TAG - Geological Society of Australia

Essay Review A guide for mineral exploration through the regolithin the Yilgarn Craton, Western Australia, (AJES Vol 57; No. 8) RR Anand and CRM Butt30 |Asustained, multi-decadal researchprogram into the development of theYilgarn Craton’s regolith and relatedimplications for mineral exploration by theCSIRO and collaborating organisations hasarguably been one of the most successful inthe history of mineral exploration relatedresearch, particularly if success is measured byimpact on industry practice. The majority ofthis research was carried by a Co-operativeResearch Centre, CRC LEME and its successor,colloquially referred to as CRC LEME II, over afourteen year period during the 1990s and2000s. A hallmark of this research programwas very close interaction with industry anda very strong focus on delivering pragmaticoutcomes to help exploration. Therefore, as anindustry, we are very fortunate that RaviAnand and Charles Butt (two of the giants ofthis field, along with Ray Smith) have writtenan extremely comprehensive review for us.They have chosen to publish this review inAJES and their paper (published in December2010) comprises the entirety of Issue 8 inVolume 57 of the journal. This choice nodoubt reflects both the standing of AJES as ajournal and a desire to ensure that this workis easily and widely accessible for manydecades to come, something that might notbe achieved with a stand-alone publication.At 99 pages, this paper is epic in scopecovering topics from the long-termweathering history of the landscape throughto detailed issues relating to practicalsampling strategies. In essence it is a“geochemical exploration manual” for anyexplorer in the Yilgarn targeting gold, nickel,base-metal or uranium deposits. In mostcontexts, this paper alone will provide theexplorer with a ‘one-stop’ shop with respectto the knowledge required to plan andexecute effective geochemical explorationstrategies, however the paper has acomprehensive reference list if more detail issought on a particular topic, or for a particulararea. Importantly, in an era where effectivevisual presentation of geological concepts iscritical to maintaining management orinvestor support for exploration, this paper isbeautifully illustrated. These illustrationsinclude high-quality photographs of relevantregolith materials, real examples of maps andsections and schematic block diagrams. Thelatter are superb visual syntheses of muchcomplex research that rapidly communicateoptimum sampling strategies and relatedcritical issues in a comprehensive range oflikely real-world exploration scenarios. TheseTAG September 2011diagrams are imminently suitable for inclusionin an Exploration Manager’s PowerPointpresentation aimed at explaining a particularproject concept to investors.The paper is very well organised, comprisingten sections: Introduction, Regolith (anoverview of its character and evolution,together with important contextual issuessuch as patterns of regional groundwatercomposition), Exploration Challenges andStrategies, Regional-District ScaleExploration- General Principles, ExplorationStrategies for Gold Deposits, ExplorationStrategies for VHMS Base-Metal Deposits,Exploration Strategies for Nickel Deposits,Exploration Strategies for Surficial UraniumDeposits, Sample Preparation, Analysis andAnalytical Control, and Conclusions.The Regolith section provides a succinct andaccessible summary of a vast body of researchon this topic (carried out over many decades)into a coherent synthesis that must underpinany thinking about geochemical exploration inthe Yilgarn. However, as the above outlinemakes clear, the strong focus of this paper isnot simply on just providing a conceptualframework for exploration but also on givingus detailed guidance on how to actuallysample and explore. The paper includeswell-organised flow charts that lay out boththe process of regolith mapping and theprocess of selecting the optimum geochemicalsampling strategy for a particular area. I wasparticularly impressed with the level ofpractical detail provided about samplingstrategies, with guidance provided on topicsas diverse as to how to sample ground-water(and importantly how to treat your sampleafter you have taken it to ensure meaningfulresults) and the need to store sulphide-richanalytical reference standards in a desiccatorto avoid degradation. Some of the potentialpitfalls highlighted when collecting sampleswere also interesting. We all know we have tobe careful about gold wedding rings but howmany of us would automatically think aboutthe potential Zn or Cr contamination thatmight be derived from the red or green painton our shovel?The paper also addresses some of the majortopics of debate in geochemical samplingstrategies in the Yilgarn over the years.The authors find little evidence for verticalgas-related metal transport and littlesuggestion that partial leach geochemicalmethods provide reliably superior results whensampling in covered terranes. They also findlittle evidence in support of collecting amagnetic lag fraction, as in most cases thenon-magnetic goethite fraction tends to be abetter concentrator of target metals. Althoughmagnetic concentration might assist whenthere is significant eolian dilution of thesample, they suggest that screening out thewind-blown sand fraction might be a betterapproach.Some of the more recent innovative advancesin regolith mapping are also discussed. Theseinclude using the spectral response ofkaolinite to discriminate transported clay-richsediments from clay-rich in situ saprolite(often fiendishly difficult in the field) and theapplication of scale-specific multi-scaleparameters to resample SRTM data in order toreveal very subtle features such as sheetwashfans (where surface sampling is likely to beineffective) within a basically flat landscape.Although the focus of the paper is thedevelopment of geochemical samplingstrategies, I would not want the reader to gainthe impression that the role of geophysics hasbeen ignored. There are lucid discussions ofthe role that various geophysical technologiescan play in helping to map the regolith,particularly in the sub-surface. Geophysicalmethods discussed include gravity, magnetics,EM, SAM, resistivity mapping andradiometrics. The role of remote sensingapproaches is also discussed.It is hard to be critical of such a magnificentpiece of work as that represented by thispaper. However, there are a few topics whereField photograph showing different group of sedimentsunderlain by an in situ weathered profile, Lancefield,Figure 14, Australian Journal of Earth Sciences, Volume57/8, 2010.