CRC LEME
Open File Report 155
ABSTRACT
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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