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Open File Report 150
Preface and Executive Summary

Regolith Studies at Edoldeh Tank (ET) Gold Prospect, Gawler Craton, South Australia

M. J. Lintern, I. J. Tapley, M. J. Sheard, M. A. Craig, G. Gouthas and A. J. Cornelius

Compiled by M. J. Lintern and M. A. Craig

The Edoldeh Tank (ET) Regolith Project is part of the broader “South Australia Regolith Project” of CRC LEME, the principal aim of which is “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 mineralisation”.

This project is a follow-on to an initial study of regolith geology and geochemistry in the Gawler Craton, sponsored by CRC LEME, PIRSA and Gawler Joint Venture exploration tenement holders completed in 2002. It was developed after extensive consultation with industry geologists, and an excursion to various sites. The principal problems identified relate to verification of the thickness of transported overburden and to procedures for exploration in terrains where transported overburden is present.

The principal objective of the ET Regolith Project was to evaluate the use of components of transported overburden for detecting buried Au deposits in the western Gawler Craton. Five prospects (ET, Monsoon, South Hilga, Golf Bore and Jumbuck) were initially investigated. The objective was to determine the most suitable site for a detailed study. A single traverse, several hundred metres in length, was selected across each prospect that had mineralisation, transported overburden and adequate drill spoil. The regolith stratigraphy was described at each site and drill spoil collected for multi-element analysis. An existing company geochemical database for each prospect was also used. The results of the five prospect survey are reported in Lintern et al, 2002. The initial studies (Lintern et al, 2002) indicated that ET has the following properties:

  1. two hundred drill holes spread over the prospect with cuttings in good condition;
  2. a large Au in calcrete anomaly that had not been linked to a primary source, leaving potential for additional areas of investigation and possible drill targets;
  3. indications that mineralisation might be expressed in the transported overburden;
  4. a greater spread and thickness of transported overburden compared with other sites; and
  5. sand dunes, typical of the western Gawler Craton, and a hindrance to exploration in this region so there are few, if any, previous studies in this type of terrain.

Regolith mapping at ET was based on the identification and detailed representation of the distribution of surficial material and the broader surface regolith units. The map was not only based on datasets commonly used in regolith map construction e.g., Landsat TM, aerial photography and DEM, but also processed AIRSAR, and HyMapTM data sets played an important role in determining the final regolithlandform relationships. Regolith stratigraphy was determined using field observations, collection and analysis of drill cuttings from chip trays, and PIMA analysis. These data determined relationships between materials, weathering, landform history and the geochemical data. Drill cuttings, calcrete, soil and vegetation were collected to determine the distribution of Au and other elements, which was visualised in 3D and 2D.

The principal results indicate that:

  1. In in situ regolith, Au was found to be concentrated in calcrete near the surface and above an interpreted leached zone of low Au concentration (leached zone?) that can extend down several tens of metres into the weathered regolith. The surficial Au anomaly extends from the ridge into transported regolith and, in places with thin (<5 m) sand cover, appears to be locally enhanced, possibly due to Au additions to the surface from underlying buried mineralisation in the transported overburden. Calcrete appears to be the best sample medium for Au exploration over soil, vegetation and silcrete. Gold appears to be the best target element although As may also provide information on the location of mineralisation.
  2. Data sets obtained by remote sensing were particular useful in discriminating regolith units and in determining surface mineralogies. They were important in assisting in the construction of the regolith-landform map.
  3. Landsat TM was most useful in mapping at the 1:5000 scale required for this study. A DEM was generated from aerial photographs and was useful in distinguishing landforms. Ground-truthing was important in delineating features, such as the extent and type of lag deposits, and in the final construction of the map. iv
  4. The complimentary use of PIMA and visual logging of chip trays enabled determination of selected clay mineralogy, colour and lithology of the regolith including the boundary between transported and in situ regolith. 3D and 2D visualisations of the data assisted in determining the regolith stratigraphy and were able to integrate the information from PIMA, geochemistry and other data sets.

The study has led to a better understanding of Au dispersion in sand dunes, improved interpretation of calcrete-hosted Au anomalies, and indicated potential new drilling targets.

M. J. Lintern and I. J. Tapley
Project Leaders April 2003


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