Geochemical exploration in regolithdominated terrains global perspective. Ravi Anand May 2014

Transcription

1 Geochemical exploration in regolithdominated terrains global perspective Ravi Anand May 2014

2 Acknowledgements Numerous mining companies CSIRO/CRC LEME/MDU AMIRA

3 Why regolith research for mineral exploration 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.

4 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?

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

6 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

7 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 Rainforest, Stone line, Amazon Arid, calcrete, Yilgarn, WA Adriana Horbe

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

9 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)

10 Mapping regolith and landforms Complete profiles (R) Truncated (E) Depositional (D) Arid Inverted landscape Savanna Rainforest Costa, 1993 Adriana Horbe

11 Factual regolith-landform map

12 Interpretative regolith-landform map (Sampling strategy map) Regime Sample Relict Erosional Depositional Lateritic duricrust and/or gravel 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

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

14 Dispersion model: Relict regime (Complete profile preserved) Lateritic residuum (Residual nodules and pisoliths) Cu in cortices Biogenic Au 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

15 Buried lateritic residuum below cover is effective sample media Anand and Smith

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

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

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

19 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)

20 Dispersion mechanism: Vegetation Metal uptake by deep tap root system and laterals

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

22 Vegetation can form anomaly through 30 m transported cover, Freddo Au deposit, Yilgarn Craton Biogenic particulate Au in Eucalyptus leaves: varies from 2 to 68 ppb in a single tree Organic Gold particles (red) within leaves

23 Dispersion mechanism: Termites Moolart Well Au deposit, Yilgarn Craton (Aqua regia) 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 Jaguar VMS deposit: Termite mandibles Mnn M n Br MZn n

24 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

25 Dispersion mechanism: Gaseous 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

26 Microbial processes are important in anomaly formation: example from VMS (Cu-Zn-Ag) deposit Mineralised Background % similarity background mineralised 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

27 Pit experiments: Anomalies can form quite quickly Pit Experiment Water extraction 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

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

29 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.

30 AMIRA/ADIMB P1123: Geochemical exploration in regolith-dominated terrains A global perspective MINERALS DOWN UNDER FLAGSHIP

31 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

32 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