CRC LEME
Open File Report 156
Preface & Executive Summary
The South Australian Regolith Project Final Report - Summary and
Synthesis
Compiled by M. J. Lintern
Preface
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
|
