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Open File Report 155

Regolith characterisation and geochemistry as an aid to mineral exploration in the Harris Greenstone Belt, Central Gawler Craton, South Australia. Includes Regolith landform map of the Lake Harris Region 1: 10 000

Sheard, M.J. and Robertson, I.D.M.

Preface and Executive Summary

The Harris Greenstone Belt, southeast of Tarcoola in the Central Gawler Craton, is a newly outlined greenstone province that has been the focus of recent drilling investigation to determine its extent and potential to host mineralisation, in particular nickel and gold. Over 95% of the greenstone, inferred from geophysical interpretation, is under regolith cover.

Investigations detailed in this report focus on the regolith units and complement the bedrock drilling programs reported elsewhere.

The objectives were to:

  • evaluate the use of components of the transported cover and in situ weathered bedrock to identify the presence of underlying fresh mafic and ultramafic rocks of the greenstone package.
  • evaluate the use of surface and shallow soil geochemistry as a means of outlining subsurface greenstone and to locate areas of anomalous metal concentration in bedrock sources.

The approach adopted was:

  • characterisation of the regolith at 3 key sites using drill cuttings augmented by purpose-drilled, fully-cored reference holes through the regolith and including petrography and geochemistry of those drill cores.
  • regolith mapping at one key site, Lake Harris, together with characterisation of surface regolith samples and shallow soil and surface sample geochemistry over a portion of the mapped area.
  • review of landscape evolution based on previous work and placing each key site in context with regard to this review.

The results show that:

  • greenstone packages are commonly more deeply weathered than adjacent felsic rocks and in places are preferentially eroded and filled by up to 80m of younger channel fill sediments.
  • shallow transported cover (<5m) over buried greenstone usually contains components of the underlying bedrock that can be used to identify it by geochemistry of appropriate sample media. These areas are expected to also show element anomaly related to shallow bedrock mineralisation. Regolith mapping that incorporates cover thickness is an important aid for interpretation of any geochemical data.
  • recognition of the unconformity boundary between transported cover and in situ weathered bedrock proved difficult at one site due to the presence of a debris flow deposit of similar mineralogy and weathering to underlying in situ bedrock. The availability of core samples was critical to recognition of this unit. Zircon contents may be helpful in defining the transported/in situ boundary on weathered greenstone.
  • pedolith zone on greenstone is typically thinly developed or poorly preserved (commonly <5m). The usually thicker saprolite zone (up to 90m) is readily identified as formed from greenstone protolith by mineralogy (smectite), chemistry (elevated Cr, Ni, Cu, Mg, etc) and petrography (spinifex textures preserved in metakomatiites).

M.J. Sheard and I.D.M. Robertson,
May 2004


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