Ravi Anand May 2014

Geochemical exploration in regolith-dominated terrains – global perspective

Acknowledgements

• Numerous mining companies

• CSIRO/CRC LEME/MDU

• AMIRA

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?

Impact: Discovery of Bronzewing gold deposit by buried lateritic residuum sampling

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

Factual regolith-landform map

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

Buried lateritic residuum below cover is effective sample media

Anand and Smith

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

Dispersion mechanism: Vegetation

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

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

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

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