and landforms
Adriana Horbe
Interpretative regolith-landform map (Sampling strategy map)
Lateritic duricrust and/or gravel
Regime Sample
Relict
Erosional
Depositional
Ferruginous lag & saprolite
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)
Residual soil
Saprolite
Anomaly in soil and lag due to: Bioturbation, Residual and Chemical dispersion Dispersion halo is narrow (50-100 m)
Regolith profile: Erosional regime
Sample media: Soil Lag Saprolite
Dispersion model: Relict regime (Complete profile preserved)
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
Lateritic residuum (Residual nodules and pisoliths)
Cu in cortices Biogenic Au
The Challenge - Seeing through transported cover in a cost effective manner
Australia Brazil
Red clays
Mottled clays
Grey clays
Belterra clay
Clays and gravel cover
Mineralisation
Adriana Horbe
Paleochannel clays, sand and gravel
Surface techniques have tremendous advantages for mineral exploration Partial extractions had limited success Poor understanding of vertical metal migration processes
Understanding mechanisms that can form anomalies through transported cover in various climatic zones
Dispersion mechanism: Electrochemical dispersion
Glaciated terrain
High water table Transported overburden (30-50 m) overlying sulphide mineralisation
Cross Lake VMS deposit, northern Ontario, Canada
VMS mineralisation
0-10 cm soil
10-20 cm
Cameron et al. (2004)
Cameron et al 2004; Kelley et al, (2006)
Dispersion mechanism: Seismic pumping in neotectonic active areas
Vertical fracture in saline soil
Spence Cu deposit, Northern Chile
Over 250 m of gravel overlying mineralisation
Earthquake prone area
Movement of metals along vertical fractures
Mapping of ore-related elements by PIXE and Synchrotron in leaves and roots from various deposits
Au
Cu-Zn-Ag
Vegetation can form anomaly through 30 m transported cover, Freddo Au deposit, Yilgarn Craton
4 68
7 2
1
2 2
6 45
5
Biogenic particulate Au in Eucalyptus leaves: varies from 2 to 68 ppb in a single tree
Gold particles (red) within leaves Organic
Dispersion mechanism: Termites
5-15 m of transported cover
Response in termite mounds but not in soil using aqua regia
or partial extractions
Response in termite mounds
in shallow cover only
Moolart Well Au deposit, Yilgarn Craton
Jaguar VMS deposit: Termite mandibles
Mn
Mnn Mn
Br Zn
(Aqua regia)
Why soil anomaly not always formed despite anomaly in vegetation, termite mounds or gas collectors?
Erosion by sheetwash, flooding
and wind
Erosion > Input = No anomaly in soil
Normal environment Dust storm
Dispersion mechanism: Gaseous
North Miitel Ni deposit, Yilgran Craton
15 m transported cover Highly saline water Anomaly only in gas collectors but not in soil or vegetation
Ni on activated carbon
Microbial processes are important in anomaly formation: example from VMS (Cu-Zn-Ag) deposit
Mineralised Background
min
eralise
d
backgro
un
d
% 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
Pit Experiment
Column experiments Six pits were dug Ores (VMS-Cu-Zn-Ag, Au) and salts buried under stagnant and non-stagnant environments Elevated concentrations of Zn, Cu and Au in soil after 7 months in stagnant environments.
Seasonal variations in metal migration
Water extraction
Ore was placed on tray
Conclusions
• 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.
AMIRA/ADIMB P1123: Geochemical exploration in regolith-dominated terrains – A global perspective
• 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
Benefits to sponsors
• 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