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Open File Report 156
Preface & Executive Summary

The South Australian Regolith Project Final Report - Summary and Synthesis

Compiled by M. J. Lintern


One of the original objectives of the CRC for Landscape Evolution and Mineral Exploration (CRC LEME 1) at its commencement in 1995 was to establish a research node in Adelaide, as part of an overall strategy to develop mineral exploration procedures for the principal regolith-dominated mineral districts in Australia. Planning for this node anticipated the upsurge in exploration activity in the Gawler Craton, and was greatly assisted by the Department of Mines and Energy South Australia (MESA; now Primary Industries and Resources South Australia, PIRSA) , which was a supporting participant of the CRC. One of the principal drivers was to assist industry in the use of calcrete geochemistry, which was by then being widely used for regional to local scale exploration for gold on the Gawler Craton. Melvyn Lintern had undertaken much of the original research on calcrete geochemistry in the Yilgarn Craton and he transferred from Perth to the MESA office in Adelaide in October 1996 to continue his work in this new district. In collaboration with Malcolm Sheard at MESA, he developed a number of research activities under the broad banner of the South Australian Regolith Project. This involved scientific collaboration between CRC LEME (Perth, Adelaide and Canberra), MESA, universities and the exploration industry from 1996 to 2001. Studies were conducted at sixteen prospects and deposits in the Gawler Craton and Curnamona Province of South Australia. Research themes included regolith-landform mapping, regolith geochemistry, biogeochemistry and hydrogeochemistry, although not all of these were pursued at each site. A number of commodities were examined, but gold was the principal focus. The results of this research are summarized in this report.

In addition to the scientific findings, one of the most important outcomes of the establishment of the CRC node in Adelaide project has been the interest in regolith geoscience that it engendered. A consequence has been that PIRSA, the University of Adelaide and CSIRO Land and Water became Core Partners to CRC for Landscape Environments and Mineral Exploration (CRC LEME2) from July 2000.

Dennis Gee - CEO, CRC LEME2 &
Charles Butt - Leader, Shields Program, CRC LEME1

23rd March 2004

Executive Summary

The principal aim of the South Australian Regolith (SAR) Project was “to develop technically efficient procedures for mineral exploration in the major Cratons of South Australia through a comprehensive understanding of the processes of regolith development and landscape evolution and their effects on the surface expression of concealed mineralization”. The Project involves scientific collaboration between CRC LEME, PIRSA, universities and the exploration industry from 1996 to 2001. Industry input was a key element throughout the project since most of the case studies herein were based on data provided by companies.

Studies were conducted at sixteen prospects and/or deposits in the Gawler Craton and Curnamona Province of South Australia. In addition, regional studies into dating and isotopes were undertaken. Research themes were regolith mapping, regolith geochemistry, surface geochemistry, biogeochemistry, and hydrogeochemistry. Not all of the themes were included for each study due to different sub-project objectives and resource limitations. A number of commodities were examined including Au, Cu, Ag and Pb, with the overwhelming proportion concerned with Au. Similarities and differences of the research between sites were developed, and models of geochemical dispersion produced. Some of the more important conclusions and recommendations are summarized below:

1. Surficial geochemical sampling programs are sensitive to regolith materials and depth of transported overburden and it is therefore important to establish the regolith stratigraphy and landforms for the area being explored. Remote sensing methods such as radiometrics, aerial photography, Landsat TM, ASTER and AIRSAR can give important information on the nature of the land surface and as an aid to mapping the distribution of surficial materials. Ground penetrating methods such as AEM may give sub-surface information such as the presence and depth of palaeochannels.

2. The construction of large scale regolith-landform maps (preferably more detailed than 1:10000) is recommended. These maps provide information on the distribution of regolith materials but should additionally provide some indication as to the extent of transported materials and thickness at the prospect scale. Small scale regolith maps e.g. 1:100000 provide an overview but have insufficient detail for any sampling programmes.

3. Distinguishing in situ from transported regolith is important for exploration as geochemical responses will differ depending on the depth of cover. The presence of cover may be inferred from field regolith-landform relationships, although drilling can provide definitive information. The use of PIMA spectra can in some cases establish transported-in situ boundaries. In some circumstances geochemistry can also discriminate between cover sequences and weathered crystalline basement.

4. Calcrete is the best near surface sampling medium for Au and should be used as a first pass geochemical sampling technique. It occurs usually within a metre of the surface and is readily identifiable using dilute acid. It works best as a guide to mineralization where transported overburden is absent or thin (<5 m), and where there is development of saprolite rather than fresh rock close to the surface. Local topography may lead to the development of transported anomalies located away from their source mineralization. For Cu, specific environments (high water table, acidic groundwaters and <5 m of transported material) may lead to upward dispersion of Cu with a precipitation in alunite at the base of the calcrete horizon due to a pH change.

5. Hilly terrain is well suited to stream sediment sampling, and orientation surveys investigating the most appropriate size fraction(s) are recommended at each site.

6. Biogeochemical methods were shown to be of limited application in the Gawler Craton. Understanding over how and why metals accumulate in plants and form anomalies remains limited. Several unexplained anomalies require further testing.

7. In the absence of calcrete, other sample media for geochemical exploration may be used but responses are either weaker or more erratic. Silcrete has been demonstrated to be a credible sample medium for Au exploration provided that it has developed within in situ materials. Soil commonly has an aeolian component, so the use of fine or coarse size fractions is recommended in order to remove sand that acts as the chief diluent to elements of interest. Groundwater as a sampling medium was not investigated to any great extent.

8. Although they may have some merit for investigating the nature of anomalies and how they form, partial extractions per se are not recommended as conventional total extraction procedures were found to be equally as satisfactory, easier to interpret and more cost-effective. Selective extractions are potentially of greater benefit since they may be used to understand the behaviour of elements in regolith materials. They indicate if it is worth targeting a particular mineral or size fraction in a sample. There are many different types of selective extraction procedures and a few were tested during the course of this project. Tests for utility in finding buried Cu and Co mineralization were unsuccessful.

9. Multi-element geochemistry should be used with caution. Understanding the nature of the mineralization being sought, potential associated pathfinders, the type of regolith material being sampled and the extra cost are important considerations for its use. For Au in the western Gawler Craton, multielement

geochemistry was of limited utility since mineralization was not usually associated with rich concentrations of pathfinder elements such as As or Cu, as may be found in the Yilgarn Craton, for example. Furthermore, the paucity of Fe-rich regolith materials, such as lateritic duricrust or ferruginous lag, meant that these metal-scavenging materials cannot be used systematically in an exploration program.

10. For calcrete, isotope data are consistent with a predominantly marine source for the Ca and a biological origin for the C. This is consistent with other studies on calcretes from South Australia and in other parts of the world. The S isotopes suggest a marine source although the distribution of discrete accumulations of gypsum in certain portions of the regolith at Challenger Gold Deposit are problematic.

M.J. Lintern
Project Leader

March 2004


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